Anthropic dark soils horizons in western Siberian taiga: origin, soil chemistry and sustainability of organic matter

Abstract

• Existence of soils similar to Amazonian Dark Earths and Plaggic soils in Siberian taiga. • New approach to studying organic matter in anthropogenic soils using Rock-Eval analysis. • Soil organic matter stability should be considered in the Anthrosols classification (WRB). The impact of human activity on soil carbon stock and soil fertility is at the forefront of scientific research. In the past, human practices improved soil fertility and increased carbon storage over long periods of time. Studying the resulting anthrosols provides access to their evolution over time scales that are unavailable otherwise. These archeological Anthrosols have been extensively investigated in Amazonia and Europe, but are virtually unknown in Siberia. Here we examined four soil profiles from two archeological sites in western Siberia, along with two corresponding control profiles. The first site, Ketskii Ostrog, is an ancient 18th century fortress occupied by an agricultural population. The second site, Shaitan III, is a historical settlement of metallurgists and horse-breeders active from the 10th through the 17th century. We aimed to understand whether human activity at these two sites modified soil carbon and chemical fertility, and to compare these soils with other studied global anthrosols. We revealed a contrasting impact of human activity for these two sites. At Ketskii Ostrog, ancient agricultural practices improved soil chemical fertility, with the effects persisting until present day. Human activities exhibited a beneficial influence on deep horizons through improvements in their cation exchange capacity, pH and nutrient (P and Ca) concentrations. The Ketskii Ostrog soils were similar to plaggic soils documented in Northern Europe, which are characterized by the addition of fresh organic matter (OM) in the form of manure which was purposefully added to improve soil fertility. At Shaitan III, chemical fertility has also been improved, but to a lesser extent. In contrast to Ketskii Ostrog, Shaitan III soils are rich in charred organic matter and strongly resemble Amazonian Dark Earths with their physico-chemical properties. The Shaitan III soils also have a different origin, as the result of unintentional human impact. Anthropisation has increased carbon stocks in the soil at both sites; however, Shaitan III soils prove to be a more effective carbon sink than at Ketskii Ostrog. This two-fold dichotomy (intentional/unintentional vs. fresh OM/charred OM) is not limited by geographical borders (Amazonia, Europe, or Siberia) and soil typology as currently proposed in the WRB. This highlights the need for concerted studies on anthropised archaeological soils in regard to their importance for both soil fertility and carbon storage.