Foliar and soil δN values reveal increased nitrogen partitioning among species in diverse grassland communities

Plant and soil nitrogen isotope ratios (δN) were studied in experimental grassland plots of varying species richness. We hypothesized that partitioning of different sources of soil nitrogen among four plant functional groups (legumes, grasses, small herbs, tall herbs) should increase with diversity. Four years after sowing, all soils were depleted in N in the top 5cm whereas in non-legume plots soils were enriched in N at 5-25cm depth. Decreasing foliar δN and ΔδN (=foliar δN-soil δN) values in legumes indicated increasing symbiotic N fixation with increasing diversity. In grasses, foliar ΔδN also decreased with increasing diversity suggesting enhanced uptake of N depleted in N. Foliar ΔδN values of small and tall herbs were unaffected by diversity. Foliar ΔδN values of grasses were also reduced in plots containing legumes, indicating direct use of legume-derived N depleted in N. Increased foliar N concentrations of tall and small herbs in plots containing legumes without reduced foliar δN indicated that these species obtained additional mineral soil N that was not consumed by legumes. These functional group and species specific shifts in the uptake of different N sources with increasing diversity indicate complementary resource use in diverse communities. © 2011 Blackwell Publishing Ltd.