Nitrogen dynamics in alpine ecosystems of the northern Caucasus

Net N mineralization, nitrification, microbial biomass N and ¹⁵N natural abundance were studied in a toposequence of representative soils and plant communities in the alpine zone of the northern Caucasus. The toposequence was represented by (1) low-productive alpine lichen heath (ALH) of wind-exposed ridge and upper slope; (2) more productive Festuca varia grassland (FG) of middle slope; (3) most productive Geranium gymnocaulon/Hedusarum caucasicummeadow (GHM) of lower slope; (4) low-productive snowbed community (SBC) of the slope bottom. N availability, net N mineralization and nitrification were higher in soils of alpine grassland and meadow of the middle part of the toposequence compared with soils of lichen heath and snowbed community of extreme habitats in the alpine zone. There was no correlation between intensities of N transformation processes and favorable (low soil acidity, low C/N ratio, long vegetation period, relatively high temperature, absence of hydromorphic features) and unfavorable (opposite) factors, indicating that the intensity of N mineralization and nitrification in the alpine soils is controlled by a complex combination of these factors. Potential net N mineralization and nitrification in alpine soils determined in the short-term laboratory incubation were considerably higher than those determined in the long-term field incubation. The differences of potential nitrification between soils of various plant communities did not correspond to the field determined pattern indicating the importance of on-site climatic conditions for control of nitrification in high mountains. The result of comparison of N transformation potentials in incubated and native soils indicated that nitrification potential was significantly increased after long-term soil incubation. It means that net nitrification determined in the field was probably overestimated, especially in the meadow soils. A soil translocation experiment indicated that low temperature was an important factor limiting net N mineralization and nitrification in alpine soils: net N mineralization and especially nitrification increased when alpine soils were translocated into the subalpine zone and mean annual temperature increased by about 3°C. Additional N input increased N availability (NH₄ ⁺-N) and potential nitrification in soils of the lower part of the toposequense (GHM and SBC), and potential net N mineralization in two soils of extreme habitats (ALH and SBC). A positive correlation was found between soil ¹⁵N and net N mineralization and nitrification; the relative ¹⁵N enrichment was characteristic of grassland and meadow ecosystems. ¹⁵N of total soil N pool increased during the field mineralization experiment; there was a positive tendency between the change in ¹⁵N and net N mineralization and nitrification, however the relationship was not significant. Foliar ¹⁵N of dominant plant species varied widely within community, however, a tendency of higher foliar ¹⁵N for species growing on the soils with higher net N mineralization, nitrification and ¹⁵N was observed.