Effect of urease and nitrification inhibitors on N transformation, gaseous emissions of ammonia and nitrous oxide, pasture yield and N uptake in grazed pasture system

Nitrogen (N) losses via nitrate (NO3−) leaching, ammonia (NH3) volatilization and nitrous oxide (N2O) emissions from grazed pastures in New Zealand are one of the major contributors to environmental degradation. The use of N inhibitors (urease and nitrification inhibitors) may have a role in mitigating these N losses. A one-year field experiment was conducted on a permanent dairy-grazed pasture site at Massey University, Palmerston North, New Zealand to quantify these N losses and to assess the effect of N inhibitors in reducing such losses during May 2005–2006. Cow urine at 600 kg N ha−1 rate with or without urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) or (trade name “Agrotain”) (3 L ha−1), nitrification inhibitor dicyandiamide (DCD) (7 kg ha−1) and the use of double inhibitor (DI) containing a combination of both Agrotain and DCD (3:7) were applied to field plots in autumn, spring and summer. Pasture production, NH3 and N2O fluxes, soil mineral N concentrations, microbial biomass C and N, and soil pH were measured following the application of treatments during each season. All measured parameters, except soil microbial biomass C and N, were influenced by the added inhibitors during the three seasons. Agrotain reduced NH3 emissions over urine alone by 29%, 93% and 31% in autumn, spring and summer respectively but had little effect on N2O emission. DCD reduced N2O emission over urine alone by 52%, 39% and 16% in autumn, spring and summer respectively but increased NH3 emission by 56%, 9% and 17% over urine alone during those three seasons. The double inhibitor reduced NH3 by 14%, 78% and 9% and N2O emissions by 37%, 67% and 28% over urine alone in autumn, spring and summer respectively. The double inhibitor also increased pasture dry matter by 10%, 11% and 8% and N uptake by the 17%, 28% and 10% over urine alone during autumn, spring and summer respectively. Changes in soil mineral N and pH suggested a delay in urine-N hydrolysis with Agrotain, and reduced nitrification with DCD. The combination of Agrotain and DCD was more effective in reducing both NH3 and N2O emissions, improving pasture production, controlling urea hydrolysis and retaining N in NH4+ form. These results suggest that the combination of both urease and nitrification inhibitors may have the most potential to reduce N losses if losses are associated with urine and improve pasture production in intensively grazed systems.