The 15N natural abundance (δ15N) of ecosystem samples reflects measures of water availability

Abstract

We assembled a globally-derived data set for site-averaged foliar δ15N, the δ15N of whole surface mineral soil and corresponding site factors (mean annual rainfall and temperature, latitude, altitude and soil pH). The δ15N of whole soil was related to all of the site variables (including foliar δ15 N) except altitude and, when regressed on latitude and rainfall, provided the best model of these data, accounting for 49% of the variation in whole soil δ15N. As single linear regressions, site-averaged foliar δ15N was more strongly related to rainfall than was whole soil δ15N. A smaller data set showed similar, negative correlations between whole soil δ15N, site-averaged foliar δ15N and soil moisture variations during a single growing season. The negative correlation between water availability (measured here by rainfall and temperature) and soil or plant δ15N fails at the landscape scale, where wet spots are δ15N-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 δ15N and, newly, a mechanism accounting for the highly δ15N-depleted values found in the foliage and soils of many wet/cold ecosystems. These hypotheses are complemented by documentation of the present gaps in knowledge, suggesting lines of research which will provide new insights into terrestrial N-cycling. Our conclusions are consistent with those of Austin and Vitousek (1998) that foliar (and soil) δ15N appear to be related to the residence time of whole ecosystem N.