Tillage and Nitrogen Fertilization Effects on Soil Organic Carbon under Semiarid Mediterranean Conditions.

The soil organic carbon (SOC) content is controlled by the balance between outputs and inputs of organic C. In semiarid Mediterranean agroecosystems, a limited crop residue input together with the historically used of intensive soil tillage practices have led to low SOC contents. Previous studies carried out in these Mediterranean systems have shown that reductions in the intensity of tillage lead to the decrease of SOC decomposition. At the same time, though little information exists in these agroecosystems, we hypothesized that an increase in the N fertilizer rates may result in an increase of the crop residue production and accordingly in the C input to the soil. Consequently, this study was aimed to show whether a decrease in tillage intensity and an increase in N fertilization enhanced SOC content in a Mediterranean semiarid agroecosystem. The study was conducted in a long-term tillage and N fertilization experiment located in Agramunt (NE Spain) and established in 1996. Three tillage systems were compared: conventional tillage (CT) with moldboard ploughing, reduced tillage (RT) with chiseling and no-tillage (NT); and three N fertilization rates: zero, medium (60 kg N ha^-1) and high (120 kg N ha^-1). Soils samples were taken up to 40 cm depth. After 10 years of different tillage and N fertilization management, differences in SOC were mainly observed in soil surface (0-5 and 5-10 cm soil depths). A 70% more of SOC was observed in NT compared with CT in the 0- to 5-cm depth. Nevertheless, different N fertilization rates has a lesser effect on the SOC content of this semiarid agroecosystem. Under Mediterranean conditions, a reduction of tillage intensity is an interesting option to increase SOC contents, though this increase has only been observed in soil surface. In contrast, N fertilization management has little impact on SOC.