Soil quality indicators in a Rhodic Paleudult under long term tillage systems

• Long-term no-till maintains soil C stocks at similar levels to the native grassland.
• Soil N stocks increased independently of system management relative to those of native grassland because of long-term soil fertilization.
• Multivariate analysis was effective to evaluate the impact of tillage systems on soil quality.
• Microbial biomass C was the most efficient soil quality indicator.

Many investigations have focused in defining soil quality indicator components. However, for understanding better the impact of soil tillage systems on soil quality, a broader approach is demanded, with simultaneous soil attributes evaluations, by using multivariate analysis. The objective of this experiment was to evaluate the long-term effect of two tillage systems on the soil C and N stocks in comparison to those of native grassland and to identify the most suitable soil attributes for characterizing soil quality. The experiment was established in 1988 at the Agronomic Experimental Station of the Federal University of Rio Grande do Sul, Southern Brazil. Treatments consisted of two tillage systems (no-tillage and conventional tillage) and a reference area under native grassland. Soil analysis was performed up to 20 cm. Soil organic matter, C and N, physical fractionation was performed and carbon management index calculated. Microbial biomass C and N was determined as well as soil aggregate stability. From soil aggregation, soil average diameters and mass were grouped, and classes determined to calculate mean weight diameter. After 18 years, conventionally tilled soil showed lower total C and N stocks in comparison to no-tilled soil, which did not differ from native grassland soil. Soil C stocks ranged from 44.3 to 34.1 Mg ha−1 for the native grassland and conventional tillage system and total nitrogen ranged from 5.2 to 4.1 Mg ha−1 for the native grassland and no-tillage system, in the 0–20 cm layer. Among the tested soil quality indicators, the microbial biomass-C, total and particulate C stocks, particulate C and N stocks, and mean weight diameter were the indexes that best indicated soil tillage system effects, and they are therefore recommended for future use in evaluating soil quality.