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) irrigate d onto soils we applied two loading rates (standard DFE, high DFE) onto a s ilt clay loam. Additionally, we investigated the microbial availability of the soluble DFE fraction compared with that of a glucose plus ammonium solu tion. The soil was incubated at 25 degreesC for up to 244 days. A water-ame nded treatment was used as the control. Net carbon mineralisation in the st andard 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 un til the end of the measurements (Day 50) and reached 48.4 +/-0.5%. DFE appl ication at the standard rate had only a minor effect on soil microbial biom ass, but the high DFE application supported a higher microbial biomass over a longer period. Nitrogen immobilisation persisted in the standard DFE tre atment throughout the experiment, while the high DFE treatment shifted to t he net mineralisation phase by Day 113. Approximately 60% of the amended C was mineralised within 13 days in both the soluble DFE and glucose treatmen ts, indicating similar microbial availability. Microbial growth, however, o ccurred only in the glucose treatment.