The outcome of plant-microbial competition for N in a wheat system and the implications for yield and N₂O mitigation

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.

However, typical crop N uptake efficiency (NupE) means approximately half the fertiliser doesn’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. 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% ‘yield gap’ 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. 

 

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 ‘little and often’ 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.

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.