Foliar δ ¹⁵N values characterize soil N cycling and reflect nitrate or ammonium preference of plants along a temperate grassland gradient

The natural abundance of stable N-15 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 N-15 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 N-15 (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 NO₃- - or NH4+ uptake explains variability in foliar Delta delta N-15 among different plant species within an ecosystem; and (3) test if variability in foliar Delta delta N-15 among species or functional group is consistent across 18 grassland sites. Delta delta N-15 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' NO₃- to NH4+ uptake ratios. Interestingly, the slope of this relationship differed in direction from previously published studies. Finally, the variability in foliar Delta delta N-15 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 N-15 among different plant species. In particular we were able to show that within a site, foliar delta N-15 values nicely reflect a plant's N source but that the direction of the relationship between NO₃- to NH4+ uptake and foliar Delta delta N-15 values is not universal. Using a large set of data, our study highlights that foliar Delta delta N-15 values are valuable tools to assess plant N uptake patterns and to characterize the soil N cycle across different ecosystems. [References: 45