Comparison of soil organic matter dynamics at five temperate deciduous forests with physical fractionation and radiocarbon measurements

Forest soils represent a significant pool for carbon sequestration and storage, but the factors con- trolling soil carbon cycling are not well constrained. We compared soil carbon dynamics at five broadleaf forests in the Eastern US that vary in climate, soil type, and soil ecology: two sites at the University of Michigan Biological Station (MI-Coarse, sandy; MI-Fine, loamy); Bartlett Experimental Forest (NH-BF); Harvard Forest (MA-HF); and Baskett Wildlife Recreation and Educa- tion Area (MO-OZ). We quantified soil carbon stocks and measured bulk soil radiocarbon to at least 60 cm depth. We determined surface (0-15 cm) soil carbon distribution and turnover times in free light (unpro- tected), occluded light (intra-aggregate), and dense (mineral-associated) soil fractions. Total soil carbon stocks ranged from 55 ± 4t o 229± 42 Mg C ha -1 and were lowest at MI-Coarse and MO-OZ and highest at MI-Fine and NH-BF. Differences in climate only partly explained differences in soil organic matter 14 C and mean turnover times, which were 75-260 year for free-light fractions, 70-625 year for occluded-light fractions, and 90-480 year for dense fractions. Turn- over times were shortest at the warmest site, but longest at the northeastern sites (NH-BF and MA-HF), rather than the coldest sites (MI-Coarse and MI-Fine). Soil texture, mineralogy, drainage, and macrofaunal activity may be at least as important as climate in determining soil carbon dynamics in temperate broadleaf forests.