Role of soil heterogeneity and hydrological variability in atmospheric hydrogen uptake

The soil sink of atmospheric hydrogen is the biggest uncertainty related to the climatic impacts of H₂ emissions and the projection of hydrogen-based energy scenarios. In this context, a crucial role is played by H₂-oxidising bacteria spread basically in every soil and accounting for about 80% of the atmospheric hydrogen removal. 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 the present work, we extend the dimension of these previous models, taking into account the horizontal spatial diffusion of both soil moisture and hydrogen. This addition opens the way to investigate the key effects of soil spatial heterogeneities and the related occurrence of spatial patterns in soil hydrogen uptake dynamics. Early results show a clear dependence of the H₂ atmospheric flux on the presence of the horizontal diffusion terms, with a different behaviour according to the hydroclimatic conditions chosen. Furthermore, spatio-temporal averages computed neglecting the complete coupled dynamics of soil moisture and hydrogen are found to lead to significant errors due to the non-linearities encoded in the model. This latter result is enhanced in climates with poor rainfall events (semi-arid ecosystems), while it is quite negligible in wet cases, where the non-linearities are smoothed down.
The findings of this study improve the understandings of soil hydrogen dynamics and underscore possible biases associated with coarse-resolution global modelling.