Soil organic carbon pool changes following land-use conversions

Carbon (C) can be sequestered in the mineral soil after the conversion of intensively cropped agricultural fields to more extensive land uses such as afforested and natural succession ecosystems. Three land-use treatments from the long-term ecological research site at Kellogg biological station in Michigan were compared with a nearby deciduous forest. Treatments included a conventionally tilled cropland, a former cropland afforested with poplar for 10 years and an old field (10 years) succession. We used soil aggregate and soil organic matter fractionation techniques to isolate C pools that (1) have a high potential for C storage and (2) accumulate C at a fast rate during afforestation or succession. These fractions could serve as sensitive indicators for the total change in C content due to land-use changes. At the mineral soil surface (0-7 cm), afforesting significantly increased soil aggregation to levels similar to native forest. However, surface soil (0-7 cm) C did not follow this trend: soil C of the native forest site (22.9 t C ha^-1) was still significantly greater than the afforested (12.6 t C ha^-1) and succession (15.4 t C ha^-1) treatments. However, when the 0-50 cm soil layer was considered, no differences in total soil C were observed between the cropland and the poplar afforested system, while the successional system increased total soil C (0-50 cm) at a rate of 0.786 t C ha^-1 yr^-1. Afforested soils sequestered C mainly in the fine intraaggregate particulate organic matter (POM) (53-250 (mu)m), whereas the successional soils sequestered C preferentially in the mineral-associated organic matter and fine intra-aggregate POM C pools.