Formation of soil humus profile under long-derivative forests on the Vasyugan Plain (West Siberia, Russia)

Abstract

Humus profile formation in southern Vasyugan plain is closely related to its parent rocks ranging from carbonaceous clays and heavy loams. Carbonates ensure slowing down of restoration cycles of the indigenous vegetation at the stage of grass deciduous forests (for more than one generation), because well developed grass cover dominated by bunch grasses impedes restoration of dark coniferous species. This gives an insular pattern to some features of climatic zonality due to different soil-vegetation relationship between zonal and successional derivative communities. Plant and soil cover determine local conditions for soil drainage and leaching. In contrast to zonal forest communities with tree storey composed of dark conifers, growing  on sod podsolic soils with residual humus horizons, under revegetating deciduous forests soil formation takes place under higher soil biological activity due to differing quantity and quality of plant matter input. The well drained areas under aspen and birch forest with understorey dominated by herbs and grasses (the most abundant species being  Betula pubescens, Populus tremula, Carex macroura, Aegopodium podagraria, Calamagrostis obtusata, Equisetum pratense, Rubus saxatilis) are occupied by organic matter accumulating soddy podzolized soils with carbonate-containing layers close to the surface. In semihydromorphic and hydromorphic biotopes under smallreed aspen-birch forests (the most abundant species being Betula pubescens, Populus tremula, Calamagrostis phragmitoides Carex globularis, Equisetum sylvaticum, Filipendula ulmaria) organic matter accumulating soils with varying thickness of the humus/peat horizons are common. The species composition of such plant communities corresponds to the one typical for the southern taiga, but their production characteristics are closer to those common for the more southern communities of the northern forest-steppe with 3–5 t/ha (dry mass) of phytomass. Phytomass mineralization rate is higher, and leaves and grass decomposition products are less acidic as compared with zonal forests, where plant litter is rich in needles and mosses. As a result, organic matter accumulating soils under such communities developed some features, more common for their southern counterparts, distinguishing them from the soddy podzolic soils and making them more like the humus-accumulating soils in the northern forest-steppe. These soils have weak granulometric differentiation and well developed structure of the humus horizon. Humus content in quite high, reaching in the southernmost part of the soil area 6–8% in humus accumulating horizons (AU) 50 cm thick. The northernmost parts of the area see the decreased thickness of the dark humus horizon where the latter is substituted with brownish-gray (AY) horizon common for the taiga soils with 4–6% humus content. Organic matter accumulating soil profiles were found to be contrasting in pH(H2O), as the latter increased from 5.5–6.0 in layers immediately below the litter to the neutral values of 6.0-7.0 in humus horizons and 7.0–7.5 in subsoil horizons. High location of carbonates and solonetz properties of parent rock results in high (75-95%) saturation of soil absorbing complex with basic elements, also preventing drastic degradation of humus profile, as only the topmost part of the humus horizon was found to be affected by acidic products of forest litter decomposition. The relic part of the soil profile can be seen in all studied soils. The radiocarbon age of the relic horizons, as determined by humic acids, ranged 4,000–6,000 years. Humus quality reflects differences in humus horizon genesis. Contemporary horizon of soddy podzolised soils is formed due to climatic conditions of the southern taiga zone and has humic to fulvic acids ratio of 1.2–1.4 and low content of Ca-bound acids, while residual humus horizons were found to have increased humic acid content, decreased fulvic acids content of aggressive 1 and 1a fractions, the highest ratio of humic to fulvic acid carbon of 1.8–2.1 and increased content of their Ca-bound fractions. These features, as well as a very low boundary of humus horizon indicate relic humus formation under more southern, i.e. forest-steppe, environmental conditions. The microbiomorph fraction of humus horizons was found to be dominated by plant detritus and phytoliths. The phytoliths assemblages of the contemporary and relic humus horizons were quite similar with notable absence of coniferous phytoliths. All these suggest that organic matter accumulating soils formation under different climatic conditions in late Holocene was related to phytocenoses similar to grassy deciduous forests. The study revealed that both contemporary humus accumulation and relic humus preservation are negatively correlated with eluvial process rate. So successional dynamics of forest communities determines the pendulum nature of soil evolution. The stage of native dark coniferous forests is characterized by humus horizon degradation under acidic forest litter. The restoration stage of grassy birch forests accelerates humus accumulation and partial restoration of both contemporary and relic humus profiles, which is more explicit with the longer duration of this successional stage.