Spatial variations in larch needle and soil δ<sup>15</sup>N at a forest–grassland boundary in northern Mongolia

<p>The spatial patterns of plant and soil δ<sup>15</sup>N and associated processes in the N cycle were investigated at a forest–grassland boundary in northern Mongolia. Needles of <i>Larix sibirica</i> Ledeb. and soils collected from two study areas were analysed to calculate the differences in δ<sup>15</sup>N between needle and soil (Δδ<sup>15</sup>N). Δδ<sup>15</sup>N 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 <sup>15</sup>N-depleted N uptake by larch and <sup>15</sup>N-enriched N immobilization by microorganisms was proposed to cause large Δδ<sup>15</sup>N, whereas in the grassland boundary, small Δδ<sup>15</sup>N 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 Δδ<sup>15</sup>N.</p>