Soil enzyme activities and microbial biomass response to crop types on the terraces of the Loess Plateau, China

PURPOSE: Soil microbes play a critical role in the recycling of soil nutrients and soil fertility. Different crop types greatly influence the soil environment, including the carbon and nitrogen biogeochemical cycles, and the effects may be particularly important on the terraces of the Loess Plateau. In this study, the characteristics of soil physicochemical properties and microbial activity were investigated under different crop types to establish relationships among soil microbial biomass, enzyme activities, and physicochemical properties. MATERIALS AND METHODS: Soils under four typical crops (potato, corn, apple, and intercropping (apple and potatoes)) grown on terraces constructed during a similar period were examined at an experimental site on the Loess Plateau, with 5-year abandoned land as the control. The activities of soil invertase, urease, and alkaline phosphatase, soil microbial biomass carbon (MBC), soil microbial biomass nitrogen (MBN), and the ratio MBC/MBN, and their relationships with soil physicochemical properties were investigated using redundancy analysis and partial redundancy analysis. RESULTS AND DISCUSSION: (1) For soil physicochemical properties, including organic carbon, total nitrogen, total phosphorus, nitrate nitrogen, and available phosphorus, the lowest concentrations were in abandoned land and the highest contents were under apple and intercropping; (2) the activities of soil invertase were in the order intercropping > corn > apple > potato > abandoned land, whereas the activities of soil urease in apple and intercropping were more than twofold higher than those in abandoned land; (3) soil microbial biomass carbon and the ratio of MBC/MBN were highest in abandoned land, which indicated that abandoned land had a high proportion of bacterial to fungal biomass; (4) soil microbial activity was affected by the application of organic manure under terrace crop systems. CONCLUSIONS: The contents of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and available phosphorus (AP) in abandoned land were significantly lower than those under all crop types; thus, directly abandoned farmland was not favorable for soil microbial activity in the typical loess hilly-gully region. Soil TP, moisture (MS), and NO3⁻–N were identified as the key factors affecting soil microbial activity in the 0–20-cm soil layer, and soil TP, AP, NH4⁺–N, and NO3⁻–N were the significant factors in the 20–40-cm layer.