Advancing phosphorus use efficiency: Fate and relative bioavailability of struvite fertiliser in New Zealand grasslands soils : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Continued phosphorus (P) inputs, mainly in the form of water-soluble mineral fertilisers derived from phosphate rock, are required to increase and maintain agroecosystem productivity. However, P input utilisation efficiency is commonly very low (10-25%), which results in the accumulation of significant quantities of P in soil over time (legacy P), which in turn increases the risk of environmental degradation of waterways due to enhanced diffuse P transfer. One option to improve overall P use efficiency is to partially or fully replace phosphate rock-derived fertilisers with P recovered from waste, including precipitation of struvite (NH4MgPO4·6H2O) from municipal and industrial wastewater. The main objective of the research described in this thesis was to investigate and quantify the reactions and bioavailability of sparingly soluble struvite-P in different grassland soils compared with water-soluble monocalcium phosphate [Ca(H2PO4)2] in single superphosphate (SSP). The first experiment examined Olsen-P dynamics following struvite application in three acidic soils differing in P retention capacities, under a plant-less incubation. Across all soils, struvite significantly increased Olsen-P concentrations, comparable to conventional P fertilisers (SSP and DAP). Moreover, P release was influenced by soil retention capacity, with the Pallic soil (lowest retention) showing the highest and rapid Olsen-P increases compared with the Allophanic soil (highest retention). In the second experiment, the agronomic performance of struvite was evaluated under greenhouse conditions, with perennial ryegrass growing in soils with low and high P retention. Despite its low water-solubility, struvite achieved similar or higher Olsen-P concentrations than SSP, suggesting a potential carryover effect able to support long-term P availability. Ryegrass yields were comparable between struvite and SSP, with the more pronounced response in the low P retention soil. On average, struvite increased plant P uptake by 22% and was 49% more effective than SSP across soils and P rates, especially at low application rates and in the high P-retention soil. The third experiment evaluated the immediate and residual effects of struvite-P fertiliser in a perennial ryegrass pasture under field conditions. In Year 1, both P fertilisers significantly increased ryegrass yield by 49% compared with controls. However, P uptake was significantly higher for struvite (63.4 kg P ha⁻¹) compared with SSP (50.9 kg P ha⁻¹) at the high P rate (80 kg P ha-1). In Year 2, the 80 kg P ha⁻¹ struvite maintained significantly higher yield (23.5 Mg ha⁻¹) and P uptake (63.4 kg ha⁻¹) compared with SSP (17.4 Mg ha⁻¹ and 50.9 kg P ha⁻¹). Additionally, soil plant-available P was consistently higher for struvite compared with SSP, confirming the results from the second experiment. The findings of this experiment demonstrated that the immediate agronomic performance of struvite was equivalent to that of SSP, while the immediate and residual effectiveness of struvite at the higher P application rate was superior to that of SSP. The fourth experiment compared P leaching losses from struvite and SSP in soils with contrasting P retention capacities. In the low retention soil, SSP lost ~80% more than struvite. Phosphorus losses were also higher for SSP in the high P retention soil, though not significant, which was partly explained by preferential flow in the high P-retention sandy-loam soil compared with the low P-retention silt-loam soil. Collectively, these results demonstrated that wastewater-derived struvite can be an effective and sustainable P fertiliser for acidic New Zealand grassland soils, including those with high P retention capacity. Further research should investigate the mechanisms controlling struvite dissolution and phosphorus release in greater detail, as well as the processes governing struvite-derived phosphorus acquisition by plants.