Soil quality assessed by carbon management index in a subtropical Acrisol subjected to tillage systems and irrigation

The combined influence of no-till and irrigation on soil quality in tropical and subtropical regions is still to be better clarified. The objective of this study was to evaluate the influence of sprinkler irrigation on soil quality of a southern Brazilian sandy loam Paleudult subjected to conventional tillage and no-till for 8 years. The soil quality indicator was the carbon management index (CMI), based on variations in the total C stock (expressed by the C pool index, CPI) and in the C lability (expressed by the C lability index, LI) related to the reference native grassland soil. The C lability was given by the ratio between the concentration of labile C, separated with NaI solution (1.8 Mg/m3), and non-labile C, obtained from the difference between total C and labile C. The total C stock, and thus the CPI, in the 0–200 mm layer were affected neither by tillage system nor by irrigation. On the other hand, the concentration of labile C, and thus the C lability and LI, were lower in conventional tillage than in no-till, and in irrigated than in non-irrigated systems. The effect of irrigation in decreasing the C lability was more pronounced in no-till than in conventional tillage soil. A combination of residue accumulation and greater water availability on the no-till soil surface probably provided suitable conditions to increase microbial mineralisation activity on the light fraction of the organic matter. The results of CMI, whose variations were caused mainly by LI, indicate that soil quality was improved with adoption of no-till in substitution of conventional tillage, but not with adoption of irrigation. No-till soils subjected to irrigation require a higher phytomass addition than non-irrigated soils.