Hydrogen emission from nodulated soybeans [Glycine max (L.) Merr.] and consequences for the productivity of a subsequent maize (Zea mays L.) crop

Hydrogen (H₂) is a by-product of the symbiotic nitrogen fixation (N₂ fixation) between legumes and root-nodule bacteria (rhizobia). Some rhizobial strains have an uptake hydrogenase enzyme (commonly referred to as Hup⁺) that recycles H₂ within the nodules. Other rhizobia, described as Hup^?, do not have the enzyme and the H₂ produced diffuses from the nodules into the soil where it is consumed by microorganisms. The effect of this phenomenon on the soil biota and on the soil itself, and consequent stimulation of plant growth, has been demonstrated previously. Soybeans Glycine max (L.) Merr.] cv. Leichhardt, inoculated with either a Hup⁺ strain (CB1809) or one of two Hup^? strains (USDA442 or USDA16) of Bradyrhizobium japonicum and uninoculated soybeans, plus a non-legume control capsicum (Capsicum annuum L.)] were grown in the field at Ayr, North Queensland, Australia. The objectives were to examine (1) relationships between N₂ fixation and H₂ emission, and (2) the influence H₂-induced changes in soil might have during the legume phase and/or on the performance of a following crop. Strains CB1809 and USDA442 were highly effective in N₂ fixation (“good” fixers); USDA16 was partly effective (“poor” fixer). The soil had a large but non-uniformly distributed naturalised population of B. japonicum and most uninoculated control plants formed nodules that fixed some N₂. These naturalised strains were classified as “poor fixers” of N₂ and were Hup⁺. H₂ emissions from nodules were assessed for all treatments when the soybean crop was 62 days old. Other parameters of symbiotic N₂ fixation and plant productivity were measured when the crop was 62 and 96 days old and at crop maturity. Immediately after final harvest, the land was sown to a crop of maize (Zea mays L.) in order to determine the consequences of H₂ emission from the soybean crop on maize growth. It was estimated that soybeans inoculated with USDA442, the highly effective Hup^– strain of B. japonicum, fixed 117 kg shoot N/ha (or about 195 kg total N/ha if the fixed N associated with roots and nodules was taken into account), and contributed about 215,000 l H₂ gas per hectare to the ecosystem over the life of the crop. The volume of H₂ evolved from soybeans nodulated by the Hup⁺ strain CB1809 was only 6% of that emitted by the USDA442 treatment, but there was no indication that soybean inoculated with USDA442 benefited from the additional H₂ input. The shoot biomass, grain yield, and amounts of N fixed (105 kg shoot N/ha, 175 kg total N/ha) by the CB1809 treatment were little less than for USDA442 plants. Three days after the soybean crop was harvested, the plots were over-sown with maize along the same row lines in which the soybeans had grown. This procedure exposed the maize roots to whatever influence soybean H₂ emission might have had on the soil and/or the soil microflora immediately surrounding soybean nodules. The evidence for a positive effect of soybean H₂ emission on maize production was equivocal. While the consistent differences between those pre-treatments that emitted H₂ and those that did not indicated a trend, only one difference (out of the 12 parameters of maize productivity that were measured) was statistically significant at P?<?0.05. The findings need substantiation by further investigation.