Soil carbon under arable and mixed dairy cropping in a long-term trial in SE Norway

Soil organic carbon (SOC) was studied at 0-45 cm depth after 28 years of cropping with arable and mixed dairy rotations on a soil with an initial SOC level of 2.6% at 0-30 cm. Measurements included both carbon concentration (SOC%) and soil bulk density (BD). Gross C input was calculated from yields. Averaged over all systems, topsoil SOC% declined significantly (-0.20% at 0-15 cm, p = 0.04, -0.39% at 15-30 cm, p = 0.05), but changed little at 30-45 cm (+0.11%, p = 0.15). Declines in topsoil SOC% tended to be greater in arable systems than in mixed dairy systems. Changes in BD were negatively related to those in SOC%, emphasizing the need to measure both when assessing SOC stocks. The overall SOC mass at 0-45 cm declined significantly from 98 to 89 Mg ha(-1), representing a loss of 0.3% yr(-1) of the initial SOC. Variability within systems was high, but arable cropping showed tendencies of high SOC losses, whilst SOC stocks appeared to be little changed in conventional mixed dairy with 50% ley and organic mixed dairy with 75% ley. The changes were related to the level of C input. Mean C input was 22% higher in mixed dairy than in arable systems.