Contribution of hydrogen production by biological nitrogen fixation to the global hydrogen budget

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Abstract
Measurements taken at two field stations in Germany have shown that H2 is evolved from the soil into the atmosphere in the presence of leguminous plants. H2 evolution rates were found to be dependent on N2 fixation rates and decreased rapidly when the plants (Trifolium hybridum) were topped, kept in darkness, or fertilized with NH4Cl. Maximum H2 evolution rates were observed during the main vegetation period, from April to June, with average rates of 5–7×10−8 cm3 cm−2 s−1. During this period, soil covered with legumes represented a net source of atmospheric H2 with an estimated global source strength of 0.9–1.2 Tg yr−1. Assuming that other biological N2 fixation processes contribute similarly to the global H2 budget, the total H2 source due to biological N2 fixation is estimated to be 2.4–4.9 Tg yr−1. The average H2 deposition velocity was found to be 7×10−2 cm s−1, resulting in a global H2 sink strength of soil of 70–110 Tg yr−1. The global H2 budget is discussed with respect to its various sources and sinks.