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Changes in net ecosystem productivity and greenhouse gas exchange with fertilization of Douglas fir:.. Grant, Robert F. 2011

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Changes in net ecosystem productivity and greenhouse gas exchange with fertilization of Douglas fir: Mathematical modeling in ecosys R. F. Grant,1 T. A. Black,2 R. S. Jassal,2 and C. Bruemmer2 Received 6 July 2009; revised 19 November 2009; accepted 1 April 2010; published 8 October 2010. [1] The application of nitrogen fertilizers to Douglas fir forests is known to raise net ecosystem productivity (NEP), but also N2O emissions, the CO2 equivalent of which may offset gains in NEP when accounting for net greenhouse gas (GHG) exchange. However, total changes in NEP and N2O emissions caused by fertilizer between times of application and harvest, while needed for national GHG inventories, are difficult to quantify except through modeling. In this study, integrated hypotheses for soil and plant N processes within the ecosystem model ecosys were tested against changes in CO2 and N2O fluxes recorded with eddy covariance (EC) and surface flux chambers for 1 year after applying 20 g N m−2 of urea to a mature Douglas fir stand in British Columbia. Parameters from annual regressions of hourly modeled versus measured CO2 fluxes conducted before and after fertilization were unchanged (b = 1.0, R2 = 0.8, RMSD = 3.4 mmol m−2 s−1), indicating that model hypotheses for soil and plant N processes did not introduce bias into CO2 fluxes modeled after fertilization. These model hypotheses were then used to project changes in NEP and GHG exchange attributed to the fertilizer during the following 10 years until likely harvest of the Douglas fir stand. Increased CO2 uptake caused modeled and EC‐derived annual NEP to rise from 443 and 386 g C m−2 in the year before fertilization to 591 and 547 g C m−2 in the year after. These gains contributed to a sustained rise in modeled wood C production with fertilization, which was partly offset by a decline in soil C attributed in the model to reduced root C productivity and litterfall. Gains in net CO2 uptake were further offset in the model by a rise of 0.74 g N m−2 yr−1 in N2O emissions during the first year after fertilization, which was consistent with one of 1.05 g N m−2 yr−1 estimated from surface flux chamber measurements. Further N2O emissions were neither modeled nor measured after the first year. At the end of the 11 year model projection, a total C sequestration of 1045 g C m−2 was attributed to the 20 g N m−2 of fertilizer. However, only 119 g C m−2 of this was sequestered in stocks that would remain on site after harvest (foliage, root, litter, soil). The remainder was sequestered as harvested wood, the duration of which would depend on use of the wood product. The direct and indirect CO2‐equivalent costs of this application, including N2O emission, were estimated to offset almost all non‐harvested C sequestration attributed to the fertilizer. Citation: Grant, R. F., T. A. Black, R. S. Jassal, and C. Bruemmer (2010), Changes in net ecosystem productivity and greenhouse gas exchange with fertilization of Douglas fir: Mathematical modeling in ecosys, J. Geophys. Res., 115, G04009, doi:10.1029/2009JG001094. 1. Introduction [2] Most temperate and boreal forest ecosystems are considered to be nitrogen limited, so that N fertilizer application may increase forest CO2 uptake and consequent C storage [Aber et al., 1989; Johnson and Curtis, 2001], slowing the rise in atmospheric CO2 concentration and increasing wood supply. However, N fertilizer production involves the emission of CO2, and its application may also increase emission of N2O [Jassal et al., 2008; Matson et al., 1992; Sitaula et al., 1995b], an important greenhouse gas (GHG). These emissions offset the effects of increased CO2 uptake on net GHG exchange and thereby on the radiative properties of the atmosphere thought to drive climate change. Therefore, a full accounting of net GHG exchange 1Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada. 2Biometeorology and Soil Physics Group, Faculty of Land and Food Systems, University of British Columbia, Vancouver, British Columbia, Canada. Copyright 2010 by the American Geophysical Union. 0148‐0227/10/2009JG001094 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, G04009, doi:10.1029/2009JG001094, 2010 G04009 1 of 17 from fertilizer application in forests needs to include effects on net CO2 uptake by trees as well as on CO2 and N2O emissions from N fertilizer production and application. [3] Gains in net CO2 uptake from fertilization of forest stands are thought to be caused by root uptake and root‐ shoot transfer of fertilizer N, resulting in increased foliar N concentrations that raise photosynthetic capacity and hence CO2 fixation for several years after application [Hopmans and Chappell, 1994]. However, these gains are highly var- iable because they are affected by the biochemical and hydrologic characteristics of the fertilized stands. Gains may be smaller with lower C/N ratios in tree foliage or the soil LFH layer when N limitations are less severe [Edmonds and Hsiang, 1987; Hopmans and Chappell, 1994]. Gains may also be smaller when uptake of fertilizer N is reduced by rapid nitrification of fertilizer products [Matson et al., 1992] and subsequent leaching of nitrate [Flint et al., 2008]. [4] Gains in net CO2 uptake from fertilization are also affected by changes in C allocation to organs with differing turnover rates within trees. With improved N supply, trees typically allocate more resources to shoot growth, much of it in boles with slower turnover, than to root growth with more rapid turnover. This reallocation is based on a theory of functional equilibrium between shoots and roots in which nutrient allocation is based on proximity to sites of acqui- sition and on rates of consumption [Ericsson et al., 1996]. Reduced root allocation is consistent with reduced soil respiration frequently measured in fertilized forest stands [Giardina et al., 2004; Olsson et al., 2005]. Changes in root‐shoot allocation with fertilization have been shown in coniferous seedlings [e.g., Iivonen et al., 2001; Kaakinen et al., 2004]; however, evidence for such changes in mature stands remains inconclusive. Iivonen et al. [2006] found small decreases in allocation of C and N to fine roots with fertilization, but little change in allocation of C and N to aboveground organs or coarse roots in spite of a large increase in growth. Earlier studies have found both increases and no changes in allocation to roots with fertilization. In a further complication, some studies have indicated that improved N supply causes shoot growth to be allocated more to foliage and branches with more rapid turnover than to boles with slower turnover [Gower et al., 1992; Valinger, 1993]. These changes in allocation require that gains in net CO2 uptake and C storage attributed to fertilization be evaluated in comprehensive ecosystem studies rather than from simple estimates of gains in wood C. [5] Increases in N2O emissions from fertilization of forest stands are thought to be caused by nitrification and deni- trification of fertilizer N and its mineral products [Martikainen and de Boer, 1993; Matson et al., 1992]. N2O emissions from nitrification are known to be favored by low pH found in acidic forest floors that develop under conif- erous stands [Martikainen and de Boer, 1993; Sitaula et al., 1995b], suggesting that these emissions may be supple- mented by chemodenitrification [e.g., Mørkved et al., 2007]. Estimates of N2O emissions derived from surface flux measurements in forests are highly variable. Although these emissions are thought to be small [Matson et al., 1992], the growing use of N fertilizer may cause them to rise. Jassal et al. [2008] measured substantial N2O emissions (5% of ad- ded N) during several months after a fertilizing a Douglas fir stand in British Columbia, although other researchers have measured less (e.g., 0.5% by Sitaula et al. [1995b] during 1 month after fertilizing Scots pine, 0.35% by Matson et al. [1992] during a growing season after fertilizing Douglas fir, and 0.2% by Bowden et al. [1991] during 1 year after Figure 1. Conceptual model of key C and N transfers in ecosys. Numbers in brackets refer to equations in Appendix A. