Conversation of cropland into grassland : implications for soil organic-carbon stocks in two soils with different texture

Soil organic-carbon (SOC) stocks are expected to increase after conversion of cropland into grassland. Two adjacent cropland and grassland sites—one with a Vertisol with 23 y after conversion and one with an Arenosol 29 y after conversion—were sampled down to 60 cm depth. Concentrations of SOC and total nitrogen (Ntot) were measured before and after density fractionation in two light fractions and a mineral-associated fraction with C adsorbed on mineral surfaces. For the soil profiles, SOC stocks and radiocarbon (14C) concentrations of mineral associated C were determined. Carbon stocks and mineral-associated SOC concentrations were increased in the upper 10 cm of the grassland soil compared to the cropland. This corresponded to the root-biomass distribution, with 59% and 86% of the total root biomass at 0–5cm soil depth of the grasslands. However, at the Arenosol site, at 10–20 cm depth, C in the mineral-associated fraction was lost 29 y after the conversion into grassland. Over all, SOC stocks were not significantly different between grassland and cropland at both sites when the whole profile was taken into account. At the Arenosol site, the impact of land-use conversion on SOC accumulation was limited by low total clay surface area available for C stabilization. Subsoil C (30–50 cm) at cropland of the Vertisol site comprised 32% of the total SOC stocks with high 14C concentrations below the plowing horizon. We concluded that fresh C was effectively translocated into the subsoil. Thus, subsoil C has to be taken into account when land-use change effects on SOC are assessed.