The N-15 natural abundance (delta N-15) of ecosystem samples reflects measures of water availability

We assembled a globally-derived data set for site-averaged foliar delta(15) N, the delta(15)N of whole surface mineral soil and corresponding site fact ors (mean annual rainfall and temperature, latitude, altitude and soil pH). The delta(15)N of whole soil was related to all of the site variables (inc luding foliar delta(15)N) except altitude and, when regressed on latitude a nd rainfall, provided the best model of these data, accounting for 49% of t he variation in whole soil delta(15)N. As single linear regressions, site-a veraged foliar delta(15)N was more strongly related to rainfall than was wh ole soil delta(15)N. A smaller data set showed similar, negative correlatio ns between whole soil delta(15)N, site-averaged foliar delta(15)N and soil moisture variations during a single growing season. The negative correlatio n between water availability (measured here by rainfall and temperature) an d soil or plant delta(15)N fails at the landscape scale, where wet spots ar e delta(15)N-enriched relative to their drier surroundings. Here we present global and seasonal data, postulate a proximate mechanism for the overall relationship between water availability and ecosystem delta(15)N and, newly , a mechanism accounting for the highly delta(15)N-depleted values found in the foliage and soils of many wet/cold ecosystems. These hypotheses are co mplemented by documentation of the present gaps in knowledge, suggesting li nes of research which will provide new insights into terrestrial N-cycling. Our conclusions are consistent with those of Austin and Vitousek (1998) th at foliar (and soil) delta(15)N appear to be related to the residence time of whole ecosystem N.