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

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.