Isotopic Composition of Nitrogen and Transformation of Nitrogen Compounds in Meadow-Alpine Soils

Abstract

Isotopic composition of nitrogen in soils can be an informative indicator of N transformation processes and sources of N nutrition of plants, but data on δ15N of labile N compounds are scarce. It is shown that N transformation in meadow-alpine soils (Leptic Umbrisols) of the northwestern Caucasus (Teberda Reserve, Karachay-Cherkess Republic) leads to well-expressed differences in isotopic signatures of different N compounds: δ15N of extractable organic matter > δ15Ntotal > δ15N– $${\text{NH}}_{{\text{4}}}^{ + }$$ > δ15N– $${\text{NO}}_{{\text{3}}}^{ - }.$$ The range of δ15N in this sequence reaches 25‰. Differences in δ15N within the same pool of N in soils of different alpine ecosystems, as well as seasonal dynamics of δ15N– $${\text{NH}}_{{\text{4}}}^{ + },$$ are much less pronounced (the range of δ15N is 2–4‰). The values of δ15Ntotal and δ15N– $${\text{NH}}_{{\text{4}}}^{ + }$$ positively correlate with N mineralization and nitrification and demonstrate the accumulation of heavy N isotope in soils of the alpine ecosystems with more active N transformation processes. Obviously, nitrification is the key process controlling the isotopic signature of N– $${\text{NH}}_{{\text{4}}}^{ + }.$$ The role of N mineralization in the fractionation of N isotopes is less obvious, and 15N accumulation in the extractable organic matter can be related to the significant content of “heavy” microbial N in extractable organic N pool.