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 2 of 17 fertilizing in red pine). However, even small emissions of N2O could partially offset gains in CO2 uptake when esti- mating net GHG exchange from N fertilizer application. [6] The variability and duration of changes in CO2 and N2O exchange caused by fertilizer N application in different forest stands complicate efforts in GHG accounting for fertilizer use directly from site studies. Such accounting could benefit from mathematical models based on a com- prehensive set of integrated hypotheses for the biological and physical processes driving changes in GHG exchange. In this study, integrated hypotheses for soil and plant N processes within the detailed ecosystem model ecosys are tested against changes in CO2 and N2O fluxes recorded with eddy covariance (EC) and surface flux chambers during the first year after applying 20 g N m−2 of urea to a mature Douglas fir stand near Campbell River in British Columbia. These hypotheses are then used to project changes in GHG exchange and in C and N storage from this application during the following 10 years until the likely harvest of the stand. 2. Model Development 2.1. General Overview [7] Key algorithms for C and N transformations that drive ecosystem productivity and greenhouse gas exchange in ecosys are described in detail elsewhere [Grant and Flanagan, 2007; Grant et al., 1993a, 1993b, 2005, 2006a, 2006b, 2006c, 2007a, 2007b, 2007c; Grant, 2004; Grant and Pattey, 2003]. Algorithms that directly govern the transformation, uptake, and assimilation of fertilizer N are described in summary form below with reference to equa- tions and definitions listed in Appendix A and represented in Figure 1. 2.2. Urea Hydrolysis [8] Urea fertilizer first undergoes hydrolysis to NH3 (equation (A1a)) at rates calculated as the product of specific hydrolysis activity and total microbial C (as a proxy for urease), constrained by an Arrhenius function of soil tem- perature [Moyo et al., 1989; Vlek and Carter, 1983] and a Michaelis‐Menten function of urea concentration [Lal et al., 1993] incorporating a competitive inhibition term driven by soil water content to simulate effects of soil drying [Vlek and Carter, 1983]. The hydrolysis product NH3 is main- tained in equilibrium with NH4 + according to soil or residue pH (equation (A1b), in which states they undergo other reactions described below. 2.3. Mineralization‐Immobilization [9] Each kinetic component j (j is labile or resistant) of each microbial population m (m is obligately aerobic bac- teria, obligately aerobic fungi, facultatively anaerobic deni- trifiers, anaerobic fermenters plus H2‐producing acetogens, acetotrophic methanogens, hydrogenotrophic methanogens and methanotrophs, NH3 and NO2 − oxidizers, and non- symbiotic diazotrophs) in each substrate‐microbe complex i (i is coarse woody residue, fine nonwoody residue, partic- ulate organic matter, or humus) in the surface residue and each soil layer l seeks to maintain a set population‐specific C/N ratio by mineralizing NH4 + (equation (A2a)) or by im- mobilizing NH4 + (equation (A2b)) or NO3 − (equation (A2c)). Changes in microbial C and N arise from changes in organic substrate availability and quality and in soil temperature and water content. Provision is made for C/N ratios to rise above set values during immobilization but at a cost to microbial function. Under these conditions, provision is also made for internal recycling of microbial N. These transfor- mations control the exchange of N between organic and inorganic states in soil. Equations representing these trans- formations are given in greater detail by Grant et al. [1993a, 1993b]. 2.4. Nitrification [10] Rates of NH3 and NO2 − oxidation are calculated from specific NH3 or NO2 − oxidizer activities multiplied by NH3 or NO2 − oxidizer biomasses, constrained by an Arrhenius function of soil temperature and Michaelis‐Menten func- tions of aqueous NH3 [Stark and Firestone, 1996] or NO2 − [Blackburne et al., 2007] concentrations and aqueous CO2 concentrations (equations (A3a) and (A3b)). Rates of NO2 − oxidation are inhibited by aqueous NH3 and HNO2 con- centrations [Blackburne et al., 2007] (equation (A3c)). Reduction of NO2 − to N2O by nitrifiers (equation (A3d)) is driven from demand for electron acceptors to oxidize NH3 (equation (A3a)) unmet by O2 because of diffusion limita- tions, and is constrained by Michaelis‐Menten functions of aqueous NO2 − and CO2. Equations for these processes are given in greater detail by Grant et al. [2006b]. 2.5. Biological Denitrification [11] Demand for electron acceptors from denitrifier C oxidation unmet by O2 because of diffusion limitations [Grant et al., 2006b] drives the sequential reduction of NO3 −, NO2 −, and N2O, constrained by Michaelis‐Menten function of NO3 −, NO2 −, and N2O concentrations [Yoshinari et al., 1977] (equations (A4a), (A4b), and (A4c)). All gaseous products undergo convective‐dispersive transfer and aque- ous and gaseous phases. Equations for these processes are given in greater detail by Grant et al. [2006b]. 2.6. Chemodenitrification [12] NO2 − from nitrification and denitrification is in dynamic equilibrium with HNO2 depending on soil or res- idue pH (equation (A5a)). HNO2 concentration drives first‐ order decomposition [Cleemput and Samater, 1996] (equation (A5b)) to N2O [Mørkved et al., 2007] (equation (A5c)) and other N products [Cleemput and Samater, 1996]. 2.7. Root and Mycorrhizal Uptake [13] NH4 + and NO3 − uptake by roots and mycorrhizae is calculated from mass flow plus radial diffusion through soil between adjacent roots and mycorrhizae (equations (A6a) and (A6c)) coupled with active uptake at root and mycor- rhizal surfaces (equations (A6b) and (A6d)) in multilayered soil profiles. Both fluxes depend on root length density derived from a root growth model, as described in greater detail by Grant [1998]. Uptake products in each soil layer are added to nonstructural N pools in roots and mycorrhizae. If N/C ratios of these nonstructural pools rise above those required for growth, inhibition of root and mycorrhizal N uptake is invoked to keep uptake in balance with CO2 fix- ation [Grant, 1998]. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 3 of 17 2.8. Plant Assimilation [14] Nonstructural N pools, generated from root and mycorrhizal uptake, are coupled with nonstructural C pools, generated from CO2 fixation, in mycorrhizae, roots, and branches. Transfers among these pools (equations (A7a) and (A7b)) are driven by concentration gradients generated by acquisition versus consumption of nonstructural N and C in mycorrhizae, roots, and branches [Grant, 1998] (equations (A7c) and (A7d)). Ratios of nonstructural N and C in branches govern CO2 fixation (equations (A7f), (A7g), and (A7h)) [Farquhar et al., 1980] by (1) setting ratios of structural N and C in leaves (A7e) and hence maximum carboxylation rates (equations (A7i) and (A7j) and (2) determining rubisco activation through product inhibition (A7k). Nonstructural C pools also drive autotrophic respi- ration (Ra) to meet maintenance and growth requirements (Rm and Rg) [Grant et al., 2007c, equations (C12)–(C16)]. If Ra falls below Rm, the deficit drives the withdrawal of remobilizable C and N from older leaves and supporting structures into C and N nonstructural pools, the former of which is used to sustain Rm and the latter of which can be translocated to newer foliage. All model equations for C and N are fully coupled to counterparts for phosphorus. Equa- tions for these processes are given in greater detail by Grant et al. [2007c] and in earlier references cited therein. 3. Field Experiment 3.1. Site Description [15] The field experiment was conducted in a stand dominated by Douglas fir (Pseudotsuga menziesii), but also including western red cedar (Thuja plicata), western hem- lock (Tsuga heterophylla), and red alder (Alnus rubra). This stand started to regenerate in 1949 after fires and logging in 1939 and 1943 on a well‐drained Humo‐Ferric Podzol (Quimper sandy loam, Table 1) about 10 km southwest of Campbell River (49°52′7.8″N, 125°20′6.3″W) on the east coast of Vancouver Island, Canada (mean annual tempera- ture and precipitation: 8.6°C and 1450 mm). This stand is further described in the works of Humphreys et al. [2006] and Morgenstern et al. [2004]. 