Asymbiotic nitrogen fixation in harvested and wildfire-killed lodgepole pine forests in the central interior of British Columbia

Rates of asymbiotic nitrogen fixation in woody debris, forest floor and mineral soils were investigated in stem-only harvested and wildfire-killed lodgepole pine (Pinus contorta ssp. latifolia Engelm. ex S. Wats.) stands in the central interior of British Columbia using an acetylene reduction assay. The long-term contribution of those nitrogen fixation processes to site nitrogen economy was estimated by mass loadings, associated nitrogen fixation rates and decomposition models. Asymbiotic nitrogen fixation activity was detected in all the woody and litter substrates examined. There were significant differences in nitrogenase activities between substrates. The highest activity (16.10±4.75 nm C2H4 g−1day−1) occurred in the below-ground woody debris (dead roots), followed in order by stumps, advanced or medium decay stems, litter, humus, early decay stems and, the lowest (0.07±0.03 nm C2H4 g−1day−1), mineral soil. Because of its relatively high rate of nitrogen fixation, below-ground woody debris is thought to have an important, but largely unrecognized, ecological role in the long-term site productivity of lodgepole pine forests. Moisture was the most important factor affecting the asymbiotic nitrogen fixation activity in woody debris in the study area. Asymbiotic nitrogen fixation in the above-ground woody debris was greater on the wildfire-killed sites than on the harvested sites due to much higher woody debris loadings and longer persistence on the former sites. The dynamics and role of asymbiotic nitrogen fixation over pine stand development were presented and discussed. Replacement of nitrogen removals from disturbance by asymbiotic nitrogen fixation would take 180 years for both stem-only and wildfire-killed sites. However, this could be shortened while considering other sources of nitrogen fixation in the study area such as nitrogen-fixing shrub, Sheperdia canadensis (L) Nutt and mycorrhizae. Management implications of asymbiotic nitrogen fixation were also discussed.