Soil enzyme activities in two forage systems following application of different rates of swine lagoon effluent or ammonium nitrate

Land application of swine lagoon effluent (SLE) to forage production systems is widespread in the southeastern USA and often leads to change in soil properties. Although soil enzymes are crucial to the degradation of soil organic matter and cycling of nutrients, the impacts of SLE application on soil enzyme activities have not been well characterized. We assessed the activities of soil enzymes involved in soil C, N, and P cycling in forage systems 3 years after the termination of three consecutive years of fertilization. Bermudagrass and tall fescue were supplied with SLE or ammonium nitrate (AN) at the rates of 0, 200, 400, and 600 kg plant available N (PAN) ha−1 year−1. The activities of oxidative enzymes (i.e., peroxidase and phenol oxidase) differed between soils amended with SLE versus AN. In soils amended with AN at 600 kg PAN ha−1 year−1, the activities of phenol oxidase and peroxidase were lower than or similar to those in the unfertilized control. In contrast, those activities were stimulated by the application of SLE at the rate of 600 kg PAN ha−1 year−1 except for phenol oxidase in the bermudagrass system. The activities of cellobiohydrolase, β-glucosidase, cellulase, β-glucosaminidase, protease, and acid phosphatase, however, were independent of the source, but varied with the rate of fertilization. In general, the activities of cellobiohydrolase, β-glucosidase, cellulase, β-glucosaminidase, protease, and acid phosphatase in soils with N fertilization at 200 or 400 kg PAN ha−1 year−1 were higher than those in the unfertilized control. But the activities of some hydrolytic enzymes in soils fertilized with 600 kg PAN ha−1 year−1 were similar to or lower than those in the unfertilized control. Non-metric multidimensional scaling (NMS) analysis integrated the activities of eight soil enzymes and showed significant differences between fertilized soils and the unfertilized control and between soils amended with SLE versus AN. These differences in soil integrated enzyme activity were correlated with soil pH (Pearson's correlation coefficient r = 0.76, P < 0.05) rather than with other soil properties. The activities of individual enzymes such as peroxidase, cellulase, and acid phosphatase were also significantly correlated with soil pH. Our results suggested that fertilization-associated change in soil pH could influence the potential activities of soil enzymes in the hay production systems. Application of SLE at 600 kg PAN ha−1 year−1 may be unbeneficial to soil organic matter accumulation and nutrient cycling.