Effects of Land Use and Mineral Properties On the Organic Carbon Contents, and the Amount and the Composition of Na-Pyrophosphate Soluble Organic Matter in Subsurface Soils.

Land use and mineral characteristics affect the ability of surface as well as subsurface soils to sequester organic carbon and their contribution to mitigate the greenhouse effect. While there a quite large number of studies focussing on surface soils, there only a few studies analyzing the effects of arable and forest land use as well as soil mineral properties on the amount and composition of organic matter (OM) in subsurface soils. Here we aimed to analyze the long-term impact of arable and forest land use and soil mineral parameters on subsurface soil organic carbon (SOC) contents, as well as on amount and composition of OM sequentially separated by Na-pyrophosphate solution (OM(PY)) from subsurface soil samples. Seven soils different in mineral characteristics were selected. Soil samples were taken from subsurface horizons of forest and adjacent arable sites continuously used >100 years. The OM(PY) fractions were analysed on their OC content (OC_PY) and characterized by FTIR spectroscopy. Multiple regression analyses indicate for the arable subsurface soils significant positive relationships between the SOC contents and combined effects of the i) exchangeable Ca (Ca_ex) and oxalate soluble Fe (Fe_ox), and ii) the Ca_ex and Al_ox contents. For the arable subsurface soils the increase of OC_PY* (OC_PY multiplied by the relative C=O content of OM(PY)) by increasing contents of Ca_ex indicats that OM(PY) mainly interacts with Ca²⁺ cations. For the forest subsurface soils (pH <5), the OC_PY contents were found to be related to the contents of Na-pyrophosphate soluble Fe and Al. The long-term arable and forest land use seems to result in different OM(PY)-mineral interactions in subsurfac soils. Based on this we hypothesize that a long-term land use change from arable to forest may lead to a shift from mainly OM(PY)-Ca²⁺ to mainly OM(PY)-Fe³⁺ and -Al³⁺ interactions if the pH of subsurface soils significantly decrease (i.e. <5).