Influence of Soil Management on the Biochemical Status of a Vertisol.

Dryland cropping systems in Spain are characterized by low soil organic matter contents, in part due to the climatic conditions that lead to continuous losses of organic matter. This study aims at comparing the effect of direct drilling (DD) and conventional tillage (CT) on the stratification of soil organic matter and biochemical properties in a dryland cereal-sunflower-legume crop rotation. Soil biochemical status was evaluated by measuring enzymatic activity (dehydrogenase, β-glucosidase, alkaline phosphatase and arylsulphatase) in May 2007. Soil samples were collected at three depths (0-5, 5-10 and 10-20 cm).
The soils managed by DD showed a higher increase in total organic carbon content and enzymatic activity than the soils managed by CT. Direct drilling contributed more efficiently to the improvement of total organic carbon (TOC) than conventional tillage, especially at the soil surface (0-5 cm depth). A decrease of TOC and enzymatic activity with increasing soil depth was observed. However, no significant differences in soil enzymatic activity could be found among the three depths, which was attributed to the high clay content of these soils. Enzymatic activity showed a high correlation with total organic carbon contents. Enzymatic activities were significantly (p< 0.01) correlated among each other. Clear differences between the treatments (DD and CT) in TOC and of enzymatic activities stratification ratio were not observed. This was attributed to the stabilizing and protecting effect of the clay minerals on TOC and soil enzymatic activity.
Long-term direct drill soil management in a dryland cereal-sunflower-legume crop rotation improved substantially the soil quality of the upper soil layer, enhancing its organic matter content and its biological status.