Erosion-transported OC and DOC loads from agricultural soil

A global decline in agricultural topsoil organic carbon (OC) content has been discovered weakening soil structure and increasing the risk of erosion, with negative impacts on soil productivity and quality of surface waters. OC loss via discharge includes both dissolved organic carbon (DOC) and erosion-transported OC. Intensive tillage increases erosion, and thus, is likely to decrease soil OC content, while agricultural managements that increase topsoil OC content may increase the annual DOC loads. The aims of this research were to quantify discharge-transported OC loads from agricultural land in the Boreal zone, and to study the effects of soil managements on OC loss. We sampled both surface runoff and subsurface drainage waters for two years (2015-2017) on two clay soil sites with different agricultural management in Finland. The studied managements on cultivated plots were ploughing, notillage, and mineral and manure fertilization. In addition, we studied permanent grassland plots. Annual discharge-exported total OC loads from cultivated plots were 20–70 kg ha-1 and from permanent grassland 50–83 kg ha-1, and were substantially
affected by annual precipitation. Total OC loads were higher from ploughed soil compared to no-tillage management only on the first year, but did not differ between mineral and manure fertilization. The share of DOC load was dominant in the total OC load (67–96% for cultivated and 90% for grassland plots) compared to erosion-transported OC load. Topsoil (0-5 cm) OC% correlated negatively with erosion-transported OC load and positively with DOC load, when precipitation was normal.