15N Natural Abundance of Soil Microbial Biomass in Alpine and Tundra Ecosystems

Abstract Isotopic composition of nitrogen in soil microbial biomass (δ 15 N micr ) is connected with the transformation of nitrogen compounds and with the balance of carbon and nitrogen availability for microorganisms. We have studied the dependence of δ 15 N micr on nitrogen isotopic composition in the substrate (δ 15 N of total and extractable nitrogen), as well as the dependence of δ 15 N micr and 15 N-enrichment of microbial biomass (Δ 15 N micr = δ 15 N micr – δ 15 N substr ) on nitrogen availability parameters (the C/N ratio in soil, the N-mineralization activity, the content of extractable nitrogen, and the nitrogen use efficiency) in soils of four alpine ecosystems in the North Caucasus and four tundra ecosystems in the Khibiny Mountains. It has been shown that δ 15 N miсr varies from –0.2 to +8.4‰ and may be characterized by both 15 N-enrichment and depletion (negative Δ 15 N miсr values) relative to the total and extractable soil nitrogen. As a rule, Δ 15 N micr is 1.5–3.1‰ relative to 15 N total and 0.6–4.8‰ relative to 15 N extr . However, under the most N-deficiency conditions in soils of mountain tundra lichen and shrub heaths, N micr does not accumulate an increased amount of 15 N. We have not revealed a close correlation of δ 15 N micr and Δ 15 N micr with the C/N ratio. The accumulation of 15 N in microbial biomass is much stronger related to N-mineralization (positively) and the nitrogen use efficiency (negatively). This testifies to the important role of microbial nitrogen dissimilation in controlling the isotopic composition of soil microbial biomass nitrogen.