Disturbance and topography regulate nitrogen availability and stable isotopes over long-term forest succession
Conceptual models of forest nitrogen (N) cycling often emphasize that disturbance and long-term succession towards old-growth can increase N availability and loss. In turn, more open N cycling increases biological N-isotope discrimination and fractionating N loss from soil, which is recorded as elevated soil δ15N values. We examined soil and foliar patterns in N and δ15N, and soil N mineralization, across 800+ years of forest succession in a topographically complex montane landscape in western Oregon, USA. We focus particular attention on characterizing how both natural (wildfire) and human (logging with burning) disturbances influence N cycling and stable isotopes in this unpolluted temperate forest landscape. We also examine how the rate of forest succession after disturbance shapes N cycling metrics.
In contrast to expectations, we found that disturbance caused declines in surface mineral soil δ15N values, both in logged forests measured 40 to 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 δ15N values after disturbance. A more important role for symbiotic N fixation is suggested by lower soil δ15N values in slow successional sites with slow canopy closure, which favors early-successional N fixers. Soil δ15N 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 δ15N values, steady-state mass balance calculations suggest that wildfire combustion of 15N-depleted vegetation and detritus can dominate long-term N loss and increase whole-ecosystem δ15N. On steeper topography, declining soil δ15N values highlight erosion and accelerated soil turnover as an additional abiotic control on N balances. We conclude for N-limited montane forests that soil δ15N and N availability are less influenced by nitrate leaching and denitrification loss than by interactions between disturbance, N fixation, and erosion.