Significance of inhibitor volume in on-farm mitigation of nitrous oxide emission from dairy cattle urine patches

Technologies are being developed for the targeted mitigation of nitrogen (N) losses from livestock urine patches using urease and nitrification inhibitors (UIs and Nls). In our earlier study, we identified a major limitation for inhibitor efficiency, specifically, the application of a 40 mL volume of inhibitor solution to a 2L of urine patch (i.e., 1:50, based on New Zealand recommended dicyandiamide [DCD] application rate of 10 kg DCD dissolved in 800 L water ha^-1).This ongoing research evaluates the effect of inhibitor treatments by varying the inhibitor: urine volume ratio from 1:50 to 1:10 (200 mL of inhibitor to the 2L of urine patch) on nitrous oxide (N₂O) mitigation of five nitrification inhibitors: DCD, 3,4-dimethylpyrazole phosphate (DMPP), 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin), and two confidential compounds (named A and C, provided by AgResearch). These inhibitors were applied 24 hours after creating 2L simulated urine patches (within 0.5 m² chambers) in two dairy-grazed pasture soils with contrasting drainage (poorly vs well drained). Results showed that the N₂O emissions reduction efficiency from urine patches was the highest (35.8%–46.7%) with DCD followed by inhibitor C (26.9%–27.9%). The reductions in emission from the other inhibitors were not significant (11.0%–23.0% with DMPP and nitrapyrin, respectively; and 1.5%–15.6% with inhibitor A). In this study, diluting the inhibitor solutions resulted in retention of only 3% to 18% of the NIs by the pasture canopy compared with up to 59% (with 1:50) in our previous study. This dilution increases the amount of inhibitor reaching the soil, offering a potential option for effectively reducing N₂O emissions from cattle urine patches. However, dilution may result in concentrations below threshold levels of DMPP, nitrapyrin and inhibitor A, compromising their effectiveness. These results warrant further research to optimise inhibitor application rate and volume and measure inhibitor residues for developing best practice for targeted application of inhibitors to urine patches while addressing unintended food and human health risks.