Immobilisation and mineralisation of carbon and nitrogen from dairy farm effluent during laboratory soil incubations

To investigate the microbial turnover of dairy farm effluent (DFE) irrigated onto soils we applied two loading rates (standard DFE, high DFE) onto a silt clay loam. Additionally, we investigated the microbial availability of the soluble DFE fraction compared with that of a glucose plus ammonium solution. The soil was incubated at 25 degreesC for up to 244 days. A water-amended treatment was used as the control. Net carbon mineralisation in the standard DFE treatment was finished within 13 days and amounted to 29.7 +/-2.4% of the carbon applied, whereas in the high DFE treatment it continued until the end of the measurements (Day 50) and reached 48.4 +/-0.5%. DFE application at the standard rate had only a minor effect on soil microbial biomass, but the high DFE application supported a higher microbial biomass over a longer period. Nitrogen immobilisation persisted in the standard DFE treatment throughout the experiment, while the high DFE treatment shifted to the net mineralisation phase by Day 113. Approximately 60% of the amended C was mineralised within 13 days in both the soluble DFE and glucose treatments, indicating similar microbial availability. Microbial growth, however, occurred only in the glucose treatment.