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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.

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) with a global warming potential 298 times that of carbon dioxide (CO2). Measurements of soil N2O emissions typically use manual chambers, with samples taken at low temporal resolution over long durations (months), or at higher temporal resolution (multiple samples per day) over short durations. Automated GHG flux systems have allowed the measurement of high frequency (sub-daily) N2O fluxes over longer periods (weeks to months), revealing that emissions can vary diurnally by up to 400% in agricultural soils.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). Improvements to agricultural GHG emission estimates have increased the accuracy of GHG reporting, but N2O emissions from agricultural soils still contribute 25% of the uncertainty of total GHG emissions across all sectors. Our project, diurnal variation in soil nitrous oxide emissions (DIVINE), combines field and laboratory experiments that exploit high-resolution, robotic and continuous N2O measurement technology, to investigate the drivers and mechanisms underpinning diurnal variation in N2O.We will present work from a field study investigating the effect of soil properties and nitrogen (N) fertiliser management on diurnal variation in N2O emissions from a wheat crop. We assess how N fertiliser application (rate and frequency) and soil gas diffusivity (determined by bulk density and rain events), affect the depth of N2O production and N2O transport in the soil, and resultant impacts on the peak timing and amplitude of diurnal N2O emissions across the crop life cycle and seasons.N2O emissions will be compared in paired transects with contrasting bulk density but similar soil texture and history, with three ammonium nitrate fertiliser scenarios. N2O is being measured continuously using SkyLine2D automated flux chamber technology. To resolve depth/gas transfer coefficients after N fertiliser and rain events, we will measure soil N2O concentration profiles across the rooting zone in discrete campaigns during the crop life cycle.Further, we will discuss how our data will be used to improve the accuracy of N2O emission factors by accounting for environmental and diurnal variation. Bayesian statistical modelling will be used to represent the spatial and temporal distribution of emissions following fertilisation, and the effects of known environmental factors (e.g. temperature, soil moisture, light intensity), as well as the residual effect explicable by the diurnal cycle.

Diurnal variations in soil nitrous oxide (N­2O) emissions have been documented for nearly four decades yet consensus on their significance is still lacking. Resolving this question is important as soil N2O emissions have some of the highest uncertainties in national greenhouse gas inventories. A major challenge for understanding diurnal variation is that conventional measurements rarely operate at temporal frequencies that can observe and report this phenomenon. Some higher frequency studies have observed daytime peaking of soil N­2O emissions and often ascribe it to the diurnal oscillation of soil temperature. However, night-time peaking and irregular diurnal N­2O patterns have also been reported in a number of studies.To investigate the prevalence and characteristics of diurnal N­2O variability, we systematically reviewed published studies that measured N2O at high temporal frequencies (≥ 5 times/day). We identified 46 published studies covering cropland, grassland and forest soils; and extracted sub-daily N­2O flux data and other soil parameters, yielding 286 individual days of data. Diurnal variability of N­2O emissions were found in ~80% of the data, with ~60% peaking during the day and ~20% at night. Diurnal N­2O patterns were associated with non-diurnal factors including soil texture and land use but the relationship between soil temperature and N­2O flux was inconsistent, with strong positive correlations (R > 0.7) only found in one-third of the datasets.This talk explores the implications of the review results on the time of sampling using conventional approaches (single time-point flux measurements), and the potential drivers of diurnal N­2O variations for future research. In addition, this talk will also introduce a novel automated measurement technique allowing flux measurements at high temporal resolutions and how its application could enable experimental investigations of potential drivers of diurnal N­2O variability.