Ministry of Forests

Cariboo Region Research Section

Pacific NW Forest & Rangeland Soil Organism Symposium - Les Paul & Bill Chapman

This study investigates the nitrogen fixing capabilities of tuberculate ectomycorrhizae (TEM) on lodgepole pine (Pinus contorta) in the SBPSxc biogeoclimatic subzone in the central interior of British Columbia, Canada. This region is characterized by cold, dry winters and cool dry summers. Forest floors are typically thin (<4cm) and decomposition is slow. The overall impression is one of marginal fertility and climate for tree growth. This study is part of a larger project that examines the importance of coarse woody debris (CWD) in this biogeoclimatic subzone. The first objective of this study was to determine nitrogenase activity in TEM on lodgepole pine in two age class stands (<40 yr;:>140 yr) crossed with three soil types. The three soil types are a moister loam textured soil derived from primarily basaltic parent material, a drier loam textured soil derived from primarily basaltic material, and a dry sandy-loam textured soil derived from granitic parent material. The soils derived from basaltic parent material tend to have high base saturation. The second objective was to determine the amount of TEM present in different substrates in the soil, including woody debris and from that estimate the contribution of TEM nitrogen fixation to the total nitrogen budget.

Nitrogenase activity, as indicated by the acetylene reduction method, was found in situ in tuberculate mycorrhizae on pine. Nitrogen fixing bacteria was isolated from both the outer surface and inside of the TEM. The amount of acetylene reduced per gram of mycorrhizae was higher in the>140 year old stands (0.13-0.54 nmoles/g) than in the younger stands. The rates of acetylene reduction were higher in both moisture classes of site located on basaltic sandy loam parent materials (0.33-0.54 nmoles/g) versus the dry, sandy-loam granitic parent material site (0.09-0.13 nmoles/g). The fresh mass of TEM per m of CWD was higher in the <40 yr; old stands (0.06-0.23g/m) than the > 140 yr. stands (0.03 nmoles per m). The rate of nitrogen fixation per unit of CWD was lowest in dry basaltic parent material site (0.03 nmoles per m), intermediate in the dry granitic site (0.03-0.06 nmoles per m) and highest in the wet basaltic site (0.03-0.15 nmoles per m).

These results suggest that there is a link between coarse woody debris levels and the amount of Nitrogen fixed per hectare in association tuberculate mycorrhizae. There is a potential for significant amounts of N to be fixed in association with tuberculate mycorrhizae in wood but other site factors such as mineral soil characteristics or moisture levels affect the rates. In the forests in this study, woody debris is an important substrate for tuberculate mycorrhizae formation and tuberculate mycorrhizae are potentially important sites of nitrogen fixation.

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