Global patterns of the effects of land-use changes on soil carbon stocks

Despite hundreds of field studies and at least a dozen literature reviews, there is still considerable disagreement about the direction and magnitude of changes in soil C stocks with land use change. This paper reviews the literature on the effects of land use conversions on soil C stocks, based on a synthesis of 103 recent publications, including 160 sites in 29 countries, with the aims of determining the factors responsible for soil C sequestration and quantifying changes in soil C stocks from seven land use conversions. The results show that as an overall average across all land use change examined, land use conversions have significantly reduced soil C stocks (0.39 Mg ha‑1 yr‑1). Soil C stocks significantly increased after conversions from farmland to grassland (0.30 Mg ha‑1 yr‑1) and forest to grassland (0.68 Mg ha‑1 yr‑1), but significantly declined after conversion from grassland to farmland (0.89 Mg ha‑1 yr‑1), forest to farmland (1.74 Mg ha‑1 yr‑1), and forest to forest (0.63 Mg ha‑1 yr‑1). And after conversion from farmland to forest and grassland to forest, soil C stocks did not change significantly. Globally, soil C sequestration showed a significant negative correlation with initial soil C stocks (P<0.05), and the effects of climatic factors (mean annual temperature and mean annual precipitation) on soil C sequestration varied between the land use conversion types. Also, the relationships between soil C sequestration and age since land use conversion varied in different land use change types. Generally, where the land use changes decreased soil C, the reverse process usually increased soil C stocks and vice versa. Soil C sequestration dynamics were not determined by age since land use conversion at the global level when all land use change types were combined.