Disturbance and Topography Shape Nitrogen Availability and δ15N over Long-Term Forest Succession

Forest disturbance and long-term succession towards old-growth are thought to increase nitrogen (N) availability and N loss, which should increase soil delta N-15 values. We examined soil and foliar patterns in N and delta N-15, and soil N mineralization, across 800 years of forest succession in a topographically complex montane landscape influenced by human logging and wildfire. In contrast to expectations, we found that disturbance caused declines in surface mineral soil delta N-15 values, both in logged forests measured 40-50 years after disturbance, and in unlogged forests disturbed by severe wildfire within the last 200 years. Both symbiotic N fixation and N transfers from disturbed vegetation and detritus could lower soil delta N-15 values after disturbance. A more important role for symbiotic N fixation is suggested by lower soil delta N-15 values in slow-successional sites with slow canopy closure, which favors early-successional N fixers. Soil delta N-15 values increased only marginally throughout 800 years of succession, reflecting soil N uptake by vegetation and strong overall N retention. Although post-disturbance N inputs lowered surface soil delta N-15 values, steady-state mass balance calculations suggest that wildfire combustion of vegetation and detritus can dominate long-term N loss and increase whole-ecosystem delta N-15. On steeper topography, declining soil delta N-15 values highlight erosion and accelerated soil turnover as an additional abiotic control on N balances. We conclude for N-limited montane forests that soil delta N-15 and N availability are less influenced by nitrate leaching and denitrification loss than by interactions between disturbance, N fixation, and erosion.