Copper contamination of fruit orchards soils: Biotic Impacts : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Extensive use of fungicide copper (Cu) has a more recent history in New Zealand than in many other parts of the world where long-lasting Cu accumulation in soil has become a major environmental issue. However, Cu is extensively applied in New Zealand orchards, including organic orchards, with some awareness that the consequences of its current and future accumulation on soil health are relatively unknown and under-explored. This doctoral study aimed to investigate the impact on soil functional processes and plants of soil copper contamination associated with cherry, apple and kiwifruit orchards, vineyards and hops. The research encompassed experimental work on soil respiration, plant growth, earthworms, soil microbial activity, root growth and plant cell culture through a combination of fieldwork, glasshouse and laboratory studies. The central hypothesis of this study was that accumulation and persistence of Cu in orchard soils are likely to adversely affect critical aspects of soil biology and functionality. Following a detailed survey of accumulation and spatial variability of soil Cu across different fruit orchards up to 73 years old, practical investigations involved soil respirometry, analysis of microbial carbon (C) and nitrogen (N), and rhizobox and pot plant growth assays, as much as possible using in-situ field measurements or soils collected from orchards and transferred to the glasshouse and laboratory. In an earthworm behavioural and Cu-uptake study, a native anecic species was exposed to soils from the same orchard with differing histories of fungicide use. Three hop varieties (Cascade, Nelson Sauvin and Riwaka) were used for plant growth trials on the same soils. Plant stress responses were investigated using callus incubation trials on cell lines isolated from three apple cultivars (Braeburn, Fuji, and Cripps Pink) grown on a Cu-spiked growth medium. All practical work was carried out from 2020 to 2023. The results showed that soil Cu concentrations in orchards frequently and substantially exceeded most published threshold limits. Whilst soil Cu concentrations could largely be explained by modelling the age of the orchards, fruit type and soil organic matter (SOM) also had a large role in Cu retention. When SOM and existing Cu concentrations were amended in four soils from different blocks of the same cherry orchard, the ecotoxicological impact differed, and it was found that SOM could be a more powerful determinant than Cu of the biotic responses. Earthworm survivorship and growth in these soils were significantly determined by both SOM and Cu; earthworms exhibited a preference for soils with concentrations of Cu elevated substantially above background (to 160 mg kg-1), where SOM content was also high. A variable impact of Cu contamination on soil microbial activity was recorded across soils with elevated Cu concentrations (from 195 to 405 mg kg-1). Only a weak correlation was found between soil total Cu concentration and soil respiration when data for all orchards were combined, but the impact of Cu was more evident when each type of fruit orchard was evaluated separately. Microbial biomass carbon (MBC) and nitrogen (MBN) analyses similarly provided only a weak or negligible correlation with soil Cu, but artificially spiked soils provided a more consistent response to elevated Cu. Confounding factors appeared to relate to vegetation cover within and between rows and the amount of cultivation of the soils used to manage weeds (and bronze beetle in apples). The influence of management variables requires a more detailed study. Root growth in hop varieties was negatively affected at Cu concentrations exceeding 263 mg kg-1, whilst best growth was observed at 160 mg kg-1 in conjunction with abundant SOM. In callus culture assays, Cu negatively impacted the growth of Braeburn and Fuji apple varieties at concentrations exceeding 15 mg kg-1, while Cripps Pink had higher Cu tolerance. The value of using biological and ecological indices to assess the impacts of agricultural chemicals and contaminated soils is discussed. The findings have identified detrimental biotic impacts of soil Cu concentrations that already exist in orchards, which are probably reflected in a negative influence on soil health. Transfer rates of Cu to fruits through uptake from soil or from foliar absorption are negligible, but stress responses in plants and soil fauna and impacts on soil biology and ecology have been detected. Currently, the deleterious impact of elevated Cu is largely mitigated by SOM content in combination with the avoidance of low pH in orchard soils. Whilst this implies there is a potential avenue for amelioration of toxicity and maintenance or restoration of soil health, residual fungicide Cu will not significantly dissipate and is likely to continue to accumulate. Sustainable soil health management in New Zealand's orchards is not viable with longer-term continued usage of Cu fungicides.