II. Temporal and spatial evolution of enzymatic activities and physico-chemical properties in an agricultural soil

Enzymatic activities are candidate "sensors" of soil stress to management practice that may sensitively warn us about soil degradation. In the present study, we explore whether a specific spatio-temporal pattern of interactions occurs between soil physico-chemical properties and enzyme activities in order to assess which physical and chemical soil conditions characterize more comprehensively the soil status with respect to soil enzymatic activities. Accordingly, we analyzed the relationships between several enzyme activities representative of main nutrient cycles (C, N, P) and soil physico-chemical properties, as a function of time and soil depth, from an initial soil status (T0) and at two sampling times during a soybean [Glycine max (L.) Merr.] crop (T1, flowering stage; T2, preharvest period). Three properties among the physico-chemical ones, organic carbon (OC), total nitrogen (TN), and water-filled pore space (WFPS), exhibited strong relationships with the enzymatic activities measured (acid and alkaline phosphatases, β-glucosidase, urease, FDA hydrolytic activity, dehydrogenase) irrespective of season and presence of crop. This finding was concomitant with strong correlation among the enzymes themselves, being FDA hydrolytic activity the most sensitive to crop and season in depth D1 (5-10 cm soil depth). The results obtained are in agreement with enzymatic activities playing an integrative role between physical, chemical and microbial soil properties. In the latter context, the concept of "hard-core" or group of strongly associated soil properties that exhibit high and persistent interactions over time, is introduced.