Exports and inputs of organic carbon on agricultural soils in Germany

The quantity and quality of organic carbon (C_org) input drive soil C_org stocks and thus fertility and climate mitigation potential of soils. To estimate fluxes of C_org as net primary production (NPP), exports, and inputs on German arable and grassland soils, we used field management data surveyed within the Agricultural Soil Inventory (n = 27.404 cases of sites multiplied by years). Further, we refined the concept of yield-based C_org allocation coefficients and delivered a new regionalized method applicable for agricultural soils in Central Europe. Mean total NPP calculated for arable and grassland soils was 6.9 ± 2.3 and 5.9 ± 2.9 Mg C_org ha^-1 yr^-1, respectively, of which approximately half was exported. On average, total C_org input calculated did not differ between arable (3.7 ± 1.8 Mg ha^-1 yr^-1) and grassland soils (3.7 ± 1.3 Mg ha^-1 yr^-1) but C_org sources were different: Grasslands received 1.4 times more C_org from root material than arable soils and we suggest that this difference in quality rather than quantity drives differences in soil C_org stocks between land use systems. On arable soils, side products were exported in 43% of the site * years. Cover crops were cultivated in 11% of site * years and contributed on average 3% of the mean annual total NPP. Across arable crops, total NPP drove C_org input (R² = 0.47) stronger than organic fertilization (R² = 0.11). Thus, maximizing plant growth enhances C_org input to soil. Our results are reliable estimates of management related C_org fluxes on agricultural soils in Germany.