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Fertilizer efficiency and incorporation and soil dynamics in forest ecosystems of northern Vancouver Island

University of British Columbia

To better understand the nutritional problems affecting conifer regeneration on western redcedar (Thuja plicata Donn ex D. Don) - western hemlock (Tsuga heterophylla (Raf.) Sarg.) (CH type) cutover sites, this thesis was aimed to: (1) investigate the effect of salal (Gaultheria shallon Pursh.) competition on tree growth and nutrition, particularly the fate of fertilizer N; (2) determine the distribution and availability of residual fertilizer N (fertilizer N not lost from the soil after certain time period following application) in forest soils; and (3) study if soil microbial competition for nutrients was also a factor limiting early conifer growth on CH cutover sites. The study sites were located on Tree Farm License (TFL) 25 near Port McNeill, on northern Vancouver Island, which is part of the very wet maritime Coastal Western Hemlock biogeochmatic subzone. One experiment had single-tree plots (1 m radius) planted (in 1987) with western redcedar, western hemlock, and Sitka spruce (Picea sitchensis (Bong.) carr.), with understory (mainly salal) either removed or remaining. Understory removal was initiated just after planting and was maintained throughout the experiment by repeated clipping. Plots were fertilized with 200 kg N ha⁻¹ in the form of (¹⁵NH₄)₂SO₄ in spring 1991 when the trees were four years old, and destructively sampled in fall 1992. Understory removal significantly increased total height, root collar diameter and biomass growth in all of the tree components (1- and 2-year-old foliage and branches, ≥3-year-old branches, and various sized roots); however, biomass allocation among the tree components was not altered. Better growth in the understory removal treatment resulted from reduced uptake and immobilization of nutrients by the competing vegetation. Chemical analysis showed that the majority of the ¹⁵N in the trees two growing seasons after fertilizer application was in the 1-year-old needles. Understory removal did not affect ¹⁵N abundance (atom %) but significantly increased ¹⁵N contents (mg plot⁻¹) in the aboveground tree components. Total ¹⁵N recovery in trees in the treated plots was 1.9, 8.9, and 2.1 times that in the control plots, for western redcedar, western hemlock, and Sitka spruce, respectively. Understory in the control (salal remaining) plots immobilized 14.8, 24.6, and 13.5% of the applied N for plots planted with western redcedar, western hemlock, and Sitka spruce, respectively. Total recoveries of ¹⁵N in the soil-plant systems ranged from 57 to 87%; of the total amount recovered, 59 to 82% was in the soil compartments. Results clearly showed that trees competed poorly with the understory and soil microbial populations for the applied N. A second experiment examined the transformations and extractabilities of the residual N in forest floor sampled 24 hours, 7 months, and 31 months after N fertilizer application. Net mineralization of total and applied N in a 42-day aerobic incubation was greatest in the samples from the 24-hr treatment followed by those from the 31-month treatment (p<0.05), indicating that recently immobilized ¹⁵N was more remineralizable. The percentage of applied N found in the total N mineralized (net) ranged from 76.6 to 87.4%, 13.1 to 42.0% and 10.6 to 14.0% in samples from the 24-hr and 7- and 31-month treatments, respectively, showing reduced relative availability of applied N with increased residence time. Both total and applied N in the extractable organic N fraction and in the N flushed after fumigation with chloroform had the following order: 24-hr > 7-month > 31-month treatment. The results confirmed that N fertilizer was being immobilized by the soil organic matter within hours after application and that the immobilized N had a low mineralization potential one growing season after N application. The extractability of the residual ¹⁵N was studied using 2 M KCl 0.5 M K₂SO₄, autoclaving, and acidic permanganate (of different strength) extractions and fumigation-extraction methods. The incorporation of ¹⁵N into the classical fulvic (FA), humic (HA) and humin fractions was also studied. Greater amounts of total and applied N were extracted from the 24-hr than from the 7- and 31-month treatments (p<0.05), with the difference between the last two nonsignificant. The extracted fractions were always enriched with ¹⁵N relative to the bulk soil. A greater (p<0.05) percentage of the total recovered ¹⁵N was in the 24-hr than in the other two treatments in the FA and the relationship for the humin fraction was reversed. The results agree with the mineralization studies and showed that the extractability of residual ¹⁵N was quickly reduced with increased residence time due to its incorporation into the stable humin fraction of the soil organic matter (SOM); however, residual ¹⁵N remained more extractable than the bulk soil N regardless of the length of residence time. A third experiment studied the dynamics of microbial biomass and N in old-growth CH forests, and in 3- and 10-yr-old western redcedar plantations. Three forest floor layers: F (partly decomposed litter material), woody F (Fw) and H (well-decomposed, amorphous organic matter) were sampled four times in May, July, August, and October of 1992. Microbial biomass C and N were relatively constant throughout the sampling period. Microbial C content was in the order: old-growth forests > 10-yr-old plantations > 3-yr-old plantations. Microbial N content was significantly greater in the old-growth forest than in the young plantations, for both F (p<0.001) and H (p<0.05), but was not different between the plantations. Therefore, the hypothesis that the microbial biomass acted as a net sink in the 10-yr-old plantations by immobilizing N into the microbial N pool was rejected. Microbial C/N ratios were greater (p<0.05) in the 10-yr-old plantations than in the old-growth forests and in the 3-yr-old plantations in H and on July 16 in F, indicating that microbial competition for N was probably a factor in the growth decline in the 10- yr-old plantations. Extractable C and N, and mineralizable N were generally higher in the oldgrowth forests than in the 3-yr-old plantations and higher in the 3-yr-old than in the 10-yr-old plantations. As a result of better nutritional conditions, tree and understory foliage in the 3-yr-old plantations had higher N concentrations and lower C/N ratios than in the 10-yr-old plantations. It was concluded that (1) salal was strongly competitive for the applied fertilizer N in the CH cutover sites; (2) salal competition greatly reduced crop tree growth; (3) N incorporation into SOM further reduced the availability of fertilizer N; and (4) microbial competition for N might have contributed to the N shortage problems in the CH cutover sites.

Thesis/Dissertation

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2009-03-20

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http://hdl.handle.net/2429/6272

Forestry

University of British Columbia

UBCV

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