Critical limits for microbiological properties and soil quality in a subtropical Acrisol in southern Brazil

Given the natural fragility of sandy soils, Brazil's intensification of agricultural use of subtropical Acrisols must be carefully monitored. Nevertheless, reference values for microbiological properties have not yet been determined for these soils. Therefore, this study sought to determine the critical limits (CLs) and interpretative classes for soil microbiological properties, establish a minimum set of soil chemical and biological indicators based on statistical associations between these indicators and maize yield, and identify which management systems contribute to the quality of a subtropical Acrisol. Soil samples were collected from two experiments that were selected to evaluate the effects of tillage and cropping systems on maize production for >30 years. Principles to determine CLs for chemical properties were used to interpret microbiological properties, and a minimum data set was established by principal component analysis. Interpretative classes were determined for soil enzymes arylsulfatase and β-glucosidase, microbial biomass carbon and nitrogen, and soil basal respiration as a function of soil organic carbon content and maize yield. By analyzing the minimum data set, we were able to identify properties to compose a soil quality index (SQI) composed of arylsulfatase activity, microbial biomass carbon, total soil nitrogen, soil organic carbon, and pH in water. The SQI revealed that maize cultivation on subtropical Acrisols in Brazil is more sustainable under no-till and with legumes as cover crops.