Reducing phosphate losses into water by treating farm dairy effluent before application to land : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Intensive dairy farming has resulted in the production of large amounts of farm dairy effluent (FDE). FDE is a mixture of water, urine, dung, soil, feed, cleaning chemicals, and milk. Land application of FDE has become a primary method of FDE management in many countries due to its high nutrient content. However, this method can result in high concentrations of phosphorus (P) in the soil, which increases the risk of P losses from the agricultural system and eutrophication of waterways. This poses a threat to water quality, especially through subsurface tile-drain systems. Additionally, FDE contains a large number of pathogenic bacteria that can pose a risk to human health. To address these issues, a new FDE treatment method using poly-ferric sulphate (PFS) has been developed. This new method can clarify water for recycling; reduce water wastage, and reduce health risks and environmental contamination from FDE applied to land. However, there is a lack of detailed knowledge on the effect of applying treated FDE (TE) on the P leaching losses through soil with tile drains. This research aims to improve knowledge and understanding of the effects on P and Escherichia coli (E. coli) leaching losses, soil P fractions, pasture yield, and plant P uptake following the application of TE. To achieve this aim, two drainage model studies were conducted at Lincoln University and a soil and pasture study was conducted at Lincoln University Research Dairy Farm (LURDF). The objective of drainage experiment 1 was to determine the impact of applying fresh TE and pond-stored TE (TE-S) compared to untreated FDE on P and E. coli leaching losses through subsurface drainage model units. The study found that the cumulative dissolved reactive phosphorus (DRP) lost to drainage water from the TE and TE-S treatments was significantly lower by 93.1% and 92.2%, respectively, compared to the untreated FDE treatment. Additionally, the drainage water from the TE and TE-S treatments showed a significant reduction of 98.3% and 99.9% in E. coli concentrations compared to the FDE treatment. The different treatments did not affect plant biomass and P uptake. The reduction in P and E. coli leaching loss was due to the formation of insoluble iron phosphate in TE, resulting in a decrease in DRP concentration and amount in the drainage water. The acidic nature of PFS and the encapsulation of bacteria within the floc also contributed to the reduction of E. coli. These results demonstrate that PFS-treated effluent application on drained pasture soils can significantly reduce P and E. coli concentrations and amounts in drainage water resulting in substantial environmental benefits, without negatively impacting plant growth. The objective of drainage experiment 2 was to determine the effect of soil compaction by simulated cattle treading on the P leaching losses following repeated TE application to the soil with subsurface drains. Drainage experiment 2 found that repeated application of TE significantly reduced amounts of P leaching losses by 87.1 to 99.6% compared to the repeated application of FDE, both with or without soil compaction by cattle treading. These reductions were attributed to the less soluble P form in TE compared to FDE and that the less soluble P compounds were less prone to being leached. Soil compaction by simulated cattle treading significantly reduced P leaching losses only with the repeated FDE application but did not affect the P loss from TE applied soil. The compaction induced reduction in P leaching loss from the FDE was attributed to a reduction in macroporosity and preferential flow through the soil. The objective of the soil and pasture study was to determine the soil P fractions, pasture biomass, and plant P uptake with repeated application of TE compared to repeated application of FDE. This study found that the repeated application of TE resulted in no significant differences in most soil properties, soil P fractionations, pasture biomass and plant P uptake, except significantly increasing concentrations of soil labile P compared with repeated application of FDE. In summary, this research has improved knowledge and understanding of the benefits of applying treated effluent in reducing P and E. coli leaching losses through the subsurface drains, both fresh or stored TE, and with or without soil compaction by animal treading. The application of TE can, therefore, be used as an effective tool to help protect surface water quality without creating adverse impacts on soil P fractionations, pasture productivity or quality and repeat application of TE has the potential to improve soil fertility.