T. I. Malysheva

The forms of phosphorus in humic and fulvic acids of a toposequence of alpine soils in the northern Caucasus

Chemical fractionation and 31P nuclear magnetic resonance (NMR) spectroscopy of humic acids (HA) and fulvic acids (FA) were used to characterize the forms of phosphorus and their changes within a toposequence of alpine soils at the Mt. Malaya Khatipara (Teberda reserve, northwestern Caucasus). Sodium hydroxide extracted 66–82% of total phosphorus from A horizons and 28–51% from B horizons. Organic P amounted to 92–99% of NaOH-extractable P. HA represented the major part of extracted organic P (52–90%). The P species in HA and FA comprised phosphate monoesters (40–86%), phosphate diesters (up to 22%), phosphonates (up to 8%), sugar-diester phosphates (up to 14%), pyrophosphates (up to 11%), polyphosphates (up to 16%), and unknown compounds (up to 6%). Inorganic orthophosphate was found in appreciable proportions only in HA of organic horizons and in FA (up to 16%). The percentages of phosphonates and phosphate diesters were higher in HA, those of sugar diesters and pyrophosphates were higher in FA. Within the toposequence, the contribution of labile P species to total P in HA of surface soil layers increased with increasing thickness of snow cover during winter and correspondingly shorter vegetation periods. The P-species distributions in HA of subsoil horizons were rather similar throughout the toposequence. HA of the surface soils thus seemed to best characterize the P dynamics in these soils.

Organic phosphorus compounds in particle-size fractions of mountain soils in the northwestern Caucasus

Distributions of organic C (Corg), total P (Pt), and organic P (Porg) among particle-size fractions were investigated in Humic Cambisols and Umbric Leptosols of the northwestern Caucasus. In all investigated soils, Pt and Porg increased with decreasing particle size. In fractions <50 μm of A horizons, Porg accounted for 58–95% of Pt. In sandy fractions, the proportions of Porg varied significantly from 29% to 80%. The greatest proportions of soil Corg, Pt, and Porg in A horizons were concentrated in the 5–50 μm fraction. The C/Porg ratios decreased with decreasing particle size, indicating a relative enrichment of P in the organic matter associated with finer particle-size fractions. Phosphorus-31 nuclear magnetic resonance spectroscopy of dialyzed NaOH extracts was used to characterize the forms of phosphorus in particle-size fractions. As expected, monoesters were the major P species in all investigated particle-size separates (64–88% of extractable Porg). Diester-P (DNA+phospholipid–teichoic acids) accounted for 10.9–33.4% of extractable Porg in particle-size fractions of A horizons and for only 9.0–22.9% in fractions of B horizons. Diester-P occurred in uniform proportions in fractions of A horizons while its relative enrichment in clay fractions was evident for the B horizons. Up to 5.2% of organic P was present as phosphonates, the maximum being in the 1–5 μm fraction.

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 δ15N and net N mineralization and nitrification; the relative 15N enrichment was characteristic of grassland and meadow ecosystems. δ15N of total soil N pool increased during the field mineralization experiment; there was a positive tendency between the change in δ15N and net N mineralization and nitrification, however the relationship was not significant. Foliar δ15N of dominant plant species varied widely within community, however, a tendency of higher foliar δ15N for species growing on the soils with higher net N mineralization, nitrification and δ15N was observed.

Solubility of the labile forms of soil carbon and nitrogen in K2SO4 of different concentrations

The general pattern of the changes in the solubility of the labile carbon and nitrogen compounds with the changes in the concentration of the salt extractant (0.05 and 0.5 M K2SO4) has been determined for soils differing in their acidity and in their contents of organic matter and nitrogen. Different forms of extracted compounds react differently to changes in the salt concentration. The solubility of inorganic nitrogen compounds (NH4+ and NO3−) does not depend on the concentration of K2SO4. In most cases, the carbon and nitrogen of the microbial biomass manifest a tendency for increasing extractability with an increase in the concentration of the K2SO4 solution. A fundamental difference is characteristic of the organic carbon and nitrogen compounds, the solubility of which in 0.5 M K2SO4 increases in different soils by 1.5–3.9 times in comparison with their solubility in 0.05 M K2SO4.

Seasonal dynamics of the mineral nitrogen forms in mountain-meadow alpine soils

The good agreement between the changes in the concentrations of ammonium and nitrate nitrogen, on the one hand, and the mineralization and nitrification activities, on the other hand, was shown in the annual dynamic cycle of the mobile mineral nitrogen concentrations, the net mineralization of its organic compounds, and the net nitrification in mountain-meadow soils of different ecosystems of the Teberda Reserve. The low nitrification activity in the mountain-meadow soils results in the predominance of ammonium nitrogen in its mineral forms during the entire vegetation period. The importance of the fall-winter-spring period, when the mineralization of organic nitrogen compounds proceeds actively and results in the accumulation of ammonium nitrate in the soil up to the beginning of the vegetation period, was emphasized. Due to the long duration of this period, its average contribution to the total annual mineralization is comparable to that of the vegetation period.

