Soil organic carbon storage following conversion from cropland to grassland on sites differing in soil drainage and erosion history

Changing soil use from cropland to grassland influences organic carbon storage in a highly complex way. This includes the root/shoot allocation, the root depth distribution, the incorporation of shoot biomass and lateral organic carbon fluxes, by erosion and removal of harvested carbon, and finally the aeration by tillage. An experiment was designed allowing resampling a number of soils 18 yr after conversion to grassland (either pasture or meadow or set-aside) only 20 cm apart from the original sampling to exclude site variation. Before conversion to grassland the cropland was prone to erosion, with a mean lateral carbon flux during 20 yr prior to conversion of 13 t ha^-1. Harvest had removed another 29 t ha^-1 of carbon at eroding sites. Colluvial carbon inputs had been up to 18 t ha^-1 while harvest had removed 38 t ha^-1 at colluvial sites. The carbon fluxes by erosion were negligible during the 18 yr period after conversion. After conversion the carbon losses by harvest also ceased at set-aside grassland and pastures while the net losses on meadows were 45 t ha^-1. Conversion to grassland significantly changed depth functions of carbon, stones, bulk density and porosity. Despite the large changes in carbon fluxes, carbon stocks did only change significantly within 18 yr under poorly drained, gleyic soils. Well-aerated soils did not show a significant increase in SOC stocks. This was even true for heavily eroded soils, where conversion from cropland to grassland (without erosion) should foster dynamic replacement of SOC. The widespread drainage of wet grassland soils prior to conversion to cropland thus can cause a large release of carbon, while an influence of tillage by either increasing aeration or erosion could not be detected in this study. Therefore, fostering carbon sequestration by conversion of cropland to grassland requires restoring former draining conditions.