Diurnal Variation in Soil Nitrous oxide Emissions (DIVINE): drivers and mechanisms

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) with a global warming potential 298 times that of carbon dioxide. Agriculture contributes approximately two thirds of anthropogenic N2O emissions and is the most challenging sector for emissions reductions. Agriculture is also the largest source of uncertainty for national GHG reporting because N2O emissions are highly variable in space and time and consequently difficult to measure accurately.

To achieve the enormous reductions urgently needed, we must be able to accurately measure and predict soil N2O emissions. Our work has demonstrated that N2O emissions can vary diurnally by up to 400% in agricultural soils. This diurnal variation is not captured in current measurements and predictions of N2O emissions. In DIVINE we will investigate for the first time, the mechanistic basis of diurnal variation in N2O emissions and use modelling approaches to quantify how this diurnal variation contributes to uncertainty in N2O emission factors used for GHG inventory reporting. Our aim is to enable more accurate GHG inventory reporting and more effective testing of mitigation strategies for agricultural N2O emissions.

Our work has demonstrated that N2O emissions can vary diurnally in fertilised soils with emissions peaking in mid-afternoon. Soil N2O emissions are conventionally measured at a single time of day, leading to significant errors when measurements are scaled up to annual emission estimates. Whilst the existence of diurnal N2O variation is generally accepted, the underlying mechanisms are poorly understood. Greater daytime N2O emissions are typically attributed to increased microbial activity with increasing temperature. However, our recent research suggests light intensity is also important. We hypothesise that this is not a direct effect of light intensity, but rather differences in plant metabolism (photosynthesis and root exudation) between day and night altering soil properties (e.g. carbon availability and oxygen) and so promoting diurnal variation in N2O. It is only recently that we have been able to make these conclusions due to major technological advances that allow us to measure N2O emissions near-continuously from multiple locations over day-night cycles. Supported by this technology we will investigate the importance, drivers and mechanisms of diurnal N2O variation through four work packages. We will:
1. Conduct field experiments over whole crop life-cycles to quantify how diurnal N2O emissions vary with management practices and soil physical properties (WP1);
2. Construct outdoor mesocosms with soil temperature control and shading treatments to determine to what extent temperature and light intensity drive diurnal variations in N2O emissions (WP2).
3. Conduct laboratory experiments to identify the mechanisms by which plant metabolism affects soil nitrogen cycling and amplifies daytime N2O emissions (WP3)
4. Quantify how diurnal variation in N2O emissions affects the accuracy of national GHG inventory reporting for agriculture and use Bayesian modelling to improve the accuracy of N2O emission factors by accounting for diurnal variability (WP4).

The outcomes of the project will enable diurnal N2O variation to be factored into tools and models for reporting and predicting agricultural GHG emissions and will enable more accurate testing of agricultural N2O mitigation strategies.

Mitigating climate change is the most difficult and important challenge faced by society, with agriculture the most challenging sector for emissions reductions. The 'DIVINE' project will deliver new scientific knowledge to underpin mitigation of agricultural nitrous oxide (N2O) emissions and we have an ambitious impact plan to ensure this knowledge reaches decision makers and influences mitigation efforts.

Our research will deliver novel mechanistic understanding of diurnal variation in N2O emissions. We will first demonstrate its importance across growing seasons and show how climate and land management interact to control the magnitude of diurnal N2O variation. We will then quantify how diurnal variation affects the accuracy of national greenhouse has (GHG) inventory reporting for agriculture and develop a methodology for improved emission factors to account for diurnal variation.

Through our impact activities we will:

1. Contribute data and knowledge to inform next generation ecosystem-scale biogeochemical models, used internationally to simulate carbon and nitrogen cycling to predict climate change feedbacks
2. Inform the development of novel mitigation strategies through knowledge of the influence plant roots and root exudates have on N2O emissions
3. Demonstrate improved measurement methodologies to enable improved testing and validation of N2O mitigation strategies through engagement with agri-business.
4. Propose improved emission factors used for national GHG inventory reporting through engagement with policymakers and key organisations in UK GHG inventory reporting
5. Raise awareness of the importance of N2O and its variability in the agricultural community and food supply chains
6. Engage the public through educational and knowledge exchange activities to increase awareness of soil N2O emissions from food production, its impacts on climate change and opportunities for mitigation.

Who will benefit and how?
Researchers - The research will advance scientific knowledge for the global research community challenged to address the mitigation of agricultural greenhouse gas emissions.
Industry - Engagement with UK and globally relevant business (e.g. UK farming sector) to share and evolve understanding and co-design plans for development of climate-smart agriculture.
Policymakers - Our scientific evidence will support further improvements to predictions and reporting of national GHG emissions from agriculture.
Public - Raising awareness of the importance of soils, agriculture and climate action in the general public including schools, will support behaviour change.

To achieve these goals we will exploit stakeholder networks of the consortium to engage relevant stakeholders and potential beneficiaries including:
- Policymakers: UK government departments, agencies and committees (DEFRA, BEIS, CCC, Natural England) and devolved administrations. International policymakers: IPCC, UNFCCC.
- Industry and representative associations e.g. Arla Foods, AGRII, National Farmers Union, Comet-Farm Team, ADAS.
- Stakeholder networks involved in developing soil-relevant policy and agricultural industry action: Sustainable Soils Alliance, Agriculture and Horticulture Development Board (AHDB), Global Alliance for Climate-Smart Agriculture
- Third sector organisations: Campaign for the Farmed Environment, LEAF Linking Environment and Farming.
- Project partners (see LoS) will be important in achieving impact from this project, in particular the Cotrufo Lab at Colorado State University who have committed to collaborating with the DIVINE team to integrate our data and knowledge into the next iteration of the MEMS model of soil organic matter dynamics.