Holocene Soil Evolution in South Siberia Based on Phytolith Records and Genetic Soil Analysis (Russia)

Denis Gavrilov,Nina Klimova,Sergey Loiko

doi:10.3390/geosciences8110402

Abstract

Dark, coniferous hemiboreal forests in the south of West Siberia are located in the Holocene forest-steppe ecotone, where natural environmental conditions have been quite dynamic. This dynamic environment resulted in the contrasting evolution of regional soil cover and the development of unique soil profiles with the second humus horizon. The second humus horizon is assumed to be a relic from the dark-humus soil formation stage in the mid-Holocene. This article draws conclusions about changes in regional environmental conditions by analysing data from a geochemically interrelated coevolutionary soil series, obtained by using a combination of conventional soil studies, phytolith analyses, and accelerator mass spectrometry (AMS) dating of phytolith-occluded carbon (PhytOC) and humic acids. The results showed that, in general, phytocenoses changed from mire-meadow vegetation towards forest vegetation via the meadow stage. However, these stages had different durations, depending on the soil catenary position. The topographical divergence of soil phytolith profiles reflects the relief effect on the development of specific soil type combinations, accounting for the major elements of the regional mid-Holocene soil cover. The leading elementary soil-forming processes were humus accumulation and hydrogenic accumulation of calcium carbonates. In the hilltop site of Endocalcic Stagnic Albic Luvisols, the evolutionary changes were shown by the shift from the meadow phytocenosis (Calcic Stagnic Chernozem) to the forest phytocenosis. In the midslope site, the environment was more humid from the start, favouring a phytocenosis with features of the meadow-mire type. The shift from the meadow-mire environment (with Spodic Chernic Gleysols) to the forest type environment with leading profile-forming processes, acid hydrolysis and lessivage, was gradual, occurring via the meadow stage with Calcic Stagnic Chernozem. In the toeslope site (Calcic Stagnic Greyzemic Epidystric Umbrisols), the meadow-mire stage (with Spodic Chernic Gleysols) was succeeded by the forest stage of soil formation. The AMS-dating of PhytOC estimated that the dark-humus stage of soil formation began 6.5–5.7 years calBC. Despite the observed slight translocation of phytoliths down soil profiles and phytolith solubilisation, phytolith analysis can be used to reconstruct shifts in the soil formation environment for surface Holocene palaeosols.

Highlights

Dark, coniferous hemiboreal forests (Milio effusi—Abietetea sibiricae) [1,2] are common in the southern taiga subzone within the West Siberian Plain, where they grow on automorphic, texturally differentiated, and semihydromorphic organic matter accumulating soils that often have residualGeosciences 2018, 8, 402; doi:10.3390/geosciences8110402 www.mdpi.com/journal/geosciencesGeosciences 2018, 8, 402 humus horizons [3,4,5]
The accelerator mass spectrometry (AMS)-dating of phytolith-occluded carbon (PhytOC) estimated that the dark-humus stage of soil formation began 6.5–5.7 years calBC
The morphological traits of the processes seemed to be expressed differently depending on the relief (Figure 1), as textural differentiation is most pronounced on the hilltop and less pronounced on the toeslope

Summary

Introduction

Coniferous hemiboreal forests (Milio effusi—Abietetea sibiricae) [1,2] are common in the southern taiga subzone within the West Siberian Plain, where they grow on automorphic, texturally differentiated, and semihydromorphic organic matter accumulating soils that often have residualGeosciences 2018, 8, 402; doi:10.3390/geosciences8110402 www.mdpi.com/journal/geosciencesGeosciences 2018, 8, 402 humus horizons [3,4,5]. In the mid-Holocene, the soils of the forest-steppe ecotone underwent the dark-humus stage of soil formation, which was later succeeded by the active textural differentiation of automorphic soils and the hydrogenic transformation of soils on the margins of the increasing mires. Due to all these processes, mineral soils of the region display polygenetic features, recorded in the complex structure of their organic matter profiles. These include high humus content, differences in humus quality between the contemporary and the second humus horizons, occurrence of molecasts, residual solonetz, carbonates, and solodic features.

Methods

Results

Conclusion