Responses of soil organic matter and greenhouse gas fluxes to soil management and land use changes in a humid temperate region of southern Europe

We studied the effects of soil management and changes of land use on soils of three adjacent plots of cropland, pasture and oak (Quercus robur) forest. The pasture and the forest were established in part of the cropland, respectively, 20 and 40 yr before the study began. Soil organic matter (SOM) dynamics, water-filled pore space (WFPS), soil temperature, inorganic N and microbial C, as well as fluxes of CO2, CH4 and N2O were measured in the plots over 25 months. The transformation of the cropland to mowed pasture slightly increased the soil organic and microbial C contents, whereas afforestation significantly increased these variables. The cropland and pasture soils showed low CH4 uptake rates (<1 kg C ha−1 yr−1) and, coinciding with WFPS values >70%, episodes of CH4 emission, which could be favoured by soil compaction. In the forest site, possibly because of the changes in soil structure and microbial activity, the soil always acted as a sink for CH4 (4.7 kg C ha−1 yr−1). The N2O releases at the cropland and pasture sites (2.7 and 4.8 kg N2O-N ha−1 yr−1) were, respectively, 3 and 6 times higher than at the forest site (0.8 kg N2O-N ha−1 yr−1). The highest N2O emissions in the cultivated soils were related to fertilisation and slurry application, and always occurred when the WFPS >60%. These results show that the changes in soil properties as a consequence of the transformation of cropfield to intensive grassland do not imply substantial changes in SOM or in the dynamics of CH4 and N2O. On the contrary, afforestation resulted in increases in SOM content and CH4 uptake, as well as decreases in N2O emissions.