Phosphorus in humus acids

The distribution of phosphorus among humus acid groups in different soil types was studied. It was shown that the binding of P with acid-soluble and-insoluble organic components is determined by the acid-base status of soils, which controls the predominant interaction of P with Fe and Al or with Ca and Mg, and the solubility of the resulting organic phosphates in acid and alkali. The major part of P is associated with the group of humic acids (HAs) in the acid soils and with fulvic acids (FAs) in the neutral and calcareous soils. During the fractionation of organic matter from acid soils, the amount of P redistributed from the HA to the FA group can increase with an increasing acid or alkaline impact on the soil, which breaks the bonds of the phosphorus-bearing organic components with metals.

Symbiotic nitrogen fixation in the alpine community of a lichen heath of the Northwestern Caucasus Region (the Teberda Reserve)

The symbiotic fixation of atmospheric nitrogen by leguminous plants in the alpine community of a lichen heath at the Teberda State Biosphere Reserve is well adapted to low soil temperature characteristic for the altitude of 2800 m a.s.l. For the determination of the N fixation by isotopic methods (the method of the natural 15N abundance and the method of isotopic 15N dilution), Trifolium polyphyllum was taken as the control plant. This plant was used as it does not form symbiosis with the nitrogen-fixing bacteria in the highlands of the Northern Caucasus Region. The contribution of the N fixation to the N nutrition of different leguminous plant species as determined by the natural 15N abundance method amounted to 28–73% at δ15N0 = 0‰ and 46–117% at δ15N0 = −1‰; for the determination of the N fixation by the method of the isotopic label’s dilution, it was 34–97%. The best correlation of the results obtained by these two isotopic methods was observed for the natural fractionation of the N isotopes in the course of the N fixation in the range of −0.5 to −0.7‰. The determination of the nitrogenase activity of the roots by the acetylene method confirmed the absence of N fixation in T. polyphyllum and its different contribution to the N nutrition of different species of leguminous plants.

Influence of drying of the samples on the transformation of nitrogen and carbon compounds in mountain-meadow alpine soils

The drying of samples of mountain-meadow soils characterized by their permanently high moisture under natural conditions fundamentally changes the concentrations of the labile nitrogen and carbon compounds, as well as the patterns of their microbial transformation. When the soil samples are dried, a four- to fivefold increase in the content of the extractable organic nitrogen compounds, carbon compounds, and inorganic nitrogen compounds is observed, while the content of nitrogen and carbon of the microbial biomass decreases by two-three times. The rewetting of the dried soil launches the process of the replenishment of the nitrogen and carbon reserves in the microbial biomass. However, even after two weeks of incubation, their values were 1.5–2 times lower than the initial values typical of the natural soil. The restoration of the microbial community in the samples of the previously dried soils occurs in the absence of a deficiency of labile organic compounds and is accompanied by their active mineralization and the low uptake of ammonium nitrogen by the microorganisms.

Freeze-thaw effect on the processes of transformation of carbon and nitrogen compounds in alpine meadow soils

Freezing-thawing of alpine meadow soils results in a 1.5- to 2-fold increase in the contents of extractable organic and inorganic nitrogen and organic carbon compounds, whereas the contents of microbial biomass nitrogen and carbon slightly decrease. The latter are quickly restored in the course of subsequent incubation, but the processes of transformation of nitrogen compounds proceed differently in soils that are subject to freezing under natural conditions and in nonfreezing soils. In nonfreezing soil, an abrupt activation of organic nitrogen mineralization and nitrification takes place against the background of a relatively low level of microbial assimilation of inorganic nitrogen compounds by microorganisms.

Soils and nutrient turnover

Mountain soils are highly variable and their properties depend on several abiotic (climate, geology, slope processes etc.) as well as biotic (vegetation, animal and microbial activity etc.) factors. In this chapter we consider soils of the studied area at several spatial scales with the increasing detail: 1) main soil types of the Teberda Reserve, 2) soils of the alpine study area, 3) soils of studied toposequences (catena) of 4 alpine plant communities (alpine lichen heath, Festuca varia grassland, Geranium-Hedysarum meadow, snowbed community) which are the main objects of experimental research; 4) features of nutrient storage and turnover in alpine communities. Soil variability within the communities is described in Chapter 5.3. In a separate chapter (5.4.) we present the influence of plant species on soil nutrients in monoculture experiment.