Organic matter mitigates biotic impact of copper in fruit orchard soil

Inorganic copper (Cu) fungicides and bactericides are widely used to control disease in fruit and vegetable crops and has led to widespread accumulation of the metal in soil beyond regulatory thresholds. We aimed to elucidate the impacts of Cu on soil health within cherry orchard soils in New Zealand, focusing on three biological indicators: earthworm behaviour, soil respiration, and plant growth. We sampled soils from four blocks of different ages within a single orchard, varying in amounts of accumulated soil Cu (7-263 mg kg^-1) but also in Soil Organic Matter (SOM) content (3-10 %). Experimental work was designed to isolate the impacts of both Cu and SOM on three critical biological descriptors: earthworm behaviour, soil respiration and root growth. Soils were amended to standardise both variables in laboratory and glasshouse experiments. The results demonstrated a pronounced inhibition of soil respiration and root development, as well as adverse effects on earthworm behaviour, with increasing Cu concentrations. SOM played a mitigating role, reducing the bioavailability and toxicity of Cu to soil organisms. However, the buffering capacity of SOM is limited and long-term reliance on SOM to mitigate Cu toxicity is not sustainable. Currently Cu continues to accumulate in most orchard soils. This study highlights the importance of assessing Cu bioavailability and soil health in the context of orchard management.