3.2. Fertilizer Treatments [16] A 1115 ha area of the stand surrounding the eddy covariance (EC) flux tower was aerially fertilized with 200 kg N ha−1 urea on 13 January 2007, following standard commercial practice. A 17 ha experimental area on the southeast side of the fertilized area remained unfertilized for a field plot study to measure changes in soil N2O emissions and in plant C and N stocks caused by fertilizer. Within this area, urea was applied manually at 200 kg N ha−1 on 11 April 2007 to four of eight 100 m2 plots, while the remaining four plots remained unfertilized. In addition, eight 2 m × 2 m plots, extended by 2 m wide buffers on each side, were established near the EC flux tower in August 2006 to measure changes in soil CO2 fluxes caused by fertilizer. These plots were protected from aerial fertilization on 13 January 2007, and then four were fertilized manually with urea at 200 kg N ha−1 on 31 January 2007. For both sets of plots, manual fertilizer application achieved the same fertilizer rate as that from the aerial application, but with greater uniformity required for chamber flux measurements. 3.3. Site Measurements [17] Ecosystem CO2 and energy exchange have been measured by EC continuously since 1998 at a flux tower site (BC‐DF49) within the fertilized area as described earlier [Jassal et al., 2007; Humphreys et al., 2006; Morgenstern et al., 2004]. Soil respiration (Rs) has been measured con- tinuously since 2003 by one automated surface flux chamber near the flux tower. Rs measured with this chamber has been shown to represent well the spatial average of Rs for the stand [Jassal et al., 2007]. [18] Soil N2O effluxes were measured with a vented static chamber mounted on 21 cm diameter PVC collars installed in each 100 m2 plot within the 17 ha experimental area. N2O emissions from these plots were recorded every 2–3 weeks from fertilizer application on 11 April to the end of the year, and corroborated with emissions calculated from soil N2O concentration gradients [Jassal et al., 2008]. Soil CO2 effluxes were measured every 2–4 weeks using portable vented chambers mounted on two collars (10 cm inner diameter, 4 mm wall thickness) in each 2 m × 2 m plot near the flux tower, as described by Jassal et al. [2010]. Further details of site management and flux measurements are given by Jassal et al. [2008]. 4. Model Experiment 4.1. Model Initialization and Spinup [19] Before testing ecosys with the CO2 and N2O fluxes recorded at BC‐DF49, the model had to reproduce site conditions by simulating site history. This was accom- plished by initializing ecosys with the biological properties of Douglas fir and a pioneer bush understory and with the physical and chemical properties of the Quimper sandy loam (Table 1). Ecosys was also initialized with stocks of coarse and fine residue estimated to remain after a stand‐replacing fire presumed to have occurred in the model year 1780. This year was selected to start the model runs so that the aboveground biomass generated by the model in 1919 was consistent with that estimated from wood volumes recorded during a 1919 timber cruise. The Douglas fir and bush were seeded after the presumed 1780 fire, grown from model Table 1. Key Properties of the Quimper Sandy Loam Near Camp- bell Rivera Quimper Horizon LFH Ahe Bf Bm Bm BCc Depth to Bottom (m) 0.03 0.09 0.23 0.53 0.83 1.33 Bulk Density (Mg m−3) 0.1 1.3 1.5 1.5 1.5 1.9 Field Capacity (m3 m−3) 0.21 0.21 0.21 0.19 0.13 0.10 Wilting Point (m3 m−3) 0.14 0.08 0.08 0.06 0.04 0.03 Ksat (mm h −1) 210 10 10 100 100 1 Sand (g kg−1) 0 540 660 760 720 740 Silt (g kg−1) 0 380 270 220 250 240 Clay (g kg−1) 0 80 70 20 30 20 Coarse Frag. (m3 m−3) 0 0.33 0.33 0.33 0.33 0.33 CEC (cmol kg−1) 124 20 10 9 10 9 pH 4.7 5.0 4.9 4.9 4.7 4.8 Organic C (g kg−1) 385 74 62 17 11 9 Total N (g Mg−1) 8100 2200 1900 850 750 720 aFrom Humphreys [2004] and Soil Landscapes of Canada v. 3.1. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 4 of 17 years 1780 to 1919 under hourly weather data recorded at BC‐DF49 during 2004, then from model years 1920 to 1997 under daily weather data constructed from meteorological records [Régnière and St‐Amant, 2007], and then from 1998 to 2006 under hourly weather data recorded at BC‐DF49 during 1998–2006. The modeled Douglas fir stand was burnt in 1939, salvage logged and burnt again in 1943, re- seeded in 1949, and fertilized with 20 g N m−2 as broadcast urea in 1994 so that the age and disturbance history of the modeled stand at the end of 2006 corresponded to that at BC‐DF49 as derived from field records. 4.2. Boundary Conditions [20] During the model run, atmospheric CO2 concentra- tion (Ca) was prescribed to rise exponentially from 280 to 385 mmol mol−1, and concentrations of NH4 + and NO3 − in precipitation used to simulate wet N deposition were pre- scribed to rise from historical values based on Holland et al. [1999] to current values based on Meteorological Service of Canada [2004]. Atmospheric concentration of NH3 used to simulate dry N deposition was maintained at 0.002 mmol mol−1. A background mortality rate of 0.75% per year was applied to Douglas fir during model runs, simulating natural self‐thinning. 4.3. Model Testing [21] The model run was then continued during 2007 under hourly weather data recorded at BC‐DF49, with and without 20 g N m−2 of urea broadcast on 13 January to simulate changes in CO2 exchange, or on 11 April to simulate changes in N2O emissions. The ability of the model to simulate changes in forest productivity with N fertilizer was evaluated by testing modeled CO2 fluxes against EC CO2 flux measurements under comparable environmental con- ditions before and after fertilization. The ability of the model to simulate changes in N2O emissions with N fertilizer was evaluated by testing modeled N2O fluxes against surface chamber measurements recorded on the fertilized versus unfertilized plots near BC‐DF49. To examine longer‐term effects of the 2007 fertilization, these model runs were extended by a further 10 years until the likely harvest date of the stand, under weather recorded during 2005, a meteoro- logically near‐average year. Values of all model parameters in this study were unchanged from those in earlier studies [e.g., Grant, 2004; Grant et al., 2005, 2006a, 2006c, 2007b, 2007c]. 5. Results 5.1. Uptake and Assimilation of Fertilizer N [22] Fertilization on 13 January 2007 hastened root uptake and root‐shoot transfer of N in the model, raising average needle N content of Douglas fir by 14% from 0.022 g N g C−1 in the unfertilized stand to 0.025 g N g C−1 in the fer- tilized stand by 6 December 2007, about 11 months after fertilization (Table 2). This increase was smaller than one from 0.023 to 0.032 g N g C−1 (assuming needles are 50% C) measured at BC‐DF49 on this date by Jassal et al. [2008], but similar to one from 0.022 to 0.027 g N g C−1 measured by Nason et al. [1990] 30 weeks after applying 20 g N m−2 as urea to a nearby Douglas fir stand of similar age. [23] In the model, this rise was driven by hydrolysis of the urea fertilizer to NH3 (equation (A1a)) and equilibration with NH4 + (equation (A1b)), which drove nitrification to NO2 − (equation (A3a)), and NO3 − (equation (A3b)) [Grant, 1994], and reduction of NO2 − to N2O (A3d) [Grant, 1995]. The NH4 + and NO3 − products of hydrolysis and nitrification were also subject to competitive uptake by microbial populations (A2b and A2c) [Grant et al., 1993a, 1993b] and by root and mycorrhizal populations (equations (A6a)– (A6d)) [Grant, 1998]. Root and mycorrhizal uptake pro- ducts raised root and mycorrhizal nonstructural N con- centrations (equation (A7c)) and thereby transfers to shoot nonstructural N (equations (A7a) and (A7b)), raising shoot nonstructural N (equation (A7d)) and hence the N/C ratio of leaf structural growth (equation (A7e)) that determined the foliar N concentrations. 