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

Measurements taken at two field stations in Germany have shown that H 2 is evolved from the soil into the atmosphere in the presence of leguminous plants. H 2 evolution rates were found to be dependent on N 2 fixation rates and decreased rapidly when the plants ( Trifolium hybridum ) were topped, kept in darkness, or fertilized with NH 4 Cl. Maximum H 2 evolution rates were observed during the main vegetation period, from April to June, with average rates of 5–7×10 −8 cm 3 cm −2 s −1 . During this period, soil covered with legumes represented a net source of atmospheric H 2 with an estimated global source strength of 0.9–1.2 Tg yr −1 . Assuming that other biological N 2 fixation processes contribute similarly to the global H 2 budget, the total H 2 source due to biological N 2 fixation is estimated to be 2.4–4.9 Tg yr −1 . The average H 2 deposition velocity was found to be 7×10 −2 cm s −1 , resulting in a global H 2 sink strength of soil of 70–110 Tg yr −1 . The global H 2 budget is discussed with respect to its various sources and sinks.