How application timings of urease and nitrification inhibitors affect N losses from urine patches in pastoral system

Abstract

In intensively dairy-grazed pastoral systems, urine patches are the major source of nitrogen (N) losses via gaseous emissions of ammonia (NH3) and nitrous oxide (N2O) and nitrate (NO3−) leaching. Minimizing these N losses can therefore enable substantial economic and environmental gains. However, the current practice like the blanket application of nitrification inhibitor (NI) such as dicyandiamide (DCD) in suspension form after grazing is not effective at reducing these N losses. The objective of this study was to identify the best time to apply a combination of urease (UI) and NI inhibitors to reduce these N losses from urine patches. A field experiment on Typic Haplustepts silt loam soil, near Lincoln, Canterbury, New Zealand was conducted. The treatments included: a control (no urine or inhibitor), urine alone at 600kgNha−1, and urine with either double inhibitor (DI) in solid form which consists of a mixture (1:7 ratio w/w) of UI (N-(n-butyl) thiophosphoric triamide (nBTPT-trade name Agrotain®) and DCD or DCD alone at 10kgha−1 in suspension form. The DI or DCD was applied to undisturbed lysimeters/field plots 10 and 5 days prior to, the same day, and 5 days after urine application in autumn (May 2008) and again to new lysimeters and field plots in spring (September 2008). Overall there were 10 treatments: control, urine alone and urine with DI or DCD applied at the time of urine application, 5 days before or after urine application and 10 days before urine application. After these treatment applications in 2 seasons, soil ammonium (NH4+) and NO3− concentrations, soil pH, gaseous emissions of NH3 and N2O, and NO3− leaching were monitored for different period of time and pasture growth and N uptake were measured over a year. The DI applied 5 days prior to urine application was more effective in reducing the 3N losses of NH3 volatilization, N2O emissions and NO3− leaching than its corresponding or DCD treatment applied 5 days after urine application. The DI applied 5 days prior to urine application significantly reduced soil NH4+ and NO3− production from applied urine and thus exhibited a minimal increase in soil pH compared with urine alone or with DCD treatments for 4–6 weeks during the two seasons. DCD consistently increased NH3 volatilization when applied 5 or 10 days prior to or concurrently with urine, however it decreased N2O emissions compared to urine alone in both seasons. Applying DI 5 days prior to urine application not only decreased N2O emissions as much as DCD did, it also significantly decreased NH3 volatilization by 38% in autumn and 28% in spring compared to urine alone. Applying DCD or DI in autumn was more effective than spring applications probably because of the lower soil temperature (<10°C) in autumn. Compared to urine alone, DI and DCD applied 5 days prior to urine application in autumn significantly reduced NO3− leaching by 58% and 43%, respectively since the leaching events occurred during the time when these inhibitors were effective (1–2 weeks for nBTPT and 4–6 weeks for DCD). Neither the DI nor DCD had any such significant effect on NO3− leaching losses after their spring application because the leaching events occurred 3–5 months after inhibitor application, which was beyond the time that these inhibitors could be effective. Pasture productivity was only significantly increased by the DI after autumn application, but no significant trend was observed after spring application. These results suggest that DI applied in solid form prior to grazing has the most potential to reduce the 3 key N losses in grazed pastoral system; and it therefore warrants further research to improve its longevity to control N losses for a longer period.