5.2. Fertilization and Net CO2 Exchange [24] Greater foliar N content in the model raised photo- synthetic capacity (equations (A7i) and (A7j)), rubisco activation (equation (A7k)), and hence CO2 fixation rates (equations (A7f), (A7g), and (A7h)) [Grant et al., 2001] in the fertilized stand. Greater CO2 fixation rates caused more rapid CO2 influxes to be modeled and measured during 2007 than under comparable weather in 2001 (e.g., DOY 163–169 in Figure 2b versus 2d), the year meteorologically most similar to 2007 within the instrumental period prior to fertilization as apparent from meteorological data summa- rized in Table 3. However, CO2 effluxes modeled or gap‐ filled during 2007 were comparable to those during 2001, causing net C uptake to rise with fertilization. [25] Rapid net C uptake modeled and measured in 2007 (Figure 2b) allowed daily net ecosystem productivity (NEP) to rise with air temperature (Ta) and day length to maximum values of 5–6 g C m−2 d−1 sustained during May and June (Figure 3b). Daily NEP subsequently declined with Ta and day length after the summer solstice but remained positive (C sink) until mid‐October. Slower net C uptake modeled and measured in 2001 (Figure 2d) caused maximum daily NEP of 4–5 g C m−2 d−1 to be sustained only to mid‐May, after which NEP declined to near zero (C neutral) by mid‐ September, about 1 month earlier than in 2007 (Figures 3d versus 3b). Lower maxima and earlier declines in daily NEP were also modeled and measured in 2006, the year Table 2. Changes in Stocks and Transfers of C and N Between Fertilized and Unfertilized Stands of Douglas Fir Modeled by Ecosys and Estimated From Biometric Measurements at BC‐ DF49 on 6 December 2007 After Fertilization on 11 April 2007 (N2O) or 13 January 2007 (Others) Unit Fertilized Unfertilized Modeled Measured Modeled Measured Foliar Na (g N g C−1) 0.025 0.032 0.022 0.023 Litterfall (g C m−2 yr−1) 55 57 ± 24 86 62 ± 5b N2O emissiona (g N m−2 yr−1) 0.753 1.04 0.017 −0.014 Foliar Ca (g C m−2) 833 0.280c 745 0.223c Wood C (g C m−2) 414 1.15 ± 0.17d 216 0.78 ± 0.31d aMeasured data from Jassal et al. [2008]. bLitterfall in unfertilized plots from J. A. Trofymow (personal communication). cGrams C per 100 needles assuming dry matter is 50% C. dAnnual increment in bole diameter (mm) from Jassal et al. [2009]. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 5 of 17 before fertilization (Figure 4), as well as in other earlier years at this site [Grant et al., 2009], indicating a consistent seasonal pattern in NEP before fertilization. [26] These earlier seasonal declines in NEP prior to fer- tilization in 2007 were modeled from earlier seasonal de- clines in foliar nonstructural N content (equation (A7d)) and hence in foliar structural N content (equation (A7e)) and rubisco activation (equation (A7k)) as described in section 5.1. Seasonal declines in foliar nonstructural N content occurred when rates of nonstructural N assimilation driven from biomass growth (equation (A7e)) exceeded rates of nonstructural N transfer (equations (A7a) and (A7b)) from roots driven from mineral N uptake by roots and mycor- rhizae (equations (A6a)–(A6d)). These modeled declines were delayed in 2007 by additional mineral N uptake of fertilizer products. Net C uptake in all 3 years was adversely affected by several brief warming events (Figures 3a and 3c), as measured and modeled in other years at this site [Grant et al., 2007c, 2009]. [27] To test consistency of model performance before and after fertilization, hourly modeled CO2 fluxes were re- gressed on hourly averaged EC CO2 fluxes measured under conditions of adequate turbulence during each year from 1998 to 2007 (i.e., excluding gap‐filled values). Parameters and correlation coefficients from this regression in 2007 were very similar to those from 1998 to 2006 before fer- tilizer application (b = 1.0, R2 = 0.8, RMSD = 3.4 mmol m−2 s−1 in Table 3), indicating that the model response to added N did not introduce a bias into modeled CO2 fluxes with respect to measured values. Figure 2. (a, c) Incoming shortwave radiation and air temperature and (b, d) CO2 fluxes measured (solid symbols), gap filled (open symbols), and modeled (lines) during DOY 163–169 (12–18 June) in 2007 after fertilization (Figures 2a and 2b) and in 2001 before fertilization (Figures 2c and 2d) over a Douglas fir stand fertilized on 13 January 2007. Positive fluxes represent CO2 uptake; negative fluxes represent CO2 emission. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 6 of 17 5.3. Fertilization and Net Primary Productivity [28] Greater CO2 fixation rates modeled during 2007 with N fertilization (Figures 2 and 3) caused a 14% gain in annual gross primary productivity (GPP) from that modeled in 2007 without fertilization, consistent with a 10% gain estimated from EC measurements by Jassal et al. [2009] (Table 4). However, fertilizer N had very different effects on Ra and net primary productivity (NPP) of shoots versus roots. Shoot Ra modeled during 2007 rose 21% with fertil- ization, driven by greater Rg and Rm required for greater biomass growth and maintenance [Grant et al., 2007c, equations (C12)–(C16)]. Root Ra modeled during 2007 also rose slightly with fertilization during the first 3 months after application but decreased thereafter so that cumulative root Ra at the end of the year was slightly lower. Shoot NPP in the model rose 43% with fertilization by the end of 2007, while root NPP declined by 33% (Table 4). [29] In the model, these differences in shoot versus root responses to fertilization arose from reduced below‐ground allocation of the nonstructural C product of GPP caused by more rapid consumption of nonstructural C in shoot growth and Ra (equations (A7d) and (A7e)) with more rapid N uptake from fertilization. This consumption lowered con- centration gradients of nonstructural C that drove transfer to roots (equation (A7a)), thereby implementing the functional equilibrium by which shoot and root growth are thought to be governed. 5.4. Fertilization and Heterotrophic Respiration [30] Fertilization lowered heterotrophic respiration (Rh) by 6% from that modeled without fertilization during 2007 (Table 4). This reduction arose from reduced shoot‐root C transfer with fertilization which caused lower nonstructural C concentrations in roots and mycorrhizae and hence less exudation of nonstructural C to soil [Grant, 1993]. Fur- thermore, by raising GPP in the model, fertilization raised production of nonstructural C, thereby reducing remobili- zation of foliar and root structural C needed to meet Rm requirements and hence reducing shoot and root litterfall driven by remobilization [Grant et al., 2007c, equations (C14)–(C18)]. However, measurements in the fertilized versus unfertilized field plots at BC‐DF49 indicated no significant reduction in foliar litterfall with fertilization (Table 2). Less litterfall and exudates modeled with fertil- ization in 2007 provided less substrate for decomposition [Grant et al., 2007c, equation (A3)] and hence for Rh and microbial growth [Grant et al., 2007c, equations (A13) and (A28)]. [31] Lower Rh partially offset higher root Ra modeled during the first three months after fertilizer application, so that soil respiration (Rs = Rh + below‐ground Ra) rose only slightly during this period. Thereafter lower Rh and lower root Ra reduced Rs so that cumulative Rs modeled at the end of 2007 was about 5% lower with fertilization (Table 4). Greater Rs was also measured with surface flux chambers during the first 3–4 months after application in the fertilized versus unfertilized field plots near the BC‐DF49 flux tower, while similar Rs was measured in both treatments thereafter, so that cumulative Rs measured at the end of 2007 was estimated to have been raised by about 6% with fertilization [Jassal et al., 2010] (Table 4). Both modeled and measured results thus indicated only small changes in Rs with fertilization. 