Foliar δ15N values characterize soil N cycling and reflect nitrate or ammonium preference of plants along a temperate grassland gradient

The natural abundance of stable (15)N isotopes in soils and plants is potentially a simple tool to assess ecosystem N dynamics. Several open questions remain, however, in particular regarding the mechanisms driving the variability of foliar delta(15)N values of non-N(2) fixing plants within and across ecosystems. The goal of the work presented here was therefore to: ( 1) characterize the relationship between soil net mineralization and variability of foliar Delta delta(15)N (delta(15)Nleaf - delta(15) Nsoil) values from 20 different plant species within and across 18 grassland sites; (2) to determine in situ if a plant's preference for NO(3)(-) - or NH(4)(+) uptake explains variability in foliar Delta delta(15)N among different plant species within an ecosystem; and (3) test if variability in foliar Delta delta(15)N among species or functional group is consistent across 18 grassland sites. Delta delta(15)N values of the 20 different plant species were positively related to soil net mineralization rates across the 18 sites. We found that within a site, foliar Dd15N values increased with the species' NO(3)(-) to NH(4)(+) uptake ratios. Interestingly, the slope of this relationship differed in direction from previously published studies. Finally, the variability in foliar Delta delta(15)N values among species was not consistent across 18 grassland sites but was significantly influenced by N mineralization rates and the abundance of a particular species in a site. Our findings improve the mechanistic understanding of the commonly observed variability in foliar Delta delta(15)N among different plant species. In particular we were able to show that within a site, foliar delta(15)N values nicely reflect a plant's N source but that the direction of the relationship between NO(3)(-) to NH(4)(+) uptake and foliar Delta delta(15)N values is not universal. Using a large set of data, our study highlights that foliar Delta delta(15)N values are valuable tools to assess plant N uptake patterns and to characterize the soil N cycle across different ecosystems.