Impacts of copper contamination on nitrogen cycling in orchard and vineyard soils: Laboratory and field validation: A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

The long-term application of copper (Cu)-based fungicides in orchard and vineyard operations has resulted in widespread accumulation of Cu in soils, raising concerns regarding potential ecological risks. While these fungicides have been considered an essential tool for controlling fungal diseases, at excess concentrations Cu may adversely affect soil microbial communities, particularly those involved in nitrogen (N) cycling processes. This study aimed to assess the effects of Cu contamination on nitrification activity and nitrifying microbial abundance under both controlled and field conditions, focusing on soils from orchards and vineyards across Central Otago, Marlborough, and Nelson, New Zealand. First, laboratory incubations were conducted using soils spiked with Cu to establish concentration-response relationships for nitrification rates. A subsequent field survey of historically Cu-contaminated soils was undertaken to validate these findings under long-term exposure conditions. The incubation results showed significant inhibition of gross nitrification rates and declines in ammonia-oxidiser abundance at elevated Cu concentrations, with estimated effect thresholds for partial inhibition varying between soils. However, field survey soils exhibited considerable microbial resilience, with nitrification processes largely maintained despite elevated total and bioavailable Cu levels. Multivariate analyses indicated that soil properties such as pH, cation exchange capacity, and organic carbon content were stronger predictors of nitrification activity than Cu concentrations alone. This suggests that long-term microbial adaptation and soil chemical buffering may play importat roles in mitigating Cu toxicity under field conditions. These findings support the relevance of long-standing contamination guidelines and highlight the importance of considering soil-specific properties when evaluating ecological risks of Cu in agricultural landscapes. Future management strategies should emphasise maintaining optimal soil conditions, such as stable pH and organic matter content, alongside regular monitoring of Cu accumulation.