Effect of treated and untreated farm dairy effluents on soil fertility, microbial population growth, plant growth, and plant chemical composition: A thesis submitted in partial fulfilment of the requirements for the degree of Master of Agriculture Science at Lincoln University

The expansion of the dairy farming industry has resulted in a large amount of farm dairy effluent (FDE) being produced. Land application of FDE is used to recycle the nutrients in the FDE. ClearTech® is a new effluent treatment technology desingned to separate the solids from the liquids and thus produce treated effluent (TE) and clarified water (CW). The CW is recycled as wash water for the farm yard, while the TE is applied to land to recycle the nutrients. However, the effect of how the treated effluent on soil fertility indices, microbial population growth, plant growth, and plant chemical composition compared with land application of untreated effluent (UE) are largely unknown. Thus, the objectives of this research were: a) to determine the effects of treated and untreated FDE on soil fertility indices; b) to determine the abundance of ammonia-oxidising bacteria (AOB), ammonia-oxidising archaea (AOA), denitrifying functional genes (nirS, nirK and nosZ), general agrobacteria (16S rRNA) and fungi (18S rRNA) following the application of treated and untreated of FDE; and c) to determine the effects of the treated and untreated FDE on plant yield and plant chemical composition. A field experiment was conducted to measure key soil properties, the abundance of AOB, AOA, denitrifying functional genes (nirS, nirK and nosZ), plant yield, and the nutrient concentrations in the pasture. The trial was located at the Lincoln University Research Dairy Farm on a Templeton silt loam soil. FDE was collected from the Lincoln University Demonstration Dairy Farm. FDE was treated to produce treated effluent (TE) and clarified water (CW) by the ClearTech® treatment technology. The TE, the original untreated effluent (UE) and water (control) were applied to the soil plots. Soil samples were taken after 1 and 14 days, and 1, 2 and 3 months following each treatment application, and the pasture was harvested following typical grazing schedules. Results showed that the content of soil organic matter, total C, total N and Olsen P and the abundance of denitrifying functional genes were higher after the application of TE than UE. There were no significant differences between TE and UE in mineral N dynamics, CEC, the abundance of AOB, AOA, general agrobacteria and fungi, plant yield and the plant chemical composition. Therefore, it is concluded that the application of the TE produced from the ClearTech® treatment technology will result in higher the contents of soil organic matter, total C, total N and Olsen P and the abundance of denitrifying functional genes compared with the UE whilst mineral N dynamics, CEC, the abundance of AOB, AOA, general agrobacteria and fungi, plant yield and the plant chemical composition will be similar. Future research could assess potential effects of long-term applications of the TE, the effect of climatic conditions and different soil types on the soil properties and plant growth arising from the application of the different effluents.