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要旨

Plant and soil nitrogen isotope ratios (d¹⁵N) 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 d¹⁵N and Dd¹⁵N (= foliar d¹⁵N - soil d¹⁵N) values in legumes indicated increasing symbiotic N₂ fixation with increasing diversity. In grasses, foliar Dd¹⁵N also decreased with increasing diversity suggesting enhanced uptake of N depleted in ¹⁵N. Foliar Dd¹⁵N values of small and tall herbs were unaffected by diversity. Foliar Dd¹⁵N 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 d¹⁵N 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.