The outcome of plant-microbial competition for N in a wheat system and the implications for yield and N<sub>2</sub>O mitigation 

<p>Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) with a global warming potential 265 times that of carbon dioxide (CO₂) over 100 years. Contributing approximately 70% of global anthropogenic N₂O emissions, agriculture represents the largest area of uncertainty for GHG reporting and the most challenging sector for emissions reduction: global N₂O emissions are increasing at double the rate estimated by the Intergovernmental Panel on Climate Change (IPCC). The largest source of agricultural N₂O emissions is from application of inorganic-N fertilisers, the manufacture of which produces more than 1% of global CO₂ emissions and consumes 1% of global energy output.</p> <p>However, typical crop N uptake efficiency (NupE) means approximately half the fertiliser doesn&#8217;t reach the target plant, causing further ecological problems, such as biodiversity loss from eutrophication and atmospheric deposition. The extent to which microbial immobilisation of fertiliser N contributes to the NupE value of ca. 60% is currently unknown.<strong> </strong>If N immobilisation is found to be a large contributor to reducing N available to crops, this offers new opportunities to better manage fertiliser N inputs. Critically, with a growing global population, it is vital that we can increase food crop yields, and more efficient use of water and nutrients could help close the 70% &#8216;yield gap&#8217; between potential and actual crop yields. Finally, inorganic N is the largest single cost in gross margins for wheat production and prices are rising. Increased NupE therefore represents a key opportunity for farmers to increase their financial sustainability.&#160;</p> <p>&#160;</p> <p>We hypothesised that under the conventional management of three applications of inorganic N in the spring, crops do not have the ability to outcompete the fast-growing soil microbial community for N, and that by supplying N to the crop in a &#8216;little and often&#8217; approach, we could increase NupE by reducing immobilisation, and consequentially reduce N₂O emissions. We conducted a field study of a winter wheat crop on a northern UK farm to investigate this, which compared conventional N fertiliser management (220 kg N ha^-1 over three applications) of ammonium nitrate, to a little and often approach (220 kg N ha^-1 over six applications) and an untreated (0 kg N ha^-1) control. We followed the crop until harvest, and continuously measured N₂O emissions and net ecosystem exchange of CO₂ using a skyline2D automated flux system and also measured C and N pools in soil, plants and microbial biomass to assess changes in N uptake and allocation.</p> <p>We will present data which shows the outcome of plant-microbe competition for N in our agricultural system, and discuss the implications of different N fertiliser management for yield, profitability and GHG mitigation.</p>