Rates of soil organic carbon change in cultivated and afforested sandy soils

Considerable advances have been made in the assessment and mapping of soil organic carbon stocks, but rates of change in carbon stocks remain to be quantified for many soils and ecosystems. We sampled 145 sandy soils (mostly Psamments) under permanent cultivation and forest. We used aerial imagery to determine the period of cultivation and to calculate changes in soil organic carbon stocks. Topsoil organic carbon stocks, including the A and O horizons, were highest in soils under forest which were never cultivated (36 Mg C ha -1) and lowest in soils under red pine with prior cultivation (31 Mg C ha -1). Average soil organic carbon stocks of the A horizons of cultivated soils were 33 Mg C ha -1. To meet the 4 per 1000 international initiative, these soils need to achieve a soil organic carbon sequestration rate of 0.1 Mg C ha- 1 yr- 1. A mean rate of change of-0.16 Mg C ha- 1 year- 1 was found. The A horizon thickness increased under cultivation, but soil organic carbon concentrations decreased leading to reduced soil organic carbon stocks. The decline in soil organic carbon stock could be explained by an increased rate of organic matter decomposition due to tillage, irrigation, nitrogen applications, and lower clay and silt contents. After about 70 years of afforestation with red pine, soil organic carbon stocks increased. The O horizon accrued organic carbon at a rate of +0.19 Mg ha- 1 yr- 1, but soil organic carbon stocks were lower in the A horizon compared to cultivated soils. Afforestation of abandoned cultivated fields maintained soil organic carbon in the A horizon and gained organic carbon in the O horizon, but the soil organic carbon stocks were below soils under forest which were never cultivated.