Spatial variations in larch needle and soil δ15N at a forest–grassland boundary in northern Mongolia

The spatial patterns of plant and soil δ15N and associated processes in the N cycle were investigated at a forest–grassland boundary in northern Mongolia. Needles of Larix sibirica Ledeb. and soils collected from two study areas were analysed to calculate the differences in δ15N between needle and soil (Δδ15N). Δδ15N showed a clear variation, ranging from −8‰ in the forest to −2‰ in the grassland boundary, and corresponded to the accumulation of organic layer. In the forest, the separation of available N produced in the soil with 15N-depleted N uptake by larch and 15N-enriched N immobilization by microorganisms was proposed to cause large Δδ15N, whereas in the grassland boundary, small Δδ15N was explained by the transport of the most available N into larch. The divergence of available N between larch and microorganisms in the soil, and the accumulation of diverged N in the organic layer control the variation in Δδ15N