Clay mineral composition modifies decomposition and sequestration of organic carbon and nitrogen in fine soil fractions

The interaction between minerals and organic matter (OM) is a key to the turnover of OM in soils. In particular, clay minerals, iron oxides and charcoal are considered as important constituents affecting the sequestration of carbon (C) and nitrogen (N). Here, we incubated pre-produced artificial soils (842 days) and a natural soil (Ap, Luvisol) with C-13- and N-15-labelled plant litter over 63 days to follow OM turnover and the formation of organo-mineral associations regarding different compositions (montmorillonite (MT), illite (IL), montmorillonite + charcoal (MT+CH), illite + ferrihydrite (IL+FH)). The microbial biomass, salt extractable organic C, the isotopic C and N composition in the bulk soil and the soil fractions (combined density and particle size fractionation) were determined. By comparison of the artificial soils with the natural soil, we were able to show that the produced soil-like systems have OM dynamics comparable to the natural soil. We found out that the decomposition of the added plant litter was affected by the type of clay mineral that formed the artificial soils, as the soil MT exhibited a slower mineralisation compared to IL, which was in line with a lower microbial biomass. Although a high specific surface area (SSA) provides a high sequestration capacity for C and N, smaller amounts were sequestered in the MT soil with a higher SSA compared to the soil IL. We suppose that a more intensive decomposition is associated with a higher microbial biomass and thus leads to higher amounts of microbial products sequestered in the clay-sized fraction. Charcoal and ferrihydrite had no additional effect in this experiment.