Bioavailable Cu can influence nitrification rate in New Zealand dairy farm soils

Purpose The ammonia monooxygenase (AMO) enzyme catalyses the first step of ammonia oxidation into hydroxylamine, and copper (Cu) is the main co-factor in its functioning. We hypothesise that bioavailable Cu in soil is a limiting factor for nitrification. An increase in bioavailable Cu will increase nitrification, while a reduction in bioavailable Cu, induced through the application of Cu complexing compounds, will reduce nitrification. Hence, this study aimed to characterise the relationship between bioavailable Cu concentration and nitrification rate in dairy grazed pasture systems. Methods An incubation study was undertaken using three contrasting soils (Pumice, Pallic and Recent soil) and high molecular weight organic acids (HMWOAs). The three soils were spiked with five levels of Cu (0.1, 0.3, 0.5, 1, and 3 mg kg −1 ) and incubated for 14 days before amendment with treatments. Four treatments were separately applied to each Cu level: urea only (300 mg N kg −1 ), dicyandiamide (DCD) (10 mg kg −1 ) + urea, calcium lignosulphonate (Ca-lig) (120 mg kg −1 ) + urea, and co-poly acrylic-maleic acid (PA-MA) (10 mg kg −1 ) + urea. After 4 and 8 days of incubation, the soil was extracted and analysed for bioavailable Cu and mineral N (nitrate, ammonia) to establish the effect of bioavailable Cu on nitrification rate. The dose response of nitrification rate to Cu was also calculated. Results The nitrification rates at day-8 significantly ( P < 0.05) increased by 35%, 22%, and 33% in the Pumice, Pallic, and Recent soils, respectively when the soil added Cu increased from 0.1 to 3 mg kg −1 . Application of Ca-lig and PA-MA significantly ( P < 0.05) decreased nitrification rate with the mean reduction being 59% and 56%, 32% and 26%, and 39% and 38% in the Pumice, Pallic, and Recent soil, respectively at day-8 relative to the urea-only treatment. Conclusion These results indicated that increasing bioavailable Cu significantly ( P < 0.05) increased nitrification rate, and suggested that the application of Cu complexing compounds to soil could potentially mitigate the positive effect of soil Cu on nitrification rate and underpin new advances in the development of nitrification inhibitors.