Winery wastewater (WW) is a high-volume biowaste that requires effective disposal systems that minimise environmental impacts and potentially capture value. In New Zealand, approximately 380,000 m3 of WW is generated annually, with the Marlborough region accounting for around 280,000 m3 yr-1. There is growing recognition of the need to improve current WW disposal practices. Land application of WW is a low-cost disposal method that could significantly reduce the environmental risks associated with direct discharge into surface and groundwater bodies. This thesis aimed to determine the likely short and long-term effects of irrigating WW to land at rates where the water and nutrients may be beneficial to soil and plants, with a focus on carbon (C) retention. The specific objectives were to: 1) Quantify how manipulation of WW and receiving soils may improve soil quality and C retention, 2) Determine if organic C (OC) in WW can increase soil C via direct sorption and under what conditions, 3) Quantify the fate of OC in terms of soil organic matter changes and losses via mineralization and leaching, and 4) Elucidate the C sequestration potential via vegetation growth in WW irrigation plots.

A field survey was conducted at three Marlborough vineyards to quantify loading rates and accumulation of WW nutrients in soil. The WW irrigation added ca. 1.8 t ha−1 yr−1 of Na and 4 t ha−1 yr−1 of total OC. WW application increased soil pH, phosphorus, and Na across sites compared to controls. However, soil C decreased under WW irrigation (mean 1.7%) relative to controls (2.3%), equivalent to a loss of ~100 t C ha−1 over 600 mm depth, despite WW adding 4 t C ha−1 yr−1. Findings recommend using K-based over Na-based cleaning chemicals to reduce sodicity. Focusing on C retention and sustainable vegetation growth could increase soil C stocks to offset emissions.

Equilibrium batch experiments aimed to quantify the adsorption capacity of an Anthrosol, representative of common soil types in New Zealand's wine regions, for OC in WW. The soil sorbed up to 1729-4165 mg C kg soil-1 from WW. Increasing WW pH from 5 to 7 led to increased organic C sorption. The use of Na-based cleaners and pH adjustment may increase OC sorption compared to K-based products, but the harmful effects of exchangeable Na on soil structure and plant health may negate potential benefits.

A glasshouse mesocosm experiment investigated changes in ecosystem C balance when WW was applied to Lolium perenne (perennial ryegrass). Irrigating WW resulted in the mesocosms being a net C sink, retaining approximately 327 g C m-2 compared to 84 g C m-2 for controls over 118 days. However, gains were mostly due to high C input from WW, with small leaching losses. Signs of necrosis were observed in WW-treated Lolium perenne, corroborated by decreased above-ground biomass production. Soil respiration could mineralise up to 40% of added C from WW during peak biomass production.

The impact of WW irrigation on nutrient dynamics and biomass production in four New Zealand native plant species (Phormium tenax, Coprosma robusta, Cordyline australis, Kunzea robusta) and Lolium perenne was investigated in a glasshouse pot trial. WW irrigation significantly increased soil pH, electrical conductivity, Na, and ammonium, and decreased nitrate. No significant differences in above-ground biomass were found for native species, but Kunzea robusta exhibited necrosis. Lolium perenne biomass significantly decreased under WW treatment. There were significant increases in Na in native plant foliage, with Kunzea robusta having the highest concentration. WW application resulted in increased leachate total C, Na, Mg, Ca, and electrical conductivity.

This thesis demonstrates that land application of WW can lead to accumulation of nutrients, particularly Na, in soil and plants. While OC in WW represents a potential resource for improving soil quality and C sequestration, the fate of this C requires further investigation as significant losses were identified in long-term WW application sites. The adsorption capacity of the studied soil for WW-derived organic C indicates loading limits to prevent leaching and infiltration issues. WW application to Lolium perenne resulted in a net C sink, but with reduced plant growth, indicating the need for appropriate application strategies. Native plant species exhibited differential responses to WW irrigation, with some showing potential for phytoremediation and C sequestration.

Future research should focus on developing regional WW application guidelines based on soil properties, assessing the long-term stability of adsorbed OC, investigating Na toxicity thresholds and phytoremediation potential of native species, and optimising C retention and biomass production in WW irrigation systems. Improving WW disposal practices through effective nutrient assimilation and promoting sustainable vegetation growth could contribute to the environmental and economic sustainability of New Zealand's expanding wine industry.