Organic matter and phosphorus fractions in irrigated agroecosystems in a semi-arid region of Northeastern Brazil

The main goal of agricultural management practices in irrigated agroecosystems in the Brazilian semi-arid is either to maintain or increase soil quality. In these areas, the reduction in soil quality is mainly associated with depletion of soil organic matter (SOM), as well as of soil phosphorus (P) reserves. We aimed to evaluate changes in SOM and P fractions of a eutrophic haplic cambisol (Eutric Cambisol) under different uses and management systems in the irrigated perimeter of Jaguaribe/Apodi in Ceará State, Brazil. The evaluated irrigated agroecosystems were perennial banana (Musa ssp.) cultivation (IPBC) and annual maize (Zea mays L.) cultivation (IAMC). The respective natural vegetation (NVIB) and (NVIM) were selected and used as a reference. Soil samples were collected at 0–5, 5–15, 15–25, and 25–40 cm soil depths, and evaluated for total organic carbon (TOC), total nitrogen (TN), humic substances, fractions of oxidizible organic carbon, and organic and inorganic P fractions. The stocks of TOC, TN, and humic substances in topsoil have been reduced by cultivation, and the largest reductions were observed in the IAMC area. The degree of SOM oxidation decreased with soil depth and the greastest amounts of labile C were found in IAMC. Annual and perennial agroecosystems showed changes in reserves of soil P. Our results pointed out that irrigated agroecosystems in the Brazilian semi-arid regions need to adopt management practices that favor the building up of soil SOM contents, particularly where annual crops are cultivated.