Factors influencing mineralizable carbon in a landscape with variable topography

Abstract

Management-induced erosion has substantially increased the spatial variability in soil organic carbon (OC) stocks in landscapes with variable topography, but its impact on C dynamics is not well understood. The variability in cumulative C mineralization of samples from five positions in a landscape was examined in an incubation study and the effects of water content and depth within the A horizon were assessed. Mineralization was generally at a maximum at water contents between 80 and 95% water-filled pore space. The amount of C mineralized increased with OC content, but the proportion of the mineralizable OC decreased with increasing OC content. Few significant differences in C mineralization existed between surface and subsurface layers of the A horizon. The spatial patterns in maximum C mineralization were generally indicative of the patterns obtained when mineralization was calculated using spatial patterns of actual seasonal average water contents. This suggests the spatial pattern of maximum mineralization was not strongly influenced by variable hydrologic conditions on this site and reflects the spatial patterns in annual mineralization. The proportion of OC mineralized was not related to the proportion of the OC in particulate organic carbon (POC) or in the silt + clay fractions, but was significantly negatively related to the saturation of the clay + silt fraction with C. We speculate that the decline in the proportion of mineralized OC with increasing OC content reflects an increased proportion of the capacity of the silt + clay fraction to retain and physically protect OC within microaggregates.