Effect of land use and carbonates on organic matter stabilization and microbial communities in Mediterranean soils

Soil organic matter (SOM) is sensitive to land use and to physico-chemical soil properties as well soil microbial communities controlling SOM stabilization. Our study aimed at exploring how carbonates of Mediterranean soils - known to stabilize SOM by enhancing soil aggregation and binding SOM by calcium - are affecting SOM and microbial communities under different land uses. In soils with and without carbonates sampled in forests, cultivated and abandoned fields, we have fractionated SOM-pools according to particle sizes and determined C mineralization rates and polyvalent cations in these fractions. In agreement with larger scale assessments in the Mediterranean, our results show that SOM contents are greater in forest than in agricultural soils, but that the differences among land use types depend on carbonate contents. While SOM contents are greater in carbonated soils of forests, cultivated soils have low SOM levels independent of the carbonate content, presumably due to a reduced protection of particulate SOM by century-long intense tillage. Forest soils contain particularly large amounts of SOM associated with the coarser soil fraction. In this fraction, C mineralization was smallest in the carbonated forest soil, possibly due to a low SOM quality indicated by high C/N ratios and low delta N-15 values. Greater contents of Ca, Fe and Al in carbonated soils also suggests that stabilization through Ca and phyllosilicates was important. Mineralization of sand-associated SOM was highest in cultivated and abandoned soils, which is indicative for a small SOM stabilization, which in turn might restrict SOM accumulation after land abandonment. Microbial communities assessed by phospho-lipid fatty acid signatures and microbial carbon use efficiency were more closely associated with land use than with carbonate content - a pattern that reflects the distribution of C pools among size fractions which differed more strongly among land use types than between soils with and without carbonates.