Decomposition and stabilization of organic carbon across soil horizons and parent materials of five sites under deciduous forest

Michael Kaiser, Department of Agronomy and Horticulture, University of Nebraska-Lincoln, Lincoln, NE, Svendja Vormstein and Bernard Ludwig, Department of Environmental Chemistry, University of Kassel, Hans-Peter Piepho, Biostatistics Unit, University of Hohenheim

Background/Question/Methods
Knowledge about soil specific differences in organic matter (OM) stabilization and mineralization is critical for the precise assessment of site specific storage potentials of soil organic carbon (OC). Here we quantified the relative importance of micro-aggregate formation and adsorption to mineral surfaces for OC storage across horizons of five soils developed from different parent materials under mature beech forest. Samples were taken from pedogenetic horizons up to a depth of about 2 m. For these samples, aggregate size and density fractions were quantified for their relative contribution to OC storage and were analyzed for their composition by infrared spectroscopy. The decomposer community of the samples was quantified using microbial biomass C (Cmic) and their activity expressed as CO2-C respired within a two weeks incubation experiment.
Results/Conclusions
The relative soil OC proportions of particulate organic matter and macro-aggregates, which are indicative for rather fast cycling OM, are generally decreasing with soil depth independent from the soil parent material. The relative proportions of OC stabilized via mineral association and micro-aggregate occlusion significantly increased with soil depth but also seems to play a role in fueling the labile OM with soil depth pointing to their ambivalent soil ecological functions. We came to this conclusion based on the observation that the CO2-C/SOC and CO2-C/Cmic ratios are increasing with soil depth despite an increasing relative proportion of stabilized OM, which was most pronounced for those soils with the deepest profile development. This is corroborated by data from infrared analyses, which point towards an increasing degree of microbial OM processing expressed by an increased abundance of oxygen containing functional groups with soil depth. The labile and the stable OM pool seem to be stronger differentiated in the subsoil compared to the topsoil on the cost of the intermediate OM pool, a finding that might help to improve subsoil specific modelling approaches. Differences in the magnitude of the detected stabilization and destabilization patterns seem to be affected by parent material and soil depth. This pronounced spatial and vertical heterogeneity in the retention of OC in soils under temperate broad leaf forest should be accounted for in global modeling efforts designed to estimate terrestrial C storage potentials.