I. Spatio-temporal patterns of soil microbial and enzymatic activities in an agricultural soil

In the general context of the search of a quality index for soil as an indicator of sustainable management, we analyzed a soil recently subjected to agriculture from the "El Salado" river basin (Buenos Aires, Argentina) under no-till or conventional tillage (CT). We sought to detect whether a pattern of interactions among microbial, biochemical and physico-chemical variables in soil exists that may be distinguished and characterized. Several microbial groups, enzymatic activities and O-2 and CO2 exchange rates were monitored before planting (T-0) and during the growth cycle of soybean (Glycine max (L.) Merr.) (T-1: flowering stage; T-2: pre-harvest period). Strong relationships were evidenced by correlation r (r(2)) matrices performed with several specific groups of bacteria and fungi, and soil enzymatic activities representative of main nutrient cycles (C, N, P). A burst of biological activity was registered at T, as could be judged through: (i) a remarkable increase in the numbers of microorganisms measured at both depths tested (D-1: 5-10 cm; D-2: 15-20 cm); (ii) a peak of O-2 consumption; (iii) a maximal gradient of activity exhibited by all enzymes tested (acid and alkaline phosphatases, dehydrogenase, FDA hydrolysis, beta-glucosidase, urease). Microorganisms stratified as a function of depth particularly at T-0 and T-2, whereas at T-1 they appeared to transiently redistribute in the 5-20 cm profile unlike all enzymatic activities that showed a maximal stratification. Low respiratory quotients, RQ (congruent to0.2; RQ, defined as the ratio of q(CO2) over q(O2)), were found, following an experimental method for quantifying rates of O-2 consumption, q(O2), and CO2 production, q(CO2), by soil, and a mathematical model developed by ourselves to interpret the data [Soil Sci. 166 (2001) 68]. The biodiversity of fungi increased 16% at T-2 with respect to T-0 along with a 37% increment in species richness. The fact that several of the variables measured were strongly linked despite season and crop presence, points to the existence of a core of highly interrelated processes in soil. (C) 2001 Elsevier Science B.V. All rights reserved.