Dehydrogenase activity in Mediterranean forest soils

The dehydrogenase activity (DHA) in the upper 10 cm of forest soils was measured in three experimental plots (1 ha) in Los Alcornocales Natural Park (southern Spain). In each plot, a sylviculture treatment of thinning and shrub-clearing had been previously carried out in one half, while the other half was left as a forest control. Soil samples were taken during the dry season (July 2000) and after the first autumn rains (October 2000). The DHA of forest soil in autumn 527끭 nmol p-iodonitrotetrazolium formazan (INTF) g^-1 h^-1] was almost double that in summer (289ᇳ nmol INTF g^-1 h^-1), for one of the studied plots. During the dry season, DHA of forest control soils (324ᇩ nmol INTF g^-1 h^-1) was higher than in the thinned and shrub-cleared forest (253ᇱ nmol INTF g^-1 h^-1). During the autumn (wet season), however, the effects of the sylvicultural practices on the soil dehydrogenase were negligible. Significant differences in DHA were found between the three sites. Multiple regression analysis identified pH as the best predictor of DHA of these soils. Other soil properties (pH, K, Ca, Mg, and soil moisture) also showed significant correlations with DHA. In addition, clay content appeared to enhance the enzyme activity. Our results suggest that thinning and shrub-clearing in Mediterranean forests seem to affect negatively the soil DHA, and their impact is more marked during the dry season. However, season and site effects are better determinants of DHA than management practices.