Disturbance and topography shape nitrogen availability and ä¹⁵N over long-term forest succession

Abstract

Forest disturbance and long-term succession towards old-growth are thought to increase nitrogen (N) availability and N loss, which should increase soil ä¹⁵N values. We examined soil and foliar patterns in N and ä¹⁵N, 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 ä¹⁵N 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 ä¹⁵Nvalues after disturbance. A more important role for symbiotic N fixation is suggested by lower soil ä¹⁵N values in slow-successional sites with slow canopy closure, which favors early-successional N fixers. Soil ä¹⁵N 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 ä¹⁵N values, steady-state mass balance calculations suggest that wildfire combustion of vegetation and detritus can dominate long-term N loss and increase whole-ecosystem ä¹⁵N. On steeper topography, declining soil ä¹⁵N values highlight erosion and accelerated soil turnover as an additional abiotic control on N balances. We conclude for N-limited montane forests that soil ä¹⁵N and N availability are less influenced by nitrate leaching and denitrification loss than by interactions between disturbance, N fixation, and erosion.

Citation

Perakis, Steven S.; Tepley, Alan J.; Compton, Jana E. 2015. Disturbance and topography shape nitrogen availability and ä¹⁵N over long-term forest succession. Ecosystems. 18(4): 573-588.