Changes in carbon stocks of D anish agricultural mineral soils between 1986 and 2009

Summary To establish a national inventory of soil organic carbon ( SOC ) stocks and their change over time, soil was sampled in 1986, 1997 and 2009 in a D anish nation‐wide 7‐km grid and analysed for SOC content. The average SOC stock in 0–100‐cm depth soil was 142 t C ha −1 , with 63, 41 and 38 t C ha −1 in the 0–25, 25–50 and 50–100 cm depths, respectively. Changes at 0–25 cm were small. During 1986–97, SOC in the 25–50‐cm layer increased in sandy soils while SOC decreased in loam soils. In the subsequent period (1997–2009), most soils showed significant losses of SOC . From 1986 to 2009, SOC at 0–100 cm decreased in loam soils and tended to increase in sandy soils. This trend is ascribed to dairy farms with grass leys being abundant on sandy soils while cereal cropping dominates on loamy soils. A statistical model including soil type, land use and management was applied separately to 0–25, 25–50 and 50–100 cm depths to pinpoint drivers for SOC change. In the 0–25 cm layer, grass leys added 0.95 t C ha −1 year −1 and autumn‐sown crops with straw incorporation added 0.40 t C ha −1 year −1 . Cattle manure added 0.21 t C ha −1 year −1 . Most interestingly, grass leys contributed 0.58 t C ha −1 year −1 at 25–50 cm, confirming that inventories based only on top‐soils are incomplete. We found no significant effects in 50–100 cm. Our study indicates a small annual loss of 0.2 t C ha −1 from the 0–100 cm soil layer between 1986 and 2009.