Trends in carbon stocks in Dutch soils: datasets and modeling results

Observations on trends in soil organic carbon (SOC) in agricultural soils is an important basis for mandatory reporting of emission of greenhouse gases from land use. A decrease indicates that these soils are a source of CO2, an increase that they are a sink. IPCC Good Practice Guidance generally assumes that the SOC content of agricultural soils decreases. We evaluated large datasets on trends in SOC in the Netherlands and compared them with datasets from Belgium, England and with literature in general. Also, the soil organic matter simulation model Century was used for a better understanding of soil processes related to manure application and SOC dynamics. The IPCC assumption of decreasing SOC contents was not confirmed for Dutch agricultural soils: on soils with SOC <70 g/kg the SOC contents are constant or increase slightly. The large amounts of manure and fertilizers applied on agricultural soils in The Netherlands could explain this trend. The positive effect of manure on SOC from the calculation with Century is significant, but smaller than the calculated effect from using IPCC Guidelines for calculation of soil organic matter budgets in National Greenhouse Gas Inventories.