Long-term effects of conservation tillage on organic fractions in two soils in southwest of Spain

Long-term field experiments can provide relevant information regarding soil organic carbon sequestration under different soil tillage systems. Especially, conservation tillage (CT) has been proved to be a practice that highly contributes to improve soil quality. For that reason, the study of soil quality indicators, such as organic fractions. are useful tools to assess changes caused by different soil tillage systems in long-term field experiments. We evaluated long-term effects of conservation tillage on soil carbon fractions and biological properties in a sandy clay loam Entisol (soil A) and in a clay Vertisol (soil B) located in semi-arid SW Spain. Cereal-sunflower-legume rotations under rainfed conditions were used in both soils in which conservation tillage (CT) was compared to traditional tillage (TT). Soil samples were collected at three depths (0-5, 5-10 and 10-20 cm) four months after sowing a pea crop (Pisum arvense L.) in the Entisol and a wheat crop (Triticum aestivum L.) in the Vertisol. Labile fractions of the total organic carbon (TOC) were determined as active carbon (AC) and water soluble carbon (WSC). Biological status was evaluated using soil microbial biomass carbon (MBC) and enzymatic activities [dehydrogenase activity (DHA), o-diphenol oxidase activity (Dphox), and beta-glucosidase activity (beta-glu)]. As a rule, the contents of AC, WSC, MBC, beta-glu and Dphox in soil A and contents of TOC, AC and DHA in soil B were higher in CT than in TT, at the 0-5 cm depth. In both soils, the studied parameters decreased with depth under both tillage treatments (TT and CT). Values of AC, TOC, MBC and beta-glu were positively correlated with each other (p < 0.05) in both soils. The principal component analysis (PCA) showed that two principal components explained 44.17% and 21.2% of the total variability in the Entisol and 47.3% and 19.3% in Vertisol. The first principal component was influenced mostly by AC and beta-glucosidase in the Entisol, while it was influenced by DHA, MBC and AC in Vertisol. Discriminant analysis (DA) showed as discriminant function was strongly correlated with MBC, AC and Dphox in soil A and with TOC, AC and WSC in soil B. From both analyses (PCA and DA) in this study, AC content was the most sensitive and consistent indicator for assessing the impact of different soil managements on soil quality in our two types of soil. Long-term conservation tillage in dryland farming systems improved the quality of both soils, especially at the surface, by enhancing soil organic carbon and biological status. (C) 2009 Elsevier B.V. All rights reserved.