5.5. Fertilization, Net Ecosystem Productivity, and Growth [32] Greater gains in GPP versus ecosystem respiration (Re = Ra + Rh) caused a 39% gain in net ecosystem pro- ductivity (NEP) to be modeled with fertilization, similar to a 37% gain estimated from EC measurements by Jassal et al. [2009] (Table 4). Somewhat larger modeled versus mea- sured NEP may partly be attributed to subsurface leaching of dissolved inorganic C (DIC), which was assumed in the model to have been lost from the site, but some of which may in fact have volatilized within the fetch area and so have been detected at the EC tower. [33] By the end of 2007, fertilization caused a substantial rise in shoot C and a decline in root C in the model (Table 4), driven by a rise in shoot NPP, a decline in root NPP and declines in both shoot and root litterfall. However, fertil- ization reduced gains in soil organic C (SOC) during the first year after application because it reduced litterfall more than it did Rs. Table 3. Mean Annual Temperatures, Annual Precipitation, Intercepts, Slopes, Coefficients of Determination, Root Mean Square of Dif- ferences, and Number of Accepted Eddy Covariance fluxes From Regressions of Hourly Modeled CO2 Fluxes Versus Hourly Averaged EC CO2 Fluxes Measured at BC‐DF49 From 1998 to 2007 a Year MAT (°C) Precip (mm) a (mmol m−2 s−1) b R2 RSMD (mmol m−2 s−1) n 1998 9.10 1844 0.24 1.10 0.75 3.3 5187 1999 7.64 1913 0.40 1.09 0.77 3.1 4982 2000 8.21 1127 0.43 1.02 0.76 3.5 4954 2001 8.09 1166 0.36 1.01 0.77 3.3 4990 2002 8.48 1222 0.54 1.07 0.75 3.4 5026 2003 8.48 1621 0.33 1.04 0.77 3.4 5277 2004 8.77 1404 0.63 0.99 0.75 3.8 4362 2005 8.31 1467 0.91 0.96 0.77 3.8 4388 2006 8.40 1809 0.13 1.02 0.76 3.4 5233 2007b 7.71 1482 0.40 1.04 0.80 3.4 5235 aMAT, mean annual temperatures; Precip, annual precipitation; a, intercepts; b, slopes; R2, coefficients of determination; RMSD, root mean square of differences; EC, number of accepted eddy covariance fluxes. bUrea applied at 20 g N m−2 on 13 January 2007. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 7 of 17 5.6. Fertilization and N2O Emissions [34] Emissions of N2O modeled in the fertilized stand and measured in the fertilized field plots were temporally vari- able but continuous during most of 2007, while emissions modeled in the unfertilized stand and measured in the unfertilized field plots were negligible (Figure 5). Total emissions of N2O attributed to the fertilizer from application on 11 April 2007 to the end of the year were 0.74 g N m−2 from differences between fertilized and unfertilized stands in the model and 1.05 g N m−2 derived from differences in measurements of surface fluxes and subsurface concentra- tion gradients in the fertilized versus unfertilized field plots by Jassal et al. [2008] (Table 2). These emissions accounted for 3.7% and 5.2% of the added fertilizer N in the model and the field plots, respectively. [35] In the model, emissions were driven by urea hydro- lysis (equations (A1a) and (A1b)) which accelerated NH4 + oxidation to NO2 − (equation (A3a)) and NO3 − (equation (A3b)) [Grant, 1994], and hence NO3 − reduction to NO2 − (equation (A4a)) and NO2 − reduction to N2O (equations (A3d) and (A4b)) [Grant, 1995; Grant et al., 2006b]. These reductions were driven by demand from oxidation reactions for electron acceptors unmet by O2. However, this demand was strongly limited by aerated conditions in the surface residue to which the urea was added. Rises in NO2 − concentrations from accelerated NH4 + oxidation caused rises Figure 3. (a, c) Air temperature (line), precipitation (bars), and (b, d) daily net ecosystem productivity (NEP) calculated from gap‐filled EC measurements (symbols) or modeled (line) during 2007 after fertil- ization (Figures 3a and 3b) and during 2001 before fertilization (Figures 3c and 3d) over a Douglas fir stand fertilized on 13 January 2007. Positive NEP represents net CO2 uptake; negative NEP represents net CO2 emission. Solid or open symbols represent daily EC values derived from gap‐filling less or more than 24 half‐hourly values, respectively. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 8 of 17 in HNO2 in the model (equation (A5a)) under the low pH of Douglas fir residue (4.75 in the work of Welke and Hope [2005]). HNO2 drove NO2 − reduction through chemodeni- trification (equation (A5b)), a fraction of which generated N2O (equation (A5c)). Large HNO2 concentrations also slowed NO2 − oxidation (equation (A3c)), sustaining the HNO2 concentrations driving chemodenitrification. 5.7. Fertilization and Changes in Ecosystem C [36] Increased GPP from fertilization caused aggregated gap‐filled EC fluxes to rise by about 150 g C m−2 yr−1 during the first year after application, similar to the modeled rise (Figure 6a). In the model, NEP of the fertilized stand was predicted to decline after 2007, rapidly for the first few years and then more slowly thereafter. NEP of the unfer- tilized stand was also predicted to decline with forest age [Grant et al., 2007c] but more slowly than that of the fer- tilized stand, so that the two values of NEP gradually con- verged. The larger modeled versus EC‐derived values may be partly attributed to the accounting of DIC losses as described earlier (Table 4). [37] The modeled gains in NEP from fertilization were found mostly in wood C and partly in foliage C (Figure 6b). By 6 December 2007, about 11 months after fertilization, foliar C in the model rose 12% from 745 g C m−2 in the unfertilized stand to 833 g C m−2 in the fertilized stand, comparable to a rise in 100‐needle C of 23% from 0.223 to 0.280 g measured on this date in the unfertilized versus fertilized field plots by Jassal et al. [2008] (Table 2). By this same date wood C growth in the model rose 92% from 216 to 414 g C m−2 yr−1, which was greater than a rise of 47% in tree ring growth measured in the fertilized versus unfertil- ized plots in 2007 (Table 2). Gains in wood C growth declined during the years after fertilization, so that cumu- lative gains in wood C rose more slowly with time, while gains in foliage C were reversed after several years and eventually lost (Figure 6b). [38] Changes in tree C allocation and hence in shoot versus root NPP with fertilization caused modeled gains in wood and foliage C to be partly offset by losses in soil, root and nonstructural C during the first 3–5 years after fertil- ization (Figure 6c). These losses were caused by more rapid use of nonstructural C in foliage and wood growth (equations (A7d) and (A7e)) which reduced transfers of nonstructural C to roots and mycorrhizae (equation (A7a)) as described earlier. Declines in soil C modeled more than 2 years after fertilization were partly offset by rises in surface litter C (Figure 6c) caused by more rapid aboveground lit- terfall from larger foliar and wood phytomass. Declines in below‐ground C were eventually reversed, so that soil, root, foliage and nonstructural C gradually returned to values similar to those modeled in the unfertilized stand by 11 years after fertilization. 5.8. Fertilization and Changes in Ecosystem N [39] Changes in ecosystem C stocks modeled after fertil- ization were associated with those in ecosystem N stocks. At the end of 2007, the added fertilizer N in the model was found mostly in nonstructural and foliage N through root uptake (equations (A6a)–(A6d)) and root‐shoot transfer (equation (A7b)) (Figure 6d). These gains were partly offset by a decline in root N (Figure 6e) associated with that in root C (Figure 6c). Some of the added fertilizer was retained in soil and surface litter (Figure 6e) through litterfall [Grant et al., 2007c, equation (C18)], mineralization (equation (A2a)), and immobilization (equations (A2b) and (A2c)), or lost to groundwater mostly as NO3 − through nitrification (equations (A3a) and (A3b)) and leaching (Figure 6e), or emittedtotheatmosphereasN2OandN2(Figure6e)fromnitrifi- cation (equation (A3d)), denitrification (equation (A4b)), and Figure 4. (a) Air temperature (lines), precipitation (bars), and (b) daily net ecosystem productivity (NEP) calculated from gap‐filled EC measurements (symbols) or modeled (line) during 2006 before fer- tilization over a Douglas fir stand fertilized on 13 January 2007. Positive NEP represents net CO2 uptake; negative NEP represents net CO2 emission. Solid or open symbols represent daily EC values derived from gap‐filling less or more than 24 half‐hourly values, respectively. