Wednesday, August 5, 2009 - 3:40 PM

COS 67-7: Nitrogen isotope patterns across a wide soil N gradient in coastal Oregon conifer forests: Implications for legacy effects of N-fixing red alder

Steven Perakis, USGS Forest and Rangeland Ecosystem Science Center, Emily R. Sinkhorn, USDA Natural Resources Conservation Service, and Jana E. Compton, US EPA, NHEERL, Western Ecology Division.

Background/Question/Methods

Natural abundance nitrogen isotopes (δ15N) can provide information on long-term sources and losses of N in forest ecosystems.  We measured N pools and δ15N isotopes in vegetation and soils of ten young coastal Oregon Douglas-fir forests, and used this information to constrain a simple model that evaluates drivers of long-term N balances in these forests. 

Results/Conclusions

Total ecosystem N pools ranged from 8 – 22 Mg N/ha across sites, dominated overwhelmingly (> 97%) by soils to 1 m depth.  Soil N contents at the most N-rich sites are among the greatest ever reported globally, despite nitrate leaching losses up to 30 kg N/ha-yr.  Foliar, needle litter, and forest floor δ15N increased significantly across the range of ecosystem N content, characterized by depleted δ15N values at low N sites, increasing to maximum values of -0.5‰ at high N sites.  Total ecosystem δ15N exhibited a significant curvilinear relationship with total ecosystem N content, declining from 5‰ at low N sites to 3‰ at intermediate N sites, and increasing again to 4‰ at high N sites.  Our model results and field data together suggest that N and δ15N balances in these forests reflect large historical atmospheric N inputs from N-fixing red alder, modified by fractionating N losses at the most N-rich sites.