Abstract
Intensification of grasslands is necessary to meet the increasing demand of livestock products. The application of nitrogen (N) on grasslands affects the N balance therefore the nitrogen use efficiency (NUE). Emissions of nitrous oxide (N2O) are produced due to N fertilisation and low NUE. These emissions depend on the type and rates of N applied. In this study we have compiled data from 5 UK N fertilised grassland sites (Crichton, Drayton, North Wyke, Hillsborough and Pwllpeiran) covering a range of soil types and climates. The experiments evaluated the effect of increasing rates of inorganic N fertiliser provided as ammonium nitrate (AN) or calcium ammonium nitrate (CAN). The following fertiliser strategies were also explored for a rate of 320 kg N ha−1: using the nitrification inhibitor dicyandiamide (DCD), changing to urea as an N source and splitting fertiliser applications. We measured N2O emissions for a full year in each experiment, as well as soil mineral N, climate data, pasture yield and N offtake. N2O emissions were greater at Crichton and North Wyke whereas Drayton, Hillsborough and Pwllpeiran had the smallest emissions. The resulting average emission factor (EF) of 1.12% total N applied showed a range of values for all the sites between 0.6 and 2.08%. NUE depended on the site and for an application rate of 320 kg N ha−1, N surplus was on average higher than 80 kg N ha−1, which is proposed as a maximum by the EU Nitrogen Expert Panel. N2O emissions tended to be lower when urea was applied instead of AN or CAN, and were particularly reduced when using urea with DCD. Finally, correlations between the factors studied showed that total N input was related to Nofftake and Nexcess; while cumulative emissions and EF were related to yield scaled emissions.
Highlights
During recent decades, the demand for global food has increased rapidly as a consequence of population growth and changes in patterns of food consumption
N2O emissions were greater at Crichton and North Wyke throughout the different application rates and fertiliser strategies
The results from this study placed in the context of the EU targets for N use efficiency (NUE) showed that the values might have to be slightly adjusted for the UK, as for low N application rates the N offtake is higher than the 90% target value for all sites
Summary
The demand for global food has increased rapidly as a consequence of population growth and changes in patterns of food consumption. One of the most relevant changes in the global agro-food system has been the intensification of production systems and the increase of nitrogen (N) use and trades (Lassaletta et al, 2016). It is expected that further intensification will occur to fulfil increasing global demand for livestock products, putting pressure on farming activities that will likely result in increased N use. N fertilisation of grasslands has relevant productive and environmental effects. Fertiliser rates exceeding crop requirements lead to an N surplus, reduced N use efficiency (NUE) and losses to the environment (Van Eerd et al, 2018). An improved NUE is required in intensively managed grasslands to reduce the negative effects of an N surplus while preserving productivity and soil fertility
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