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Comparison of soil acidification and intensity of podzolization beneath decaying wood versus non-woody.. Klinka, Karel; Kayahara, Gordon J.; Chourmouzis, Christine 2001-04-23

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Scientia Silvica Extension Series, Number 30, 2001Comparison of Soil Acidification and Intensity of Podzolization BeneathDecaying Wood versus Non-woody Forest Floors in Coastal BCIntroductionForest managers concerned with maintaining soil productivity must consider the impacts of forestry practices upon thefeatures of a site.  One critical feature is the amount and type of organic matter on a site, which may affect soil development.This study addresses the question of whether CWD accumulations increase the intensity of podzolization, thus reducingthe long-term productivity of a site.Podzolization is defined as the chemical downward translocation of aluminum, iron and organic matter, resulting in theconcentration of silica in the eluviated mineral soil layer (Ae horizon), and accumulation of the these products in themineral soil layer below.  From a soil productivity perspective, podzolization is often thought to affect nutrition, resulting inforest productivity decline.  These nutritional properties include:(1) upper horizons leached of cations and consisting of nearly pure Si;(2) increased acidity;(3)  decreased nutrient availability as N and P are immobilized in organic horizons, due to the high capacity of Podzolsto fix phosphate;(4) build-up of a thick, slow-decomposing, acid mor humus form;(5) precipitated organic matter that resists decomposition; and(6)  development of impermeable layers, resulting in a water table within the rooting zone.Podzolization itself does not diminish productivity; rather, it is the ancillary soil properties.  The intensity of these processesis seen as a measure of the rate of soil development towards the six aforementioned characteristics.  Since increasedintensity of podzolization lowers site productivity, podzolization measures can be used as indicators or indices of thepotential for reduced productivity.In moist cool climates, podzolization is inevitable with or without forest management.  However, forest managers still needto know if their practices regarding CWD will alter the natural rate of change. Since the chemical nature of decayingwood is so different from other humus forms, and it can take many years for logs to fully decay in coastal BC, largeaccumulations of decaying wood persist long enough to affect the soil directly beneath. CWD accumulations may resultin a lower forest floor pH, which may be associated with more intense podzolization.  Thus CWD accumulations maynegatively affect site productivity, especially in the west coast of BC where CWD accumulations can be large.In this study, we investigated the long-term productivity implications of CWD accumulations by addressing the questions:is there (i) a thicker Ae layer, (ii) greater acidification, and (iii) greater degree of podzolization in forest floors with CWDversus those without?Study sites and methodsThree study sites were located in the watersheds north of Vancouver in the Submontane Very Wet Maritime CoastalWestern Hemlock (CWHvm1) variant.  Old-growth stands dominated by Douglas-fir, western hemlock, Pacific silver firand western redcedar, characterized by canopy trees 60 m in height, 300-750 years old, and 100-200 cm dbh wereselected.  Candidate stands were zonal, intermediate in both SNR and SMR.  Within each stand, a 1-ha site was established,and twelve 1 m2 pedons were systematically located.  One side of each pedon consisted of a forest floor layer with a decayclass IV or V log >2 m, with a diameter >30 cm, and incorporated by at least 30% (by volume) into the forest floor; theopposite side was a non-woody humus form.  If the log was on a slope >10%, then only logs perpendicular to the slopewithin 20? were chosen.  We randomly selected the side of the log to sample for the non-woody substrate.  