Microbial attributes as structural quality index for physical health of an Oxisol under compaction levels

Abstract

Microbial activity is a sensitive indicator of soil structure improvement provided by plant species in the physical recovery of compacted soils. This study aimed to assess the impacts of crop species on soil aggregate classes for soil recovery after compaction and identify which microbial attributes best describe the structural quality index for soil physical health in an Oxisol under no-tillage. The experiment comprised three crop species—maize, ruzigrass, and black oats—cultivated under four degrees of compactness produced by chiseling or machine traffic under long-term no-tillage. Soil aggregates were separated by dry sieving into five diameter classes to assess the basal respiration, microbial biomass carbon, metabolic quotient for CO2 (qCO2), labile carbon, cellulase activity, and total organic carbon. The values of microbial indicators for the bulk soil were estimated as a weighted mean of each attribute using aggregate size distribution. The lowest values for total organic carbon, labile carbon, and microbial biomass carbon were observed in aggregates > 4 mm. The basal respiration values increased from 8.2 µg C-CO2 g d−1 in the aggregates > 4 mm, to 22.1 µg C-CO2 g soil d−1 in the aggregates < 0.5 mm (+170%). Similarly, the cellulase activity increased approximately four times comparing aggregates > 4 mm (38 µg glucose g−1 d−1) to those < 1 mm (181 µg glucose g−1 d−1). The total organic carbon and microbial biomass carbon were found to be little affected by crops. However, the plots cultivated with black oats or ruzigrass exhibited higher cellulase activity, basal respiration, and qCO2 in comparison with maize, primarily due to the greater content of labile carbon. Increasing the mean weight diameter from 3 mm to 8 mm led to a decrease in the labile carbon (−27.4%), microbial biomass carbon (−25.5%), cellulase activity (−70%), and basal respiration (−21.6%), thereby revealing negative effects of soil compaction on microbial attributes. The degree of compactness did not influence the values of total organic carbon and qCO2. The response of the microbial indicators depends on the aggregate diameter classes. The results of this study indicate that cultivation of cover crops decreases the negative impacts of soil structure degradation on soil microbial quality. Cellulase activity and labile carbon were considered as new potential indicators of soil compaction for soil physical health studies—highly sensitive and quickly responsive to managements that induce soil structure changes in an Oxisol.