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 > δ¹⁵N_total > 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 δ¹⁵N_total 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.