A 1 m longtrench was dug lengthwise through the centre of the log, and extended perpendicularly for 1 m to the non-woody forestfloor, forming a 1m2 soil pit.From each side of the pedon (i.e., the two different substrates), the depth of the Ae horizon was recorded, and approximately2000 cm? was collected from (i) the decaying log; (ii) the LFH layer; (iii) the Ae horizon if present or, if absent, the top 2cm of the B horizon (separately from both substrate types); and (iv)  the upper B horizon to a depth of 10 cm. Forest floorand upper B horizon samples were analyzed for pH, total C, total N, and mineralizable N; upper B horizon samples werealso analyzed for extractable Fe and Al.  We used indicators of the intensity of podzolization based on definitions of an "f"horizon:  organically complexed Fe and Al; ratio of organic C to organically complexed Fe; ratio of the organically-complexed Fe and Al to the total free Fe and Al; and the iron activity ratio (the ratio of organically complexed plus non-crystalline Fe to total free Fe).Forest floor samples were separated yielding:  fraction A (lipids), humic acid (HA), and fulvic acid (FA).  Each fractionwas analyzed for carbon content (?C? in HA and ?C? in FA).  The C indicates the amount of fulvic acid constituentsthought to be responsible for the chelation of Al and other metals, especially Fe, and their percolation through the soil.  TheHA:FA ratio is an indicator of humification, with higher ratios reflecting more intense humification from greater biologicalactivity.  Lipids accumulate in acidic or anaerobic soil conditions where biological activity is low.ResultsSignificant differences (alpha = 0.05) were detected between the mean pH of the non-woody versus woody substrates(Table 1).  Total C and N, the C:N ratio, C in HA and C in FA were also significantly different.  The HA concentrationwas greater in the woody substrates, the FA concentration greater in the non-woody substrates, and the HA:FA ratiogreater in the woody substrates.Forest Floor     Mineral soil horizon    Ae  Upper B chemical property non-woody  mean (sterror) alpha woody  mean (sterror) beta  non-woody  mean (sterror) alpha woody  mean (sterror) beta non-woody  mean (sterror) alpha woody  mean (sterror) beta Depth (cm)        3.5 (0.7)  3.8 (0.8)         0.506 0.216   pH  3.72 (0.04)  3.41 (0.02)    4.00 (0.08)  3.95 (0.11)  4.42 (0.05)  4.36 (0.07)   0.022    0.447    0.026  0.168  0.003 total C (%)  48.87 (0.45)  58.84 (0.42)    2.95 (0.75)  3.55 (0.35)  5.43 (0.55)  5.17 (0.19)   0.001    0.369 0.029 0.713 0.036 total N (%)  1.58 (0.06)  0.62 (0.05)    0.16 (0.05)  0.15 (0.04)  0.23 (0.01)  0.19 (0.02)   <0.001    0.396 0.157 0.247 0.860 C:N  31.1 (1.4)  104.4 (9.8)    22.5 (3.4)  27.4 (4.3)  25.3 (2.8)  27.9 (2.9)   0.013   0.139  0.891 0.001  min-N (ppm)       17.0 (5.0)  13.5 (1.7)  24.6 (1.9)  21.2 (2.6)       0.403 0.267 0.386 0.212 C in HA (%)  12.73 (0.28)  15.11 (1.71)     <0.001   C in FA (%)  10.23 (0.43)  6.57 (0.53)     0.007   HA:FA   1.28 (0.06)  2.52 (0.14)     0.013   Lipids (%)  3.59 (0.15)  1.68 (0.20)     0.031*    Table 1. Differences in mean chemical properties between the non-woody humus form and the CWD(woody), and between the mineral soil beneath the substrates in the CWHvm subzone.  Standard error ofthe mean is in parentheses (n=3) and below are the alpha (treatment) p-values followed by the p-valuefor beta, where applicable.  For site ? treatment interaction p-values, an asterisk indicates a significantunidirectional interaction effect.  Cells in bold have significant alpha (< 0.05); cells in italics have significant beta( < 0.20). Min-N = mineralizable N; HA = humic acid; FA = fulvic acid.Chemical property  Mineral soil horizon  Ae upper B  non-woody  mean (sterror) alpha woody  mean (sterror) beta non-woody  mean (sterror) alpha woody  mean (sterror) beta 0.30 (0.09)  0.24 (0.04)  0.75 (0.14)  0.67 (0.05) Pyrophosphate Fe (%) 0.555 0.817 0.468  0.900 0.56 (0.11)  0.45 (0.07)  1.57 (0.18)  1.43 (0.13) (Fe + Al)(%)  0.353   0.228  0.150   0.040 61.7 (31.4)  43.3 (10.8)  9.2 (2.0)  9.1 (0.9) Pyrophosphate Fe  0.521   0.974  0.909   0.003 0.43 (0.05)  0.43 (0.06)  0.63 (0.07)  0.60 (0.07) Dithionate (Fe + Al)   0.980    <0.001  0.212   <0.001 0.47 (0.05)  0.49 (0.03)  0.62 (0.06)  0.64 (0.07) Dithionite - Fe  0.652   0.057  0.245   0.003  There were variable results for both the A horizon (or top 2 cm of the soil) and the upper 10 cm of the B horizon directlyunder each forest floor substrate (Table 2).  C:N ratio was significantly different in the lower 10 cm of the soil profile;however, the mean difference was not biologically meaningful.  