Repeated monitoring of organic carbon stocks after eight years reveals carbon losses from intensively managed agricultural soils in Western Germany

Past land‐use changes, intensive cropping with large proportions of root crops, and preferred use of mineral fertilizer have been made responsible for proceeding losses of soil organic C (SOC) in the plough layer. We hypothesized that in intensive agriculturally managed regions changes in SOC stocks would be detectable within a decade. To test this hypothesis, we tracked the temporal development of the concentrations and stocks of SOC in 268 arable sites, sampled by horizon down to 60 cm in the Cologne‐Bonn region, W Germany, in 2005 and in 2013. We then related these changes to soil management data and humus balances obtained from farmers' surveys. As we expected that changes in SOC concentrations might at least in part be minor, we fractionated soils from 38 representative sites according to particle size in order to obtain C pools of different stability. We found that SOC concentrations had increased significantly in the topsoil (from 9.4 g kg^?1 in 2005 to 9.8 g kg^?1 in 2013), but had decreased significantly in the subsoil (from 4.1 g kg^?1 in 2005 to 3.5 g kg^?­1 in 2013). Intriguingly, these changes were due to changes in mineral‐bound SOC rather than to changes in sand‐sized organic matter pools. As bulk density decreased, the overall SOC stocks in the upper 60 cm exhibited a SOC loss of nearly 0.6 t C (ha · y)^?1 after correction by the equivalent soil mass method. This loss was most pronounced for sandy soils ?0.73 t SOC (ha · y)^?1], and less pronounced for loamy soils ?0.64 t SOC (ha · y)^?1]; silty soils revealed the smallest reduction in SOC ?0.3 t SOC (ha · y)^?1]. Losses of SOC occurred even with the overall humus balances having increased positively from about 20 kg C (ha · y)^?1 (2003–2005) to about 133 kg C (ha · y)^?1 (2005–2013) due to an improved organic fertilization and intercropping. We conclude that current management may fail to raise overall SOC stocks. In our study area SOC stocks even continued to decline, despite humus conservation practice, likely because past land use conversions (before 2005) still affect SOC dynamics.