Responses of Iron-Reducing Bacteria to Salinity and Organic Matter Amendment in Paddy Soils of Thailand

In paddy soils of Thailand, the addition of organic matter (OM) is used to efficiently limit the effect of salinity on rice culture and production. OM used as an amendment; and fertilizer promotes the reduced condition and increases iron solubilization without provoking ferrous toxicity. In this study, the intricate biogeochemical role of iron-reducing bacteria (IRB) involved in the quality of water and soil of paddy fields, particularly when the paddy fields were subject to salinity and organic matter addition, were studied in paddy fields of Thailand. The results demonstrated that the addition of OM increased the proliferation of cultivable IRB and their specific activity. Cultivable IRB communities decreased in the presence of salt. The presence of salt modified the structure of the bacterial populations by favoring the development of alkaline and moderately halophilic bacteria (Virgibacillus spp., Oceanobacillus spp., and Paenibacillus spp.). The paddy soils studied contained very diversified (halosensitive, halotolerant, and halophilic) IRB populations that could be adapted to a variety of salinity conditions (0-90 g L-1 NaCl) using different organic substrates (glucose, acetate, and soil organic matter) to maintain significant activities under extreme conditions of salinity. The rhizosphere of rice stimulated IRB community growth without organic matter, whereas organic matter addition limited the rhizosphere effect on IRB cultivable number in saline condition. The interactive action of salinity and organic amendment had a negative impact on the rhizosphere effect. The presence of specific iron-reducing populations (fermentative, iron-respiring, anaerobic, and facultative anaerobic), having different behaviors under salt and redox stresses, appeared to be a key factor that contributed to the control or enhancement of the quality of water and soil in paddy fields.