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 9 of 17 chemodenitrification (equation (A5c)) (Figure 5). After 2007, there was a gradual translocation of added N out of non- structural stocks into foliage stocks and after 2010 out of nonstructural and foliage stocks into soil, wood, and root stocks (Figure 6d versus 6e). Losses of added N through leaching and gaseous emissions in the model were negligible after 2007 (Figure 6e), as was found from measurements of emissions at BC‐DF49 in 2008 [Jassal et al., 2010]. 6. Discussion 6.1. Fertilizer N and Aboveground Productivity [40] Productivity at the BC‐DF49 site was likely limited by N, as evidenced by foliar N concentrations of only 0.022 g N g C−1 modeled in the unfertilized stand and 0.023 g N g C−1 measured in the unfertilized field plots (Table 2). These concentrations were much lower than 0.029 g N g C−1 below which growth responses to N fertilization in Douglas fir were found by Hopmans and Chappell [1994]. The N concentration of the soil LFH layer at BC‐DF49 Table 4. Annual Carbon Fluxes and Changes in Key C Stocks From Fertilized and Unfertilized Stands of Douglas Fir Modeled By Ecosys and Estimated From Eddy Covariance Measurements at BC‐DF49 in 2007 Fertilized Unfertilized Modeled Measured Modeled Measured Fluxesa (g C m−2 yr−1) GPP 2459 1944 2161 1760b Ra Shoot 1012 838 Rootc 439 455 Total 1451 1293 NPP Shoot 801 560 Rootc 207 308 Total 1008 868 Litter Foliage 55 86 Other AGd 211 190 Rootc,e 264 318 Total 530 594 Rh 417 443 Rs 856 921 897 862 b Re 1868 1397 1736 1362 b NEP (atm) 591 547 425 398b DIC, DOCf 53 57 NEP (total) 538 368 Stock Changes (g C m−2) DABCc Living 476 225 Dead 59 59 DBGCh Roota −83 −7 Storage 25 −1 DSOC 58 89 DSoil CO2 3 3 Total 538 368 aAbbreviations: Ra, autotrophic respiration; Rh, heterotrophic respiration; Rs, ecosystem respiration; DIC, dissolved inorganic carbon; DOC, dissolved organic carbon; ABC, aboveground plant carbon; BGC, below‐ ground plant carbon; SOC, soil organic carbon. bFrom an empirical model fitting measured C fluxes to climate variables from 1998 to 2006 and used to estimate C fluxes from climate variables in 2007 by Jassal et al. [2009, 2010]. cAll root terms include mycorrhizae. dIncludes twig, reproductive material, coarse woody litter. eIncludes litterfall plus exudation. fDissolved inorganic and organic C (mostly aqueous CO2, carbonates) exported in runoff and drainage water. gAboveground C, including foliage, twigs, boles, reproductive material, and nonstructural C. hBelow‐ground C, including coarse and fine roots, mycorrhizae, and nonstructural C. Figure 5. N2O emissions measured (symbols) and modeled (lines) during 2007 from Douglas fir stands that were not fertilized or fertilized with 20 g N m−2 as urea on 11 April 2007 (DOY 101). Figure 6. (a) Net ecosystem productivity (NEP) and cumu- lative changes in (b) aboveground and (c) below‐ground C stocks, and (d) aboveground and (e) below‐ground N stocks and fluxes modeled (lines) and measured (symbols in Figure 6a) over 11 years in a Douglas fir stand fertilized on 11 Jan- uary 2007 versus an unfertilized Douglas fir stand. GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 10 of 17 (0.021g N g C−1 from Table 1) was also much lower than one of 0.036 g N g C−1 below which growth of Douglas fir was found by Edmonds and Hsiang [1987] to become N limited. These foliar and soil N contents indicated that a strong response of CO2 uptake to N fertilizer should be expected at BC‐DF49, as modeled and measured in Figures 2 and 3. [41] The modeled response of CO2 uptake to N fertilizer generated a gain in modeled wood C of 752 g C m−2 by 8 years after the 20 g N m−2 urea application (Figure 6b), which was consistent with gains measured under similar soil and climates in earlier field studies. This gain was larger than an average gain of ∼550 g C m−2 calculated from volume growth measured in field studies 8 years after applying 22.4 g N m−2 urea to young (<30 years) Douglas fir stands in the nearby states of Oregon and Washington [Hopmans and Chappell, 1994; Stegemoeller and Chappell, 1990]. However, most of the stands in these studies had foliar N contents before fertilization that were larger than that at BC‐DF49, so that a smaller response of growth to fertilizer might be expected. The time course of the modeled gain in wood C attributed to the fertilizer application in this study was similar to, or smaller than, ones averaging 2430 g C m−2 over 16 years measured by Adams et al. [2005] after four applications of 22.4 g N m−2 on soils in Washington State similar to that at BC‐DF49. 6.2. Fertilizer N and Below‐Ground Productivity [42] Gains in foliar and wood C with fertilization in the model were partly offset for several years by declines in root and soil C (Table 4) which were in turn only partly offset by a rise in surface litter C (Figure 6c). These declines were caused by reductions in below‐ground C allocation and hence in NPP and litterfall of roots and mycorrhizae as described earlier, so that fertilization caused net reductions in modeled root and microbial growth and hence in Ra, Rh, and Rs (Table 4). At this stage of model development, direct adverse effects of high soil N on Rh are not simulated because underlying mechanisms are not well understood [Berg and McClaugherty, 2003]. Therefore reductions in Rh with fertilization may have been underestimated when modeled only from reduced root C inputs. [43] Reductions in Rs with fertilization in the model (5% in Table 4) were consistent with, but usually smaller than, reductions of Rs measured after fertilization of tropical (18% in the work of Giardina et al. [2004]), temperate (17% in the work of Burton et al. [2004]; ∼25% in the work of Haynes and Gower [1995]), and boreal (40% in the work of Olsson et al. [2005]) forests. In some studies, Rs has been found to rise slightly with fertilization for a brief period after application and to decline thereafter [Burton et al., 2004], as was modeled in this study. As in this modeling study, de- clines in Rs measured in these experiments were attributed to reductions in below‐ground C allocation and hence in root and mycorrhizal growth and litterfall [Haynes and Gower, 1995] and in microbial growth [Burton et al., 2004], which were only partly offset by increases in aboveground litterfall [Giardina et al., 2004]. Some studies have indi- cated gains in soil C following fertilization, although with limited certainty [e.g., Adams et al., 2005]. These results indicate that gains in C sequestration attributed to fertiliza- tion must be estimated from total changes in aboveground, surface, and below‐ground C stocks, rather than from changes in wood C alone. 6.3. Retention of Fertilizer N [44] The rapid early gain in NEP from fertilizer N in the model (Figure 6a) was driven by rapid uptake of fertilizer N, much of which appeared as nonstructural and foliar N in trees within the first year after application (Figure 6d). This model result is consistent with those from many field studies showing that fertilizer N in forests is largely taken up during the first growing season after application [Mead et al., 2008]. Six months after application, the fertilized stand in the model gained 9.5 g N m−2 in aboveground phytomass (foliage, wood, and nonstructural) and 7.4 g N m−2 in soil (mostly as nitrate) above values in the unfertilized stand (Figures 6d and 6e). These gains in the model were slightly larger than ones of 5.8 and 6.0 g N m−2 in trees and soil, respectively, measured by Flint et al. [2008] 6 months after a spring application of 22.4 g N m−2 as urea on a similar Douglas fir stand in nearby Washington State. However, the gains measured in their study may have been underestimated because the gain in tree N did not account for gain in tree biomass and the gain in soil N was measured only to 0.4 m depth and excluded the coarse organic fraction. These early gains in N measured in a field study conducted under con- ditions comparable to those at BC‐DF49 corroborate the rapid uptake of fertilizer N in the model. The large nitrate accumulation modeled during 2007 (Figure 6e) arose from rapid nitrification of the added fertilizer, consistent with rapid nitrification measured in fertilized Douglas fir soils [Matson et al., 1992]. [45] After several years, N gains modeled in the fertilized trees were distributed away from nonstructural and foliar stocks into root, wood, and soil stocks (Figures 