Below-ground organic carbon and decomposition potential in a field-forest glacial-outwash landscape

Relationships of soil properties to land use and topography may vary among locales because of differences in other controlling factors. We evaluated relationships of below-ground C, defined as soil organic C plus fine-root C, and decomposition potential to site characteristics at Cedar Creek Natural History Area in east-central Minnesota, USA. The landscape, formed in glacial outwash sand, has a complex spatial pattern of grasslands and forests resulting from interactions among cultivation, agricultural abandonment, topography, and fire. Below-ground C was higher in mature forests than in either adjacent abandoned agricultural fields or uncultivated prairie for both the O Horizon (0.57 vs. 0.13 kg m(-2)) and the underlying 0-10 cm of soil (2.1 vs. 1.0 kg m(-2)) but was similar at 10-30 cm (2.0 vs. 2.0 kg m(-2)) and 30-50 cm (1.4 vs. 1.5 kg m(-2)). The higher C in surface soil under forests contrasted with published observations for finer textured substrates. Below-ground C was constant across forest summit, shoulder, and backslope positions, and increased at the toeslope position. Average fine-root (<2 mm diameter) C at the depth of 0-50 cm was 0.2 kg m(-2) and represented 4% of belowground C. In contrast to an expected trend of monotonically increasing decomposition with increased temperature, cellulose decomposition during a 60-day field incubation increased with temperature on cool forest slopes but decreased with temperature in warm fields. Nutrient availability, water availability, and microbial biomass may confound this relationship. The results indicate diverse controls on decomposition in this field-forest landscape.