[B-GM22_30AM1] Microbial ecology in earth and planetary sciences

★ [BGM22-01] Plant rhizopsphere is a hotspot for greenhouse gas emissions

*Kiwamu MINAMISAWA¹ (1.Consortium for Rhizosphere Biogeochemistry, Graduate School of Life Sciences, Tohoku University)

Keywords:methane, nitrous oxide, rhizosphere, Bradyrhiozbia, bacteria, stable isotope

Nitrous oxide (N₂O) is a greenhouse gas that also degrades stratosphere ozone. Marked N₂O emission were detected from soybean root systems with degraded nodules during late growth stage in field-grown soybeans. A model system developed to produce N₂O emissions from soybean fields. Soybean plants inoculated with nosZ mutant of Bradyrhizobium japonicum USDA110 (lacking N₂O reductase) were grown in aseptic jars. After 30 days, shoot decapitation (D, to promote nodule degradation), soil addition (S, to supply soil microbes), or both (DS) were applied. N₂O was emitted only in the DS treatment. Thus, both soil microbes and nodule degradation are required for the emission of N₂O from the soybean rhizosphere. The N₂O flux peaked at 15 days after DS treatment. A ¹⁵N tracer experiment indicated that the N₂O was derived from N fixed in the nodules. As for nitrification, the addition of nitrification inhibitors significantly reduced N₂O flux. Both AOA and AOB were detected by PCR analysis with N₂O emission profile in soybean rhizosphere. The N₂O flux from the nirKnosZ mutant rhizosphere was significantly lower than that from nosZ mutant, but was still 30% to 60% of that of nosZ mutant, suggesting that N₂O emission is due to both B. japonicum and other soil microorganisms. Only B. japonicum nosZ+ strains could take up N₂O. In particular, Fusarium spp., a soil fungus may contributed to N₂O emission in soybean rhizoshere. From these results, the organic-N inside of the nodules was mineralized to NH₄⁺, and N₂O producing processes (nitrification and denitrification) simultaneously occur in the soybean rhizopsphere. We continue to examine which microbes really mediated N₂O methabolism using isotopic techniques including ¹⁵N site preference of N₂O molecules. N₂O emissions from soybeans ecosystems can be mitigated by inoculting B. japonicum mutants with increased N₂O reductase activity (Nos++ strains). The mutation of nasS gene is responsible for the Nos++ phenotype. We propose that nasS mutation might be an effective strategy to induce higher Nos activities in N₂O-reducing rhizobia, such as indigenous isolates from local soybean fields or even from other important leguminous crops such as alfalfa, and thus to mitigate N₂O emission. Plants have mutualistic symbiotic relationships with rhizobia and fungi by the common symbiosis pathway, in which Ca₂⁺/calmodulin-dependent protein kinase (encoded by CCaMK) is a central component. Although OsCCaMK is required for fungal accommodation in rice roots, little is known about the role of OsCCaMK in rice symbiosis with bacteria. Here, we report the effect of a tos17-induced OsCCaMK mutant (NE1115) on CH₄ flux in low-nitrogen (LN) and standard-nitrogen (SN) paddy fields as compared with wild-type (WT) Nipponbare. Growth of NE1115 was significantly decreased compared with that of WT, especially in the LN field. The CH₄ flux of NE1115 in the LN field was significantly higher (156?407% in 2011 and 170?816% in 2012) than that of WT, although no difference was observed in the SN field. The copy number of pmoA was significantly higher in the roots and rhizosphere soil of WT than those of NE1115. However, mcrA copy number did not differ between WT and NE1115. These results were supported by a ¹³C-labeled CH₄-feeding experiment. In addition, the natural abundance of ¹⁵N in WT shoots (3.05 permile) was significantly lower than in NE1115 shoots (3.45 permile), suggesting higher N₂ fixation in WT due to dilution with atmospheric N₂ (0.00 permile). Thus, CH₄ oxidation and N₂ fixation were simultaneously activated in the root zone of WT rice in the LN field, and both processes are likely controlled by OsCCaMK.