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

Abstract

Nitrogen (N) losses via nitrate (NO 3 −) leaching, ammonia (NH 3) volatilization and nitrous oxide (N 2O) 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, NH 3 and N 2O 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 NH 3 emissions over urine alone by 29%, 93% and 31% in autumn, spring and summer respectively but had little effect on N 2O emission. DCD reduced N 2O emission over urine alone by 52%, 39% and 16% in autumn, spring and summer respectively but increased NH 3 emission by 56%, 9% and 17% over urine alone during those three seasons. The double inhibitor reduced NH 3 by 14%, 78% and 9% and N 2O 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 NH 3 and N 2O emissions, improving pasture production, controlling urea hydrolysis and retaining N in NH 4 + 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.