Disturbance and topography regulate nitrogen availability and stable isotopes over long-term forest succession

Background/Question/Methods

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.   

Results/Conclusions

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.