The Potential Inhibitory Effect of Copper and Nickel on Gaseous Soil N Emissions

Abstract

Nickel (Ni) and copper (Cu) are essential soil micronutrients that are beneficial for plant growth and development when applied at the right levels to deficient soils. However, they can also be of concern if excessive application leads to accumulation in soil, causing soil fertility problems that can compromise food production as well as result in human exposure to these metals if they enter the food chain. The application of Ni and Cu salts can strongly influence the nitrogen (N) cycle as they can stimulate soil microbiological activity that leads to N losses to the atmosphere. A laboratory incubation was conducted to study the effect of these metal salts on gaseous N emissions. Copper sulphate (CuSO4) and nickel sulphate (NiSO4) were applied at rates below soil toxicity (3.0 kg Ni/ha and 7.5 kg Cu/ha), together with 15N-labelled-urea (rate 100 kg N/ha) and under two soil water-filled pore space (WFPS) conditions: continuous anaerobic (75% WFPS) versus aerobic (50% WFPS) followed by an anaerobic (75% WFPS) cycle. Results showed no significant effect of the Ni application on nitric oxide (NO) emissions at the low rates applied. However, there was an indication of a meaningful (p = 0.051) production of this gas under aerobic soil conditions. No N2 emissions were observed from any treatment (only expected to occur under anaerobic conditions) during the 43 days of the incubation, which could suggest that, as for Ni, the selected Cu application rate may not clearly stimulate the complete denitrification process to produce N2. It is possible that the expected increase in N2 emissions or larger NO emissions were counteracted by the low amount of soil-extractable Cu and Ni available after application because of binding processes with the clay fraction (48% clay) and the acid nature of the Cambisol used in the incubation. Further research into the effect of metal applications to contrasting soil types, with varied application rates and repeated application events, and under differing soil moisture conditions on N emissions is required. Furthermore, experiments should also be conducted at finer scales to elucidate the specific effect of these metal application rates on the biochemical processes responsible for the N gases produced.