Abstract/Summary

Mineral nitrogen (N) fertiliser inputs are essential for achieving high crop yields in agricultural production systems and can help to drive farm profitability. However, when N fertiliser is applied to soil it can be lost to the environment and have negative consequences. Such losses include nitrous oxide (N2O) emissions, ammonia (NH3) emissions and nitrate (NO3-) leaching to waterways. This loss of N fertiliser also represents a substantial economic loss of N from the farm. The dominant N fertiliser source used on arable farms in Ireland is calcium ammonium nitrate (CAN) which, in environmental terms, principally contributes to N2O emissions and NO3- leaching. Switching from CAN to urea has the potential to reduce these N loss pathways, but can result in substantially higher NH3 emissions. Nitrogen stabilisers are compounds that can be added to N fertilisers to reduce these N losses. There are two main types of N stabilisers currently available: urease inhibitors and nitrification inhibitors. Urease inhibitors are used to regulate urea fertiliser hydrolysis and to reduce NH3 emissions and nitrification inhibitors are used to regulate the soil NO3- pool and to reduce N2O emissions and NO3- leaching. The urease inhibitor used in this study was N-(n-butyl) thiophosphoric triamide (NBPT) and the nitrification inhibitor used was dicyandiamide (DCD). An additional N fertiliser formulation, urea + the maleic – itaconic co-polymer (MICO), was added in 2014. Two field sites cropped with spring barley were established in 2013 and the overall study was conducted for three years. Nitrous oxide emissions, NH3 emissions, NO3- leaching and grain yield and N uptake were measured. The N fertilisers evaluated were CAN, urea, urea + NBPT, urea + DCD and urea + NBPT + DCD and Urea + MICO in 2014. Results showed that N2O emissions were low (over 50% lower than the IPCC default emission factor of 1%) regardless of the N fertilisers used but using the N stabilisers NBPT and DCD reduced emissions by up to 62%. There was no significant effect (P>0.05) of fertiliser formulation on NO3- leaching but there was a significant effect (P<0.05) on NH3 emissions with urea + NBPT reducing emissions compared to urea. There was no significant effect (P>0.05) of fertiliser formulation on spring barley grain yield but there was significantly lower N uptake with urea compared to CAN. Using urea + NBPT had similar N uptake levels to CAN. Overall this study showed that switching N fertiliser source from CAN to urea stabilised with the urease inhibitor NBPT can reduce environmental N losses and increase fertiliser N use efficiency (fNUE). This provides farmers with options to increase the environmental and economic sustainability of their arable farming systems while maintaining crop yields and quality.