Study On Soil N2O Emissions Associated with Soybean Symbiosis.

In a previous laboratory study, we found that soil not exposed to N₂-fixing legumes for more than 8 years showed an increased in N₂O emission up to10 fold higher when the soil was treated with H₂-enriched air for 16 weeks at an exposure rate similar to that experience by soils adjacent to many legume nodules (220 umolH₂ L^-1soil h^-1). We hypothesized that the H₂ produced as a byproduct of legume N₂ fixation could modify soil properties (especially microbial populations) thereby increasing the potential for N₂O production in N₂ fixing legume crops.

 To test this hypothesis, we established field trials of soybeans (Glycine max cv. Bravor) inoculated with either sterile culture solution (control) or isogenic strains of Bradyhizobium japonicum JH (Hup⁺, wild-type), JH47 (Hup^- mutant) or 532C, and a non-inoculating Evans cultivar on a virgin field in Ottawa (Agriculture and Agri-Food Canada, Eastern Cereal & Oilseed Res Ctr,) that had not seen legumes for at least 20 years. We measured soil atmosphere N₂O concentrations in the field, and soil N₂O emissions in the laboratory using soils collected from soybean field. Soil adjacent to nodules had significantly higher rates of N₂O production than soil near roots lacking nodules, a result consistent with the hypothesis and with previous studies using H₂ treated soil, giving evidence of enhanced N₂O emissions related to the legume symbiosis. In the field, variation of N₂O emission over sample period was found in both soil static gas chamber and tube measurements. However, integrated over the sampling season, the soybean field inoculated with Rhizobia showed higher N₂O emissions relative to non-inoculated field. In conclusion, our results give a possible explanation of why legume crops are often reported to have N₂O emission rates that are higher than can be explained by N addition.