Inhibitors application time and pasture canopy capture regulate gaseous losses of urine-N

Abstract

AbstractTechnologies have been developed for the in-situ treatment of urine patches deposited by grazing livestock to mitigate nitrogen (N) losses using N transformation inhibitors. For this mitigation to be effective, close contact between the applied inhibitors and the N in the urine patch is required (similar to N-fertilisers coated with inhibitors). This research aimed to determine the proportions of urine-N that mixed with inhibitor at or exceeding the threshold concentration (inhibitor concentration at which the nitrification rate is reduced by at least 40%) when inhibitors were applied to simulated urine patches at 4, 24 and 48 h after synthetic urine application. Three commonly used nitrification inhibitors (NIs) [dicyandiamide (DCD), 3,4-dimethylpyrazole phosphate (DMPP), and 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin)] were applied at 40 mL of inhibitor per urine-patch at two different concentrations. The field studies were undertaken in two dairy-grazed pasture soils with contrasting drainage. Large proportions of applied NIs (38%–59% DCD, 27%–58% DMPP, and 31%–58% nitrapyrin) were retained in the pasture canopy. In most cases, the inhibitor threshold concentration was present only within the top 0–20 mm of the soil, with only 16%–40% of the urine-N present. In some cases, the proportions of urine-N intercepted was 12%–15% higher when inhibitors were applied 4 h after urine application compared to delayed application of 24 and 48 h after urine application. Our results revealed that a substantial proportion of N in the urine-patch remained out of the reach of the inhibitor solution. This is possibly due to the small volume (40 mL per 2 L urine patch, 1:50) of the inhibitors applied, with up to 59% of inhibitor solution retained in the pasture canopy. The time delays (4 to 48 h) between the urine deposition and the inhibitor application could have also contributed to this poor physical mixing between inhibitor and urine. Increasing the volume of water applied with the inhibitor and assessing the effect of rainfall/irrigation on increasing urine-N and inhibitor mixing warrants further consideration.