Topic B: The Enigma of the Soil Hydrogen Sink Variability [ELGAR]

At COP 26, countries agreed to reduce their carbon dioxide (CO2) and methane (CH4) emissions, with a focus on reducing fossil fuel use. This will leave an energy gap, which many countries plan to replace using hydrogen (H2) as an energy carrier. Hydrogen is a small molecule, and susceptible to leakage at all stages of delivery from production to the end-user. Inevitably, this will increase atmospheric H2 concentrations that have remained relatively stable for the last two decades. The primary removal mechanism for atmospheric H2 is via its diffusion into soils where it is consumed by microbes. This accounts for circa two-thirds of its removal. The other sink is through its atmospheric reaction with the hydroxyl radical, and increases in atmospheric H2 will extend the lifetimes of CH4 and ozone (O3). Both are important greenhouse gases, and tropospheric O3 is also an air pollutant that impacts human health and ecosystems. In the stratosphere, increased H2 concentrations can lead to increased water, leading to depletion of protective O3.

The dominant H2 soil sink is poorly constrained and it is not clear how it will respond to increasing atmospheric H2 in a changing climate, making predictions of future H2 atmospheric impacts uncertain. The enigma of the soil H2 sink strength needs to be investigated for atmospheric modellers to develop robust forecasts of the impact of future H2 levels. To address this knowledge gap we created a team of atmospheric scientists, biogeochemists and biogeochemical modellers.

Project ELGAR will study controls and variations of soil H2 uptake rates and develop numerical algorithms for implementation into global models. Soil H2 uptake is a passive diffusion process, hence, porous soils are stronger sinks than compacted or waterlogged soil, with low diffusion rates. Many soil microbes utilise H2 as an energy source. H2 uptake rates are controlled by i.e. soil temperature, pH and carbon. Building on this knowledge, we will quantify soil H2 sink rates from a range of soil in response to soil parameters, climates, and vegetation cover: (i) Laboratory manipulations using soils from the UK (8 sites), and the tropics (min. 2 sites) will provide data on the response to soil moisture, temperature, H2 concentrations, pH, and fluxes of CO2, CH4, N2O, required for the models. (ii) We will deliver 1-year real-world observations of spatial and temporal soil H2 uptake: (a) Static chambers inform on within-field spatial and temporal variability and effects of land management. (b) Direct H2 flux measurements by the aerodynamic flux gradient method will study the relationship between H2 uptake and meteorology, and in-soil H2 concentrations and fluxes of CO2, CH4, N2O, CO at UKCEH's Easter Bush monitoring site, and (c) indirect flux measurements, derived from atmospheric H2 decay in conjunction with measurements of ozone deposition and radon accumulation, at a second, drier site.

ELGAR will develop a soil model of H2 uptake, drawing on recently published H2 modelling work. The model will run at the site, national and global scale, and be suitable to link to atmospheric chemistry and transport models. It will be constructed on well-established soil organic matter modelling approaches and use ELGAR measurement data to derive response functions and constrain model parameters. Simulations run at the global scale will investigate the impacts of soil properties, climate and vegetation types on H2 uptake and release. ELGAR will collaborate with atmospheric modellers, including those funded under Topics A and C under this call to ensure the new process understanding feeds into improved atmospheric predictions during and beyond the project lifetime. Data will be stored at a NERC data centre and we will educate the public on the importance of soils as a sink for atmospheric H2 and engage with policymakers and farmers regarding the importance of minimising soil compaction and maintaining field drains in the H2 economy.