Water-Induced Stochastic Dynamics of Hydrogen in Soils

Due to the growing interest in hydrogen (H₂) energy for climate change mitigation, hydrogen losses to the atmosphere are expected to increase the H₂ atmospheric concentration over its natural value. The problem is that, for its capacity to interact with atmospheric gases such as methane, ozone and stratospheric water vapor, hydrogen is an indirect greenhouse gas. Hence, a challenging question for the scientific community is: will the use of hydrogen-based energy be a real step toward the solution of climate change or will it effectively add to this already fundamental global issue? To answer this question, a critical piece of the puzzle has to be considered: bacteria in soils. In fact, soil bacteria account for about 80% of the atmospheric hydrogen removal and therefore represent the main global sink of H₂ . Many studies on soils and bacterial activity have been performed and multiple factors, both biotic and abiotic, have been found to influence the hydrogen uptake. Above all, soil moisture and, in particular, its temporal fluctuations have been shown to be the dominant control, conditioning both bacterial activity and hydrogen diffusion in the soil. In this thesis, we try to extend a previous depth-averaged model for moisture and H₂ (Bertagni et al. 2021, Global Biogeochem. Cycles) to the horizontal direction, in order to investigate how spatial heterogeneities and patterns influence hydrogen uptake. A 1D horizontal model is examined, especially to understand the role of horizontal diffusion in this framework and how the various terms governing the hydrogen dynamics in soils are conditioned by it. We show that, while Turing-like instabilities cannot occur in this particular system, spatial heterogeneities and diffusion processes interact to produce complex H₂ uptake patterns. In future works, an extension to 2D models possibly accounting for complex topographies is suggested as a further step, in order to better highlight critical bio-geophysical processes neglected in current formulations. Understanding how hydrogen uptake varies across ecosystems and what types of spatial patterns can arise depending on soil moisture is of crucial importance to comprehend the effects of a possible H₂ energy-based industry in the context of climate change, and this work is a critical step forward in this direction.