Manure increases soil organic carbon most when allocated to annual cropping

Soil carbon sequestration has a great potential in climate change mitigation. To maximise carbon stocks in northern agricultural soils, the carbon sequestration determinants of manure application are crucial. We quantified the effect of manure on soil organic carbon (SOC) depending on the application rate, proportion of leys relative to annuals in crop rotation, and soil texture. We compared the steady-state SOC concentration under farm-yard manure application to a control treatment with no manure application based on 56 individual longterm experiments at 27 locations north of 50 degrees north latitude. At a low application rate of manure such as 0.7 Mg C/ha/y, SOC gradually increased with increasing ley proportions. At a moderate manure rate, such as 1.3 Mg C/ha/y, an increase in the ley proportion led to a decreasing growth of, or even a decrease in, SOC. At a low fixed ley proportion in crop rotation such as 20 %, SOC linearly increased with an increasing manure application rate. As the proportion of leys increased, saturation occurred at a certain ley proportion depending on soil texture. When leys occurred on sandy soils more than once out of seven years and the proportion of leys in crop rotation on clay soils was higher than 45 %, increasing the manure application rate led to decreasing SOC accrual. Compared to the absence of manure application, high manure rates even decreased SOC. We conclude that manure application in crop rotation with a low ley proportion maximises SOC accrual in the topsoil. The most effective manure application rate and ley proportion leys in regard to SOC in the northern agricultural landscape can be determined using a statistical model that we developed, depending on the management system and soil texture.