At a Glance
Hydrogen (H2 ) is gaining international attention as an energy carrier with a low carbon footprint. But a more H2 -centered economy may also increase the amount of H2 in the atmosphere, causing indirect global warming effects (e.g., increasing methane lifetime). To sharpen future H2 projections, Porporato’s group is addressing the large uncertainties related to the H2 soil uptake by bacteria, which is currently estimated to be responsible for nearly 80% of the atmospheric H2 consumption.
Hydrogen may play an important role in the decarbonization of energy systems and provide a cost-effective option to displace fossil fuels in applications—such as heavy transport— where emission reductions are difficult. However, the largescale use of hydrogen as energy carrier may increase the H2 atmospheric concentration—currently around 530 ppb (Novelli et al., 1999)—and the environmental implications are not yet clear.
Hydrogen is not a greenhouse gas, but it has an indirect global warming potential, mainly due to the increased lifetime of methane and the cooling of the stratosphere (Paulot et al., 2021). Model estimates suggest that this global warming potential is small compared to that of fossil fuels. Nonetheless, there is still large uncertainty in the estimates due to the poorly constrained main sink of atmospheric hydrogen: the soil uptake by bacteria.
H2 soil uptake by bacteria accounts for nearly 80% of the tropospheric removal and is widespread in all ecosystems worldwide, including extreme environments (Ji et al., 2017). Complex interactions between biotic and abiotic processes govern this uptake. In particular, soil moisture plays the main role as it affects the rate at which H2 is consumed by bacteria (the bacteria metabolism) and the rate at which H2 becomes available for bacteria (H2 diffusion through the soil).
Research (e.g., Smith-Downey 2006) has shown that at very low levels of soil moisture, bacterial metabolism is inhibited because of water stress. At high values of soil moisture, the soil H2 uptake is limited by the reduced diffusivity of H2 through the moist soil. In between, there is an optimal moisture value that maximizes the H2 soil uptake (Figure 3.1a). Through a system that encompasses water and hydrogen dynamics in soil (Figure 3.1b), Porporato’s group has shown that addressing the dry-wet sequences is necessary to correctly characterize the H2 soil uptake in semi-arid regions (Bertagni et al., submitted). This highlights a challenge for global climate models that often rely on time-averaged moisture data. Results also show that H2 diffusion through the soil generally limits H2 uptake in humid temperate and tropical regions, while biotic limitations tend to occur in arid or cold regions (Figure 3.2).
Bertagni, M.B., F. Paulot, and A. Porporato. Submitted to Global Biogeochemical Cycles.
Ji, M., C. Greening, I. Vanwonterghem, C.R. Carere, R., Bay, S. K., Steen, J. A., Montgomery, K., Lines, T., Beardall, J., Van Dorst, J., et al., 2017. Atmospheric trace gases support primary production in Antarctic desert surface soil. Nature, 552(7685):400–403. (DOI:10.1038/nature25014).
Novelli, P. C., P.M. Lang, K.A. Masarie, D.F. Hurst, R. Myers, and J.W. Elkins, 1999. Molecular hydrogen in the troposphere: Global distribution and budget. Journal of Geophysical Research: Atmospheres, 104(D23):30427–30444.
Paulot, F., D. Paynter, V. Naik, S. Malyshev, R. Menzel, and L. Horowitz, 2021. Global modeling of hydrogen using GFDLAM4.1: sensitivity of soil removal and radiative forcing. International Journal of Hydrogen Energy. (DOI:10.1016/j. ijhydene.2021.01.088).
Smith-Downey, N. V., J.T. Randerson, and J.M. Eiler, 2006. Temperature and moisture dependence of soil H2 uptake measured in the laboratory. Geophysical Research Letters, 33(14). (doi:10.1029/2006GL026749).