The acid phosphatase activity (APA) is a key factor in phosphorus (P), mineralization, P nutrition of plants, and P cycling. However, the widely used method of Tabatabai and Bremner for APA determinations is seriously restricted in soils with high organic matter contents, owing to interference in photometric detection by dissolved organic matter. The objectives of this study were: A) to modify the procedure of Tabatabai and Bremner with a view to its application to forest soils rich in organic carbon, b) to determine APA contents in forest soils of the Sierra de Gata mountains, and c) to establish relationships between the APA with different soil parameters. The modified procedure was well adapted to forest soils with high organic matter contents and had good reproducibility, while maintaining the relative simplicity and accuracy of the original Tabatabai and Bremner procedure. APA in the forest soils studied was high but did not correlate with labile P. The APA in the soils studied depended principally on availability of readily degradable energy sources and soil nitrogen (N).

Two natural soil ecosystems, representative of agricultural management in a temperate Mediterranean climate, were selected: a sandy soil located at Peccioli Farm, Pisa (Italy), and a silt‐loam soil located at Salamanca, Muñovela (Spain). Within the systems, three areas were sampled (0–10 cm) during the dry season, each corresponding to a different soil use: native undisturbed (NS), cultivated (CS, crop‐pasture rotation) and intensively cultivated soil (ICS, monoculture). Total C, N, exchangeable metals (Mn, Fe, Cu, Zn), ATP, urease, benzoyl‐argininamide (BAA)‐hydrolyzing and casein‐hydrolyzing proteases, phosphatase, ß‐glucosidase, and dehydrogenase were determined and compared. Exchangeable metals, ATP, enzyme activity, C, and N were found to be significantly higher (p = 1%) in native than in cultivated and intensive‐agriculture soils. A good correlation (p = 5%) was found between ATP, hydrolases, C, and N, indicating that all these parameters are related to soil microbial biomass and activity in such “low‐energy”; ecosystems. Dehydrogenase did not correlate with C, N, and protease activities, meaning that the soil ecosystems have low concentrations of exogenous substrates to metabolize, and microorganisms were in a low state of activity. These parameters may be taken as markers of agriculture‐induced changes in soil ecosystems.