Interpretation of Microbial Indicators As a Function of Crop Yield and Organic Carbon In Cerrado Soils.

An interpretative framework for microbial biomass carbon (MBC), basal respiration and the activity of soil enzymes cellulase, b-glucosidase (BG), arylsulfatase, and acid phosphatase was developed for the clayey Oxisols of the Brazilian Cerrado. Soil samples (0 to 10 cm depth) were collected from 24 treatments from three long-term experiments and analyzed to determine their microbial attributes and soil organic carbon (SOC). These treatments presented a large range of extractable Mehlich-P and cumulative corn and soybean yields. The critical levels (CLs) for the microbial indicators were defined as a function of the relative cumulative yields (RCYs) of corn and soybean, and the SOC using linear regression models. The use of RCY to generate the interpretation tables, rather than a single yield measurement, was key to minimize the influence of factors not directly related to soil quality and reflected, more accurately, all of the changes in the soil throughout the duration of the three long-term field experiments. Adequacy classes for each microbial indicator as a function of the RCY and SOC were established based on the following criteria: equal or <40%: low; 41 to 80%: moderate; and >80%: adequate. The CLs equivalent to 80% of the RCY for MBC, basal respiration, cellulase, BG, acid phosphatase and arylsulfatase were: 375 mg C kg-1, 90 mg C-CO2 kg-1, 105 mg glucose kg-1 d-1, 115 mg p-nitrophenol kg-1 h-1, 1160 mg p-nitrophenol kg-1 h-1 and 90 mg p-nitrophenol kg-1 h-1, respectively. Similar CLs were obtained when SOC was used as the interpretation criterion. The interpretation tables provided in this study establish, for the first time, reference values for the soil microbial indicators based on crop yields and constitute a first approximation. Their applicability to other conditions must be evaluated.