Effects of dairy factory effluent application on nutrient transformation in soil

Dairy factory effluent (DFE) contains significant amounts of nutrients such as nitrogen (N), phosphorus (P), potassium (K), and sulphur (S) which are beneficial to plant growth. It also contains high amounts of carbon (C). Lately, there has been some concern that DFE application to pastoral land is adversely affecting plant growth in some regions of New Zealand. In this study, we determined the mineralisation and immobilisation of nutrients particularly C, N, S, and cations, in a DFE-treated Omebeu sandy loam soil. We report findings from laboratory-based open incubation studies carried out at 10, 20, and 30 degrees C, with four rates of DFE application (0, 150 000, 300 000, and 450 000 litres ha(-1)) alone and with added NO3- (100 kg N ha(-1)). The DFE was applied at two-weekly intervals into packed soil columns which were leached with 0.01 MCaCl2, solution. Leachates were analysed for total C, total N, SO42-, NO3-, NH4+, K+, Na+, and Mg2+. Effects of DFE application on soil microbial biomass-C, hot-water extractable-C, and anaerobically mineralisable-N were also determined. Addition of DFE increased the size of the microbial biomass pool and thereby enhanced immobilisation of nutrients, mainly N and S. The immobilisation was greater at higher temperature. At 10 degrees C, microbes were unable to utilise all of the added C, even at the lowest rate of DFE application, and 40-50% of the C was leached from soil columns. However, at 30 degrees C soil microbes either immobilised or respired between 95-97% of the C added from DFE, and only small amounts of C were measured in the leachates. Addition of NO3--N had no significant influence on the C immobilisation or respiration. Most of the added N (92-97%) from DFE remained immobilised in the soils throughout the study. A high proportion of the NO3--N added with DFE was immobilised in soils at 10 and 20 degrees C, showing the dominating influence of soluble C, added through the two-weekly application of DFE, in stimulating microbial activity and causing a prolonged immobilisation of N. There was a net mineralisation of about 100 mu g NO3--N g(-1) soil at 30 degrees C, indicating faster metabolic use of soluble C from DFE by microbes at this temperature. Between 15-35% of the SO42--S applied from DFE was either immobilised by soil microbes or was adsorbed on soil organic matter. The presence of significant amounts of NH4+ in DFE-treated soils suggests that parts of the soil columns may have become anaerobic during incubation, causing mineratisation of N from the death of aerobic microbes or decomposition of soil organic matter. A high proportion of the cations (K+, Na+, and Mg2+) that were added with DFE leached out, indicating that DFE application would have very little effect on the availability of these cations for plant uptake. This study, in part, explains that the poor performance of DFE application on pastoral soils predominantly arises through its effects on the availability of N for plant growth.