Significant differences in any chemical measure (alpha =0.05) were not detected for the upper 10 cm of the B horizon. The power of the test for lack of significant differencebetween the woody and non-woody substrates was adequate (1-beta > 0.80) for most of the measures.DiscussionDespite differences in nutrient properties and humus fractions between the non-woody and woody substrates, there werefew differences in the corresponding nutrition and podzolization measures (depth of Ae horizon, Fe and Al extractions) ofthe soil directly beneath (Figure 1).  In particular, the greater concentration of FA associated with the non-woody substratedid not translate into equivalent differences in the factors of the degree of podzolization.  FA is the main constituent oforganic material responsible for chelation of Fe and Al, and the  downward movement of this complex.  Thus, a deeper Aehorizon is not associated with accumulations of CWD for soils on zonal sites.  In fact, the potential productivity of CWDis either equal to non-woody forest floor re:  intensity of podzolization or, CWD has a lower productivity based on its lowerfulvic acid.  Either way, long term site productivity appears to be unaffected by CWD.This study was part of a larger study where sampling occurred along an elevation and latitudinal gradation in nine climaticallydifferent areas in BC.  The study areas were located in: (1) southwestern BC in each of the Very Dry Maritime CoastalWestern Hemlock (CWHxm) subzone, the Very Wet Maritime Coastal Western Hemlock (CWHvm) subzone, and theMoist Maritime Mountain Hemlock (MHmm) subzone; (2) southern central BC in each of the Very Dry Warm InteriorDouglas-fir (IDFxw) subzone, the Moist Warm Interior Cedar Hemlock (ICHmw) subzone, and the Moist Cool EngelmannSpruce - Subalpine Fir (ESSF mk) subzone; and (3) central BC in each of the Wet Cool Interior Cedar Hemlock (ICHwk)subzone, the Moist Cool Sub-Boreal Spruce (SBSmk) subzone, and the Moist Warm Boreal White and Black Spruce(BWBSmw) subzone.  These locations covered a precipitation, temperature, and continentality gradient in three respects:longitude, latitude, and altitude.  All areas had similar results to those reported here.ConclusionsBased on the similarity of the depth of Ae horizon, pH, and the chemical variables in the Bf horizon, current evidencesuggests that decaying wood does not increase acidification and eluviation of mineral soils on zonal sites in coastal BC.Table 2. Differences in mean chemical properties between the mineral soil beneath the non-woody humus form and the CWD (woody) in the CWHvm subzone.  Standard error of themean is in parentheses (n=3) and below are the a (treatment) p-values followed by the p-value for b, where applicable.  For site ? treatment interaction p-values, an asterisk indicatesa significant unidirectional interaction effect.  Cells in bold have significant alpha (< 0.05); cells initalics have significant beta (< 0.20) values. Pyrophosphate = sodium pyrophosphateextractable; dithionite = dithionite-citrate extractable.Scientia Silvica is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka ( Gordon Kayahara ( and design: Christine Chourmouzis ( support: Forest Renewal British Columbia and Fletcher Challenge CanadaFor more information contact: Gordon KayaharaCopies available from: orK. Klinka, Forest Sciences Department, UBC, 3036-2424 Main Mall, Vancouver, BC  V6T 1Z4The theory that CWD causes increased acidification and eluviation compared to non-woody forest floors appears to befalse in this case.  Based on the evidence to date, forest managers deciding to leave a legacy of CWD for habitat andbiodiversity need not be concerned about impacts on the long-term productivity of zonal sites.Reference Kayahara, G.J. 2000. The effects of coarse woody debris on site productivity of some forest sites in southwestern BritishColumbia. Ph.D. Dissertation.  University of British Columbia, Vancouver, BC.123 pp.(%)024681012fulvic acidpyro-Fenon-woody woodyBTotal N (%) woodyAFigure 1. Difference in (A) total N between the non-woody and woody substrates (open boxes) and theupper 10 cm of the mineral soil (shaded boxes) and(B) between the C in fulvic acid between the non-woody and woody substrates and pyrophosphate Fein the upper 10 cm of the mineral soil.  Error barsrepresent one standard error of the mean.


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