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.