Soil carbon storage as affected by the conversion from croplands to pastures with holistic grazing in Eastern Denmark

Agricultural management is strongly affecting soil organic carbon (SOC) stocks and cycling in half the world’s habitable land areas. Regular cropping with annual crops is known to considerably reduce soil carbon stocks, with detrimental effects to soils’ fertility and heightening atmospheric greenhouse gas concentrations. Compared to croplands, grasslands and pastures generally store more SOC. Thus, the conversion of croplands into pastures and grassland can be beneficial for soil carbon storage. The objective of the present study was to elucidate how the conversion of a cropland into holistically grazed pastures affect SOC storage. To test differences in soil OC we used a space for time approach and sampled a chronosequence consisting of a 4-year-old pasture, 10-year-old pasture and a nearby crop-field near Ringsted, Denmark. Mean annual temperature is 8.9°C and annual precipitation 695 mm, all three sites classified as a sandy loam according to Danish soil texture maps. Sampling was done in late September and early October, where 10 soil cores to 40 cm depth were taken from each field and split into samples of 0-10, 10-25 and 25-40 cm. SOC, total N, pH and bulk density (BD) was determined for each sample. SOC concentrations and stocks were calculated by equivalent soil mass approach, and comparisons of SOC and BD between managements done through linear mixed modelling, to account for spatial autocorrelation between samples from one soil core. Results show that both pastures have significantly higher SOC concentrations and lower bulk densities than the crop field. The 10-year-old pasture showed slightly higher SOC concentrations and stocks in the top 10 cm, but this difference was not statistically significant. No other significant differences were found between the two pastures. This suggests that SOC and BD changes happen rapidly after conversion from croplands to pastures, and a steady low build up of additional soil OC occurs after a certain threshold of SOC stocks is reached. However, as we used a space for time approach, one can not fully rule out differences in historic land-use and thus certain similarities between the pastures before the conversion compared to the sampled cropland.