Hydrogen evolution from field pea biological nitrogen fixation and the effect on nitrous oxide production in soil
Rhizobia in symbiosis with legumes are capable of fixing atmospheric dinitrogen (N2) into plant available N through biological N fixation (BNF), during which H2 is produced as an obligate byproduct. Some rhizobia-legume symbioses possess an uptake hydrogenase (HUP) enzyme that is capable of recycling H2 produced during BNF; this type of symbiosis is referred to as HUP+. However, many symbionts do not possess the HUP enzyme (HUP-) and are therefore unable to recycle H2. Consequently, the H2 diffuses into the soil surrounding the nodules where it can be consumed by H2-oxidizing bacteria. There is evidence to suggest that microbial consumption of H2 in the soil causes increased CO2 fixation and O2 consumption in the rhizosphere soil, which could lead to the development of anoxic or hypoxic zones in the soil. These conditions favour denitrification, a process that produces N2O. The H2 from HUP- nodules also has been associated with enhanced plant growth, which may be a non-N benefit associated with planting legumes in rotation with other crops. Two studies were conducted to look at the effect of H2 from BNF in field pea (Pisum sativum L.) on N2O production and plant growth enhancement. The first was a growth chamber study where pea plants were grown in the absence of soil. The objectives of the first study were to (i) determine if actively fixing HUP- pea nodules produced more H2 than HUP+ nodules and (ii) if a H2-enriched atmosphere around pea nodules stimulated N2O production. Indeed, actively fixing pea nodules inoculated with HUP- rhizobia produced significantly more H2 than nodules inoculated with HUP+ rhizobia; however, pea nodules inoculated with HUP- rhizobia and exposed to an enriched H2 atmosphere were not associated with elevated N2O. The second study was completed as a greenhouse experiment where pea plants were grown in soil and N2O concentrations were monitored over the course of a growing season. The objectives of the greenhouse study were to (i) determine if field pea inoculated with HUP- rhizobia produced more N2O than field pea inoculated with HUP+ rhizobia, and (ii) determine if enhanced plant growth was associated with pea inoculated with HUP- rhizobia. Rhizosphere N2O concentrations and surface N2O flux were measured over the course of a growing season. Pea inoculated with HUP- rhizobia were not associated with increased N2O production. As well, there was no enhanced plant growth observed in pea plants inoculated with HUP- rhizobia compared to HUP+ rhizobia. This study demonstrated that in the Saskatchewan soil used in this study, field pea inoculated with HUP- rhizobia did not appear to stimulate N2O production or enhance plant growth, which further supports the notion that N2O production associated with legume production is not directly related to BNF.
hydrogen, nitrous oxide, HUP, biological nitrogen fixation, legumes
Master of Science (M.Sc.)