Foliar and soil δ15N values reveal increased nitrogen partitioning among species in diverse grassland communities

Abstract

Plant and soil nitrogen isotope ratios (∂15N) 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 15N in the top 5 cm whereas in non-legume plots soils were enriched in 15N at 5–25 cm depth. Decreasing foliar ∂15N and D∂15N (= foliar ∂15N - soil ∂15N) values in legumes indicated increasing symbiotic N2 fixation with increasing diversity. In grasses, foliar Δ∂15N also decreased with increasing diversity suggesting enhanced uptake of N depleted in 15N. Foliar Δ∂15N values of small and tall herbs were unaffected by diversity. Foliar Δ∂15N values of grasses were also reduced in plots containing legumes, indicating direct use of legume-derived N depleted in 15N. Increased foliar N concentrations of tall and small herbs in plots containing legumes without reduced foliar ∂15N 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.