Changes in mineral N, microbial biomass and enzyme activities in different soil depths after surface applications of dairy shed effluent and chemical fertilizer

Abstract

A field experiment was conducted to determine the effects of surface applications of dairy shed effluent (DSE) (effluent collected from a dairy milking shed and consists of dung, urine and water) or chemical fertilizer (NH4Cl) on N dynamics, microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) in different soil depths. The DSE and NH4Cl were applied to pasture soil at an equivalent rate of 200 kg N ha−1in May and November 1996, as autumn and late spring applications, respectively. Soil samples taken from different soil depths following the autumn application were analyzed for inorganic N, microbial biomass C and N and enzyme activities, while soil samples taken following the late spring application were analyzed for inorganic N only. The soil NH4+concentration, soluble organic C, protease, deaminase and urease activities, and microbial biomass C and N significantly increased in the 0–5 cm soil depth soon after the application of DSE. During the first 30 days, the soluble organic C, microbial C and N and protease activity also increased in the 10–20 cm, while there was no such increase in deaminase and urease activities below 10 cm soil depth. After day 30, the microbial and enzyme activities decreased in the surface as well as in the sub-surface layers possibly due to the exhaustion of the available carbon substrates but remained higher compared to the NH4Cl and control. The NH4Cl application, due to lack of organic substrates, had no effect on soluble organic C, protease or urease activities and biomass C. However, it did increase the deaminase activity and microbial biomass N. The NO3− concentration in lower soil depths of NH4Cl treated soils was significantly higher than those in the DSE and control. This indicates that possible NO3− leaching were more after NH4Cl addition than after DSE. N applied in autumn had higher potential for leaching than that applied in late spring because of increased drainage, lower pasture growth and N uptake during the winter period. Being a source of organic N, DSE showed better performance in maintaining higher pasture yield and N uptake than the NH4Cl and the control. Pasture yield and N uptake were always higher following the spring application than the autumn application because of the optimal environmental condition during summer. These results showed that soil treated with DSE had higher enzyme activities and microbial biomass than soil treated with chemical fertilizers and this may result in longer availability of N for plant uptake and reduce the risk of N leaching losses.