Improving Nitrogen Use Efficiency: from Planet to Dairy Paddock

Introduction At a global scale, human activity has increased the flux of N two-fold (Vitousek et al. 1997), particularly driven by large scale fertiliser manufacturing (Fowler et al. 2013). Additionally, the ability to transport inputs and outputs cheaply and extensively has led to substantial growth in agricultural production over the past 50 years with an accompanying 40% increase in world population and extensive urbanisation. However, this has also lead to a spatial disconnection between nitrogen flows required for agricultural production systems and reduced incentives to capture and recycle nitrogen at the farm scale. Moreover, agricultural production systems are inherently in efficiency at capturing nitrogen, with excess nitrogen dissipated into the broader environment. Of the total N applied to agricultural land worldwide only 5–15% is eventually transformed into human food (Erisman et al., 2012). In cropping systems nitrogen use efficiency (NUE) will often range between 35 – 65%, while in more intensive animal systems such as dairy production, NUE will typically range from 15 – 35% (Powell et al. 2010). Major pathways of agricultural N loss to the environment are gaseous emission of ammonia and nitrous oxide, and the leaching of nitrate through soil, with various transformations causing a cascade of potential environmental problems (Galloway et al. 2008). In the past decade, measuring losses for nitrous oxide and the effectiveness of mitigation strategies have received considerable attention due to a policy focus on greenhouse gas emissions. In contrast, grazing based dairy farms in Australia and New Zealand have been encouraged to increase production through greater reliance on imported feed and fertiliser (Thorold and Doyle 2007), with likely greater nitrogen losses per ha. Growing societal expectations for air and water quality, stricter standards from international markets, and increasing costs for purchased nitrogen will mean that improving NUE and reducing nutrient losses will be a necessary part of agricultural production systems. This is likely to require difficult choices to better balance production and environmental goals, particularly for intensive livestock industries such as dairy production.