Responses of soil nutrient and enzyme activities to long-term mulched drip irrigation (MDI) after the conversion of wasteland to cropland

Soil nutrients and enzymes are essential indicators of ecological functions in terrestrial ecosystem and are also sensitive to land-use change. However, the response of soil nutrients and enzymes to long-term mulched drip irrigation (MDI) in the conversion of agroecosystems is still limited. In this study, we analyzed soil nutrient concentrations and enzyme activities at a soil depth of 0-40 cm in four typical cotton fields with different durations of MDI (8, 12, 16, and 22 years) and a wasteland (CK) as control. After the conversion of saline-alkali wasteland into cotton fields, we found that soil electrical conductivity (EC) and pH significantly decreased, but the concentrations of available soil phosphorus (AP), total carbon (TC), soil organic carbon (SOC) and total nitrogen (TN) significantly increased with increasing years of MDI application. Soil enzymes, such as polyphenol oxidase, catalase, urease, alkaline phosphatase, and cellulase were more active in the subsoil than in the topsoil. Compared to the wasteland, a significant reduction of soil enzyme activities (polyphenol oxidase, catalase, and cellulase) appeared in the young cotton field, which gradually reestablished after practicing long-term MDI. Based on redundancy analysis, the variations of the enzyme activities were obtained by pH (38.7 %), C/N (27.6 %), EC (20.9 %), TN (4.9 %), and NO3  -N (3.1 %) at 0-20 cm depth; and by AP (43.7 %), SWC (18.4 %), pH (12.6 %), and TN (12.5 %) at 20-40 cm depth. Long-term MDI has a positive effect on reducing soil salinity, increasing nutrient content in cotton fields, and can accelerate the activities of nutrient-releasing enzymes, which partly confirm the sustainability of MDI in oasis agroecosystems.