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

Abstract

Isotopic composition of nitrogen in soil microbial biomass (δ15Nmicr) 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 δ15Nmicr on nitrogen isotopic composition in the substrate (δ15N of total and extractable nitrogen), as well as the dependence of δ15Nmicr and 15N-enrichment of microbial biomass (Δ15Nmicr = δ15Nmicr – δ15Nsubstr) 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 δ15Nmiсr varies from –0.2 to +8.4‰ and may be characterized by both 15N-enrichment and depletion (negative Δ15Nmiсr values) relative to the total and extractable soil nitrogen. As a rule, Δ15Nmicr is 1.5–3.1‰ relative to 15Ntotal and 0.6–4.8‰ relative to 15Nextr. However, under the most N-deficiency conditions in soils of mountain tundra lichen and shrub heaths, Nmicr does not accumulate an increased amount of 15N. We have not revealed a close correlation of δ15Nmicr and Δ15Nmicr with the C/N ratio. The accumulation of 15N 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.