Constraints to N Fixation

The pervasiveness of terrestrial ecosytems where N availability demonstrably limits NPP and other ecosystem processes suggests that alternative pathways of N loss and additional constraints on N2 fixation should be fundamental parts of our view of the N cycle. To the extent that these are important, anthropogenic 

FIGURE 3 The elTecls on biomass and the extent of N limitation of adding constraints on N2 fixation. The constraints include differentially severe effects of grazing, P limitation, and shade intolerance on N2 fixers. In each case, a pulse of simulated N fertilizer was added beginning in year 300 (50 kg N ha year continued for 20 years) to illustrate the extent of N limitation. (A) No additional constraints to N2 fixation, losses of excess available N only. (B) No additional constraints losses of N by additional pathways. (C) N, fixation con.strained by grazing, P limitation, and shade intolerance losses of excess available N only. (D) N2 fixation constrained, and losses of N by additional pathways. Revised from Vitou.sek and Field (1999). 

This research was supported by grants from the National Science Foundation and the Andrew Mellon Foundation. 

(1992). Nitrogen cycling and nitrogen saturation in temperate forest ecosystems. Trauh F.col. Evolut. 7, 220-223. 

and Driscoll, C. T. (1997). Effects of land use, climate variation, and N deposition on N cycling and C storage in northern hardwood forests. Global Hiogeoclicm. Cycles 11,639-648. 

---