6d and 6e). The time course of the initial rise and subsequent decline of modeled gains in foliar N was consistent with that of foliar fertilizer‐derived N measured in field experiments with Douglas fir [e.g.,Mead et al., 2008]. After 10 years, fertilized trees in the model gained 3.2 and 6.3 g N m−2 in foliar and nonfoliar (wood, nonstructural) phytomass, and the fertilized soil gained 6.0 g Nm−2 as organic matter, above values in the unfertilized stand. These gains in the model were comparable to ones of 3.8 and 7.0 g N m−2 in foliar and nonfoliar (wood) phytomass and 8.0 g N m−2 in soil measured by Mead et al. [2008] 10 tears after applying 20 g N m−2 as urea to nearby Douglas fir stands of a similar age to that at BD‐DF49. The long‐term gain in soil + litter N with respect to soil + litter C in the model (Figures 6e versus 6c) caused gradual declines in soil + litter C/N ratios following fertilization as has been measured in Douglas fir stands by Prietzel et al. [2004]. These declines indicated long‐term retention of fertilizer N in soils with large initial C/N ratios, enabling long‐term increases in net N mineralization [Prietzel et al., 2004] that sustained long‐term gains in ecosystem productivity (Figures 6a and 6b) [Adams et al., 2005]. 6.4. Losses of Fertilizer N [46] Some of the fertilizer N was not retained by the forest stand, but was lost from leaching and gaseous emissions. Of the 20 g N m−2 fertilizer N added in the model on 13 January, 2.1 g N m−2 was leached and 0.8 g N m−2 emitted to the atmosphere as N2O and N2 during the first year after appli- GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 11 of 17 cation (Figure 5c). However, no fertilizer N was emitted thereafter, as was also found from N2O flux measurements taken during 2008 at the BC‐DF49 site [Jassal et al., 2010]. Most of the leaching loss modeled in 2007 occurred under heavier rainfall after mid‐October 2007 when plant growth and hence uptake had slowed (Figure 3). This loss was larger than one of 0.4 g N m−2 measured by Flint et al. [2008], most of which also occurred after mid‐October, following a spring application of 22.4 g N m−2 as urea to a Douglas fir stand in nearby Washington State under climate conditions similar to those at BC‐DF49. Substantial downward water movement occurs through the soil profile with heavier rainfall and cooler weather after mid‐October in the Pacific Northwest. These leaching losses would thus represent most of the residual mineral N not taken up during the previous summer growing season, indicating that most fertilizer N was taken up soon after application at both sites, but perhaps more so in the study of Flint et al. [2008]. [47] The fraction of fertilizer N emitted as N2O, or emission coefficient EF, during the first year after the urea application on 11 April in the model (3.74%) and in the field study (5.2%) (Figure 5) were larger than ones measured in other studies of fertilizer application in forests. Sitaula et al. [1995b] calculated an annual EF of 1.7% in boreal pine by extrapolating 1 month of measurements. Matson et al. [1992] calculated an annual EF of 0.35% in Douglas fir from a few measurements during one growing season. In both these experiments, N was applied as ammonium nitrate, only one half of which would contribute to N2O emissions through nitrification. However, Brumme and Beese [1992] measured a similar annual EF of 1.6% in an acidic beech soil from five measurements per day with automated chambers following application of ammonium sulphate. The EFs modeled and measured in this study were also larger than those commonly measured or modeled in arable soils [e.g., Grant et al., 2006b]. [48] The large N2O emissions modeled after fertilization in this study were attributed mostly to chemodenitrification of NO2 − from nitrification (70%), rather than to biological reduction of NO2 − from nitrification and denitrification (30%). These emissions were driven by the low pH of Douglas fir litter (4.75) to which the fertilizer was applied (equation (A5a)). However, the parameterization of che- modenitrification in the model, while based on literature values derived from laboratory incubations (equations (A5b) and (A5c)), needs further testing in field studies. Nonethe- less the lower pH of forest soils is known to favor pro- duction of N2O over NO3 − during nitrification [Martikainen and de Boer, 1993; Sitaula et al., 1995a]. Consequently raising forest soil pH from 4.5 to 6.5 by liming has been found to reduce N2O emissions substantially [Brumme and Beese, 1992]. N2O emissions from coniferous forest soils may therefore be more sensitive to N additions from fertil- izer or deposition than are those from arable soils with generally higher pH [Sitaula et al., 1995b]. The possibility of larger N2O EFs for fertilizer applications on forest soils with lower pH needs to be examined in further studies, given the rising use of fertilizer in forest production. 6.5. Net CO2‐Equivalent Exchange From Fertilizer N [49] Fertilization of mature Douglas fir stands would typically occur 10–12 years before harvesting (although it may be done earlier) so that net GHG exchange attributed to fertilizer application might be assessed over this period. After 11 years, a total C sequestration of 1045 g C m−2 in the model was attributed to the 20 g N m−2 of fertilizer applied on 13 January 2007 (Figures 6b and 6c). Of this amount, only 48 g C m−2 was sequestered in below‐ground pools (root, litter, soil) that would remain on site after har- vest (Figure 6c). An additional 71 g C m−2 was sequestered as foliage, some of which could be lost at harvest, depending on logging practices. Most of these pools would be respired after harvest, and only a fraction, depending on clay content, would be stabilized in long‐term humus pools. The remaining 926 g C m−2 in the model was sequestered as wood, the duration of which would depend on uses of the wood product. [50] This sequestration had a direct cost of 0.60 g N m−2 or 78 g C m−2 equivalent from N2O emission and indirect costs of 17 g C m−2 from fertilizer transport and application [Jassal et al., 2008] and a further 2.7 g C m−2 equivalent through N2O emission from fertilizer N leached to ground or surface water. These costs offset C sequestration in all below‐ground pools and most of that in foliage for the 11 years after application, so that any net gain in sequestration from this fertilizer application would depend entirely on the accounting of C from harvested wood products. Appendix A A1. Urea Hydrolysis [51] COðNH2Þ2l þ H2O ! CO2l þ 2NH3l ðA1aÞ ½NHþ4l  $ ½NH3sl  þ ½Hþl  pKa ¼ 9:25 ðA1bÞ A2. Mineralization‐Immobilization [52] UNH4i;l;m;j ¼ ðMCi;l;m;jCNj MNi;l;m;jÞðmineralizationÞ UNH4 < 0 ðA2aÞ UNH4i;l;m;j ¼ min MCi;l;m;jCNj MNi;l;m;j  n ; U 0 NH4 Ai;l;m;j NH þ 4i;l;m;j h i  NHþ4 mn    . NHþ4i;l;m;j h i  NHþ4 mn  þ KNH4 o UNH4 > 0 ðA2bÞ UNO3i;l;m;j ¼ min n MCi;l;m;jCNj  MNi;l;m;j þ UNH4i;l;m;j    ; U 0 NO3 Ai;l;m;j NO  3i;l;m;j h i  NO3 mn    . NO3i;l;m;j h i  NO3 mn  þ KNO3 o UNO3 > 0 ðA2cÞ GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 12 of 17 A3. Nitrification [53] XNH3i;l;n ¼ X 0 NH3 MCi;l;n;a NH3sl½ = NH3sl½  þ KNH3ð Þf g  CO2sl½ = CO2sl½  þ KCO2ð Þf gftsl fO2l ðA3aÞ XNO2i;l;o ¼ X 0 NO2 MCi;l;o;a NO  2l h i. NO2l h i þ KNO2o  n o  CO2sl½ = CO2sl½  þ KCO2ð Þf gftsl fO2l fil ðA3bÞ fil ¼ 1:0= 1:0þ NH3sl½ =KiNH3ð Þ 1:0þ HNO2l½ =KiHNO2ð Þf g ðA3cÞ RNO2i;l;n ¼ R 0 NO2i;l;n NO2l h i. NO2l h i þ KNO2n  n o  CO2sl½ = CO2sl½  þ KCO2ð Þf g ðA3dÞ A4. Biological Denitrification [54] RNO3i;l;d ¼ R 0 NO3 i;l;d NO3l h i. NO3l h i þ KNO3   ðA4aÞ RNO2i;l;d ¼ R 0 NO3 i;l;d  RNO3i;l;d   NO2  . NO2  þ KNO2d  ðA4bÞ RN2Oi;l;d ¼ 2 R 0 NO3i;l;d  RNO3 i;l;d  RNO2 i;l;d    N2Ol½ = N2Ol½  þ KN2Oð Þ ðA4cÞ A5. Chemodenitrification [55] HNO2l½  $ NO2l h i þ Hþl   pKa ¼ 3:3 ðA5aÞ QNO2l ¼ Q 0 NO2 HNO2l½ l ftsl ðA5bÞ QN2Ol ¼ fN2O QNO2l ðA5cÞ A6. Root and Mycorrhizal Uptake [56] UNH4s;l;r ¼ Uws;l;r NHþ4l  þ 2Ls;l;rDeNH4 NHþ4l  NHþ4s;l;rh i n. ln ds;l;r=rs;l;r  o ftsl ðA6aÞ ¼ U 0NH4As;l;r NHþ4s;l;r h i  NHþ4mn    . NHþ4s;l;r h i  NHþ4mn  þ KNH4 ftsl ðA6bÞ UNO3s;l;r ¼ Uws;l;r NO3l  þ 2Ls;l;rDeNO3 NO3l  NO3s;l;rh i n . ln ds;l;r=rs;l;r  o ftsl ðA6cÞ ¼ U 0NO3As;l;r NO3s;l;r h i  NO3mn    . NO3s;l;r h i  