Comparison of the structural stability of pasture and cultivated soils

The structural properties of two neighbouring soils from the NW of Spain were evaluated in order to elucidate the effect of management on the soil structural quality and soil organic carbon turnover. The two soils were developed on granite under a warm and humid climate, but differed in land use (pasture and cultivation). The pasture soil had more favourable structural properties than the cultivated soil, showing lower bulk density, higher porosity and water retention. Also, the pasture soil showed a higher mean aggregate diameter and aggregate stability against mechanical agitation in water, as well as lower soil loss under simulated rainfall. This increased structural stability of the pasture soil could be attributed to its higher soil organic matter (SOM) content. The effect of soil use and aggregate size on SOM mineralization was also investigated. Laboratory incubation experiments were conducted with 1–5 mm aggregates and disaggregated < 1 mm soil. More C–CO2 was released by SOM mineralization in the pasture soil than in the cultivated soil, thus indicating a higher microbial activity in the pasture soil. The respiratory quotient (C–CO2/Corg) was also higher in the pasture soil, which means that SOM in this soil is more accessible to microbial decomposition. Nevertheless no significant differences were observed between organic C mineralization in the disaggregated < 1 mm soil and the undisturbed 5–1 mm aggregates. The overall results demonstrate the need to maintain adequate levels of OM by adding organic amendments or adopting lower impact cultivation practices such as reduced tillage.