Effects of the different rates of urease and nitrification inhibitors on gaseous emissions of ammonia and nitrous oxide, nitrate leaching and pasture production from urine patches in an intensive grazed pasture system

Abstract

Minimizing nitrogen (N) losses via ammonia (NH 3) and nitrous oxide (N 2O) emissions into the atmosphere and nitrate (NO 3 −) leaching into surface and ground waters from intensively grazed pastures is essential for environmental protection worldwide. Applying urease inhibitor such as N-(n-butyl) thiophosphoric triamide (nBPT) or (Agrotain) and nitrification inhibitor dicyandiamide (DCD) to grazed pastures has the potential to mitigate such N losses. A lysimeter/mini plot experiment, using Paparua silt loam soil near Lincoln, Canterbury New Zealand, was conducted to quantify these N losses during May 2007 to July 2008. The nine treatments were: cow urine only applied at an equivalent rate of 600 kg N ha −1, urine + DCD at 5 kg ha −1, urine + DCD at 7 kg ha −1, urine + DCD at 10 kg ha −1, urine + double inhibitor (DI), i.e. both Agrotain and DCD applied at 1 L ha −1 and 7 kg ha −1, respectively (or 1:7 of v/w basis), urine + DI (1:10), urine + DI (2:7), urine + DI (2:10) and the control (no urine). These treatments were randomly applied to one set of lysimeters or mini plots in May as autumn and then to another set of lysimeters or mini plots in August as spring applications. Additional nine lysimeters received DCD only at rates equivalent to 5, 7 and 10 kg ha −1 in autumn to see if DCD has any effect on NO 3 − leaching and pasture production and N uptake from non-urine patches in autumn. Gaseous emissions of NH 3 and N 2O, NO 3 − leaching and pasture production and N uptake varied with the types and rates of the applied inhibitors during the two seasons. DCD applied at 7 and 10 kg ha −1 rates with urine was more effective than its lower rate of 5 kg ha −1 and reduced N 2O emissions by 37–53% (autumn) and 47% (spring), NO 3 − leaching losses by 57–55% (autumn) and 26–10% (spring) compared with urine alone. However DCD increased NH 3 emissions by 41% and 18% compared with urine alone treatment after autumn and spring, respectively. DCD applied at higher rates also increased pasture dry matter by 9% and 12% and N uptake by 12% and 6% after autumn and spring applications, respectively. However DCD applied at different rates without urine in autumn had no such effect on either NO 3 − leaching or pasture dry matter yield or N uptake. The DI at 1:7 ratio was more effective than the higher rates of DI and DCD in reducing losses of NH 3 (48% and 51%), N 2O (55% and 63%) and NO 3 − leaching (56% and 42%) as well as increasing pasture production (13% and 17%) and N uptake (7% and 18%) compared with urine alone treatment in autumn and spring, respectively. These results suggest that applying Agrotain + DCD at a ratio of 1:7 (v/w) may provide the best option for both mitigating N losses and improving pasture production in intensively grazed systems.