Ecosystem fluxes of hydrogen in a mid-latitude forest driven by soil microorganisms and plants.

Molecular hydrogen (H ) is an atmospheric trace gas with a large microbe-mediated soil sink, yet cycling of this compound throughout ecosystems is poorly understood. Measurements of the sources and sinks of H in various ecosystems are sparse, resulting in large uncertainties in the global H budget. Constraining the H cycle is critical to understanding its role in atmospheric chemistry and climate. We measured H fluxes at high frequency in a temperate mixed deciduous forest for 15 months using a tower-based flux-gradient approach to determine both the soil-atmosphere and the net ecosystem flux of H . We found that Harvard Forest is a net H sink (-1.4 ± 1.1 kg H  ha ) with soils as the dominant H sink (-2.0 ± 1.0 kg H  ha ) and aboveground canopy emissions as the dominant H source (+0.6 ± 0.8 kg H  ha ). Aboveground emissions of H were an unexpected and substantial component of the ecosystem H flux, reducing net ecosystem uptake by 30% of that calculated from soil uptake alone. Soil uptake was highly seasonal (July maximum, February minimum), positively correlated with soil temperature and negatively correlated with environmental variables relevant to diffusion into soils (i.e., soil moisture, snow depth, snow density). Soil microbial H uptake was correlated with rhizosphere respiration rates (r = 0.8, P < 0.001), and H metabolism yielded up to 2% of the energy gleaned by microbes from carbon substrate respiration. Here, we elucidate key processes controlling the biosphere-atmosphere exchange of H and raise new questions regarding the role of aboveground biomass as a source of atmospheric H and mechanisms linking soil H and carbon cycling. Results from this study should be incorporated into modeling efforts to predict the response of the H soil sink to changes in anthropogenic H emissions and shifting soil conditions with climate and land-use change.