Spatial variations in larch needle and soil δN at a forest-grassland boundary in northern Mongolia.

The spatial patterns of plant and soil δN 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 δN between needle and soil (ΔδN). ΔδN 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 N-depleted N uptake by larch and N-enriched N immobilization by microorganisms was proposed to cause large ΔδN, whereas in the grassland boundary, small ΔδN 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 ΔδN.