NO3mn  þ KNO3 ftsl ðA6dÞ A7. Plant Assimilation [57] Cs;l;r=MCs;l;r þ TCs;l;r ¼ Cs;b=MCs;b  TCs;l;r ðA7aÞ Ns;l;r=Cs;l;r  TNs;l;r ¼ Ns;b=Cs;b þ TNs;l;r ðA7bÞ Ns;l;rðtÞ ¼ Ns;l;rðt1Þ þ UNH4s;l;r þ UNO3s;l;r  TNs;l;r  GNs;l;r ðA7cÞ Ns;bðtÞ ¼ Ns;bðt1Þ þ X l TNs;l;r  GNs;b ðA7dÞ GNs;l;r ¼ MNs;b=MCs;b ¼ MCs;b=tf min Ns;b . Ns;b þ Cs;b=KiN   ; n  Ps;b . Ps;b þ Cs;b=KiP  gÞ ðA7eÞ GPPs ¼ X b X l X k X x X y X z min Vbs;b;l;k;x;y;z ;Vjs;b;l;k;x;y;z n o ðA7f Þ Vbs;b;l;k;x;y;z ¼ Vbmaxs;b;k ðCcs;b;l;k;x;y;z  Gs;b;kÞ =ðCcs;b;l;k;x;y;z þ KCO2sÞ fys ftbs fNs;b ðA7gÞ Vjs;b;l;k;x;y;z ¼ "Is;l;x;y;z þ Vjmaxs;b;k  "Ii;l;x;y;z þ Vjmaxs;b;k  2  4"Is;l;x;y;zVjmaxs;b;k 	0:5	, ð2ÞYs;b;l;k;x;y;z fys ftjs fNs;b ðA7hÞ Vbmaxs;b;k ¼ V 0 bs NrubiscosMNs;b;k=As;b;k ðA7iÞ Vjmaxs;b;k ¼ V 0 js NchlorophyllsMNs;b;k=As;b;k ðA7jÞ fNs;b ¼ min Ns;b . Ns;b þ Cs;b=KiN   ; Ps;b . Ps;b þ Cs;b=KiP  n o ðA7kÞ Notation A microbial or root surface area, equal to 4pr2 N (mic.) and pr2L (roots) (equations (A2a)–(A2c), (A6a)–(A6d), and (A7a)–(A7k)) (m2 m−2). GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 13 of 17 a shape parameter for response of Vj to I, equal to 0.7. Cc chloroplast [CO2] in equilibrium with Ci (equation (A7g)) (mM). [CO2S] CO2 concentration in soil solution (equations (A3a), (A3b), and (A3d)) (g C m−3). CNj maximum ratio of N to C in M, equal to 0.22 and 0.13 for labile and resistant j (equations (A2a), (A2b), and (A2c)) (g N g C−1). DeNH4 effective dispersivity‐diffusivity of NH4 + dur- ing root uptake (equation (A6a)) (m2 h−1). ds,z half distance between adjacent roots assumed equal to uptake path length, equal to (pLs,l,r/ Dz)−1/2 (equations (A6a) and (A6c)) (m) " quantum yield, equal to 0.40 (equation (A7h)) (mmol e− mmol quanta−1). fi function for inhibition of XNO2 by NH3 and HNO2 (equations (A3b) and (A3c)). fN nutrient inhibition of Vb, (equations (A7g), (A7h), and (A7k)) fN2O fraction of QNO2 released as QN2O, equal to 0.1 [Mørkved et al., 2007] (equation (A5c)). fO2 ratio of O2 supply to demand at ambient [O2] (equations (A3a) and (A3b)). fy nonstomatal water effect on Vb, Vj [Grant and Flanagan, 2007] (equations (A7g) and (A7h)). fts Arrhenius temperature function for soil pro- cesses (equations (A3a)–(A3d), (A5a)–(A5c), and (A6a)–(A6d)). ftb Arrhenius temperature function for Vb [Bernacchi et al., 2003] (equation (A7g)). ftj Arrhenius temperature funct ion for V j [Bernacchi et al., 2003] (equation (A7h)). GN sN consumed in growth (equations (A7c), (A7d), and (A7e)) (g m−2 h−1). G CO2 compensation point (equation (A7g)) (mM). [H+] hydrogen ion concentration (equations (A1c) and (A5a)) (mol L−1). I irradiance (equation (A7h)) (mmol m−2 s−1). KCO2 Michaelis‐Menten constant for CO2 reduction, equal to 0.15 (equations (A3a), (A3b), and (A3d)) (g C m−3). KCO2 Michaelis‐Menten constant for CO2 fixation (equation (A7g)) (mM). KiHNO2 [HNO2] at 1/2 inhibition of XNO2, equal to 25.0 [Blackburne et al., 2007] (equation (A3c) (g N m−3). KiNH3 [NH3] at 1/2 inhibition of XNO2, equal to 10.0 [Blackburne et al., 2007] (equation (A3c) (g N m−3). KisN inhibition constant for sCi,j versus sNi,j, equal to 100 [Grant, 1998] (equations (A7e) and (A7k)) (g C g N−1). KisP inhibition constant for sCi,j versus sPi,j, equal to 1000 [Grant, 1998] (equations (A7e) and (A7k)) (g C g P−1). KNH3 Michaelis‐Menten constant for XNH3, equal to 2.0 × 10−4 [Stark and Firestone, 1996] (equation (A3a)) (g N m−3). KNH4 Michaelis–Menten constant for INH4 or UNH4, equal to 0.40 [Barber and Silberbush, 1984] (equations (A2b) and (A6b)) (g N m−3) KNO3 Michaelis–Menten constant for INO3 or UNO3, equal to 0.35 [Barber and Silberbush, 1984] (equations (A2c) and (A6c)) (g N m−3) KNO2d Michaelis‐Menten constant for RNO2 by deni- trifiers, equal to 2.5 [Yoshinari et al., 1977] (equation (A4b)) (g N m−3). KNO2n Michaelis‐Menten constant for RNO2 by nitrifi- ers, equal to 2.5(equation (A3d)) (g N m−3). KNO2o Michaelis‐Menten constant for XNO2, equal to 0.7 [Blackburne et al., 2007] (equation (A3b)) (g N m−3). KNO3 Michaelis‐Menten constant for RNO3, equal to 2.5 [Yoshinari et al., 1977] (equation (A4a)) (g N m−3). KN2O Michaelis‐Menten for RN2O, equal to 2.5 [Yoshinari et al., 1977] (equation (A4c)) (gNm−3). kT rate constant for TN (equation (A7b)) (h −1). L root length (equations (A6a) and (A6c)) (mm−2). MC microbial, root or shoot biomass C (equations (A2a)–(A2c), (A1a), (A1b), (A3), (A7a), and (A7e)) (g C m−2). MN microbial, root or shoot biomass N (equations (A2a)–(A2c), (A1a)–(A1b), (A3a)–(A3d), and (A7e)) (g N m−2). N microbial number (m−2). Nchlorophyll fraction of leaf N in chlorophyll, equal to 0.025 (equation (A7j)). Nrubisco fraction of leaf N in rubisco, equal to 0.125 (equation (A7i)). [NH4 + mn] [NH4 +] below which UNH4 = 0, equal to 0.0125 [Barber and Silberbush, 1984] (equations (A2b) and (A6b)) (g N m−3). [NO3−mn] [NO3 −] below which UNO3 = 0, equal to 0.03 [Barber and Silberbush, 1984] (equations (A2c) and (A6c)) (g N m−3). QNO2 NO2 − reduction by chemodenitrification (equations (A5b) and (A5c)) (g N m−2 h−1). Q′NO2 rate constant for decomposition of HNO2 at 25°C, equal to 0.075 [Cleemput and Samater, 1996] (equation (A5b)) (h−1). QN2O N2O production by chemodenitrification (equation (A5c)) (g N m−2 h−1).  soil water content (equation (A5b)) (m3 m−3). RNO2 NO2 − r educ t ion under ambien t [NO2 − ] (equations (A3d) and (A4b)) (g N m−2 h−1). R′NO2 RNO2 under nonlimiting [NO2 −] (equation (A3d)) (g N m−2 h−1). RNO3 NO3 − reduct ion under ambient [NO3 −] (equations (A4a), (A4b), and (A4c)) (gNm−2 h−1). R′NO3 RNO3 under nonlimiting [NO3 −] (equations (A4a), (A4b), and (A4c)) (g N m−2 h−1). RN2O N2O reduction under ambient [N2O] (equation (A4c)) (g N m−2 h−1). r radius of root or mycorrhizae, equal to 1.0 × 10−3, 5.0 × 10−6 (equations (A6a) and (A6c)) (m). sC nonstructural C (equations (A7a), (A7b), (A7e), and (A7k)) (g C m−2). sN nonstructural N (equations (A7b), (A7c), (A7d), (A7e), and (A7k)) (g N m−2). GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 14 of 17 TC transfer of sC between shoot and root (equation (A7a)) (g N m−2 h−1). TN transfer of sN between root and shoot (equations (A7b), (A7c), and (A7d)) (g N m−2 h−1). UNH4 NH4 + uptake by microbial, root or mycorrhizal surfaces (equations (A2a)–(A2c), (A6a)–(A6d), and (A7c)) (g N m−2 h−1). U′NH4 maximum UNH4 at 25°C and nonlimiting NH4 +, equal to 5.0 × 10−3 [Barber and Silberbush, 1984] (equations (A2b) and (A6b)) (gNm−2 h−1). UNO3 NO3 − uptake by microbial, root or mycorrhizal surfaces (equations (A2a)–(A2c), (A6a)–(A6d), and (A7c)) (g N m−2 h−1). U′NO3 maximum UNO3 at 25°C and nonlimiting NO3 −, equal to 5.0 × 10−3 [Barber and Silberbush, 1984] (equations (A2c) and (A6c)) (g Nm−2 h−1). Uw water uptake by roots or mycorrhizae (equations (A6a) and (A6c)) (m3 m−2 h−1). Vb CO2‐limited leaf carboxylation (equation (A7g)) (mmol m−2 s−1). Vb′ specific rubisco carboxylation at 25°C, equal to 45 [Farquhar et al. , 1980] (equation (A7i)) (mmol g−1 rubisco s−1). Vbmax Vb at nonlimiting CO2, y;c i, Tc and N,P (equations (A7g) and (A7i)) (mmol m−2 s−1). Vj irradiance‐limited carboxylation (equation (A7h)) (mmol m−2 s−1). Vj′ specific chlorophyll e − transfer at 25°C, equal to 450 [Farquhar et al., 1980] (equation (A7j)) (mmol g−1 chlorophyll s−1). Vjmax V j at nonlimiting CO2, yc i , Tc and N,P (equation (A7h)) (mmol m−2 s−1). X′NH3 specific NH3 oxidation at 25°C and nonlimit- ing [O2], equal to 0.625 [Belser and Schmidt, 1980] (equation (A3a)) (g N g−1 M h−1). XNH3 NH3 oxidation under ambient Ts and [O2] (equation (A3a)) (g N m−2 h−1). X′NO2 specific NO2 − oxidation under at 25°C and non- limiting [O2s], equal to 0.75 [Blackburne et al., 2007] (equation (A3b)) (g C m−2 h−1). XNO2 NO2 − oxidation under ambient temperature and [O2s] (equation (A3b)) (g C m −2 h−1). 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Jassal, Biometeorology and Soil Physics Group, Faculty of Land and Food Systems, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada. R. F. Grant, Department of Renewable Resources, University of Alberta, 4‐30 Earth Science Bldg., Edmonton, AB T6G 2E3, Canada. (robert. grant@ales.ualberta.ca) GRANT ET AL.: FERTILIZATION AND NEP OF DOUGLAS FIR G04009G04009 17 of 17


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