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δ 15 N constraints on long-term nitrogen balances in temperate forests
Citation:
Perakis, S. S., E. R. Sinkhorn, AND J. E. COMPTON. δ 15 N constraints on long-term nitrogen balances in temperate forests. OECOLOGIA. Springer, New York, NY, 167:793-807, (2011).
Impact/Purpose:
Natural abundance δ15N of ecosystems integrates nitrogen (N) inputs and losses, and thus reflects factors that control the long-term development of ecosystem N balances.
Description:
Natural abundance δ15N of ecosystems integrates nitrogen (N) inputs and losses, and thus reflects factors that control the long-term development of ecosystem N balances. We here report N and carbon (C) content of forest vegetation and soils, and associated δ15N, across nine Douglas-fir forests occupying a wide gradient in soil N and C. We then use a simple dynamic simulation model to evaluate the long-term development of δ15N and N balances in these forests. Total ecosystem N ranged from 8,754 to 22,639 kg ha-1 and ecosystem C ranged from 188 to 460 Mg C ha-1, and with the highest values near the Pacific coast. Mineral soil (0 to 100 cm) accounted for 96 to 98% of ecosystem N and 60 to 83% of ecosystem C. Whole-ecosystem δ15N exhibited a curvilinear relationship with total system N content that was driven largely by mineral soil N. Dynamic simulation modeling of δ15N and N mass balances suggest that long-term average N inputs must be at least 20 kg N ha-1yr-1 to account for observed N accumulation at our sites. This input exceeds known atmospheric deposition and asymbiotic N2-fixation rates, and is consistent with higher episodic inputs N inputs from symbiotic N2-fixing red alder after disturbance. Soil water δ15NO3 declined at high N sites, suggesting a shift in relative N losses from denitrification to nitrate as system N increased, and supported by δ15N mass balance simulations. Ecosystem δ15N and N field patterns were most closely approximated by simulations that included episodic N losses from wildfire and N replacement by symbiotic N2-fixation, with additional chronic N loss as nitrate, and modest denitrification. Whereas wildfire is often considered an important pathway of N loss, our results suggest that wildfire may result in long-term N accretion in sites colonized by symbiotic N2-fixing vegetation.