Impact of saltwater intrusion on paddy soil microbial communities
The rice farming in wetlands worldwide is facing a significant threat from soil salinization caused by the infiltration of saltwater due to the rising sea levels in coastal areas. The high concentrations of salt in the soil pose a serious risk to the viability of arable land as most crops are not to...
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|Summary:||The rice farming in wetlands worldwide is facing a significant threat from soil salinization caused by the infiltration of saltwater due to the rising sea levels in coastal areas. The high concentrations of salt in the soil pose a serious risk to the viability of arable land as most crops are not tolerant to such conditions. Despite numerous studies on the growth and yield of rice in saline conditions, little is known about how soil salinity affects the microbial communities, their compositions, and functions in rice paddy soil. Rice cultivation is one of the major sources of methane emission globally due to the use of rice straw as an organic fertilizer. This source accounts for about 10% of the world's methane budget. The microbial communities in the paddy soil can effectively break down rice straw in the absence of oxygen, with the rate-limiting stage being the breakdown of bio-polymers. Seawater intrusion leads to reduced carbon availability and increased recalcitrance of organic matter, resulting in decreased soil CO2 and CH4 production and lower enzyme activity involved in the hydrolysis of cellulose and the oxidation of lignin. The aim of my thesis research is to (i) investigate how intermediate salinity affects the structure and function of methanogens, sulfate reducers, and bacterial communities in paddy soil under anoxic conditions, (ii) assess the long-term impact of NaCl and seasalt treatments on microbial communities.
At the beginning, microcosm slurry was set up with 40 gr of the Philippine paddy soil 35 ml distilled water, which amended with 0.5 gr rice straw. The bottles containing microcosm incubated 30°C for seven days as a preincubation. On day seven (week = 0) some microcosms treated with NaCl and seaalt at 150 mM then incubated up to week six. During the experiement, we measured gases and fatty acids concentrations, and also evaluated the copy numbers of three marker genes (16S rRNA, dsrB and mcrA) and their transctipts. Later, taxonomy assignment at different levels was perfomed.
The NaCl and seasalt treatments at 150 mM had a significant impact on the production of CH4 and CO2. In particular, the concentration of these gases was considerably reduced in the group that received the seasalt treatment, as compared to both the control group and the group treated with NaCl. As for the concentration of H2S gas among the three groups, it was heightened in the seasalt treated group but lower in both the control and NaCl treated groups. The processing of fatty acids digestion (namely, acetate, butyrate, and propionate) was observed to be slower in both the NaCl and seasalt treated groups when compared to the control group. The experimental findings revealed that the NaCl treatment hindered the digestion of propionate and butyrate to a greater extent than the seasalt treatment. Conversely, the digestion of acetate was found to be more impeded in the seasalt treated group compared to the NaCl treated group.
The application of the two salt treatments had a discernible impact on the genes and transcript copies of bacteria, methanogens, and sulfate-reducing bacteria, as revealed by the RT-qPCR and qPCR assays carried out using the three primer sets (namely, 16S rRNA, mcrA, and dsrB). With the exception of the dsrB gene and transcript assays, the groups treated with seasalt displayed fewer copies compared to the other two groups. Upon analyzing the 16S rRNA gene at the phylum level, it was found that the treated groups exhibited a higher relative abundance of Firmicutes in comparison to the control group. Additionally, the implementation of both the NaCl and seasalt treatments resulted in an escalation of the relative abundance of Actinobacteria and Chloroflexi in the 16S rRNA transcript's phylum level. Conversely, the utilization of the two salt treatments caused a decline in the relative abundance of Proteobacteria during the entire course of the experiment. In the dsrB gene at the phylum level, the two treatments exhibited a marked increase in the relative abundance of Firmicutes during the first and second weeks. Similarly, the application of both NaCl and seasalt treatments resulted in an increase in the abundance of Methanosarcinales at the order level of the mcrA gene. Using order-level mcrA transcripts, the analysis showed an increase in Methanomassiliicoccales and Methanosarcinales in both the NaCl and seasalt treated groups, while Methanocellales demonstrated a decrease throughout the experiment.
The present study investigated the salt treatment effects on the functional and growth characteristics of the Philippine paddy soil microbial communities, including bacteria and methanogens. Our results demonstrate that the treatments disrupted the metabolic functions of the microbial communities, as evidenced by the delayed processing of fatty acid digestion biologically, such as acetate, propionate, and butyrate, and also they mitigated the methane and carbon dioxide concentrations. Furthermore, we observed that intermediate levels of salinity (150 mM; NaCl and seasalt) had an impact on the microbial community structure in anoxic condition. Specifically, the treatments led to a higher diversity of microbial taxa at the 16S rRNA gene, transcript, and mcrA gene levels when compared to the untreated control group. Conversely, the dsrB gene and transcript levels were reduced following treatment with intermediate salinity. Of note, we observed an increase in H2S gas concentration in the seasalt-treated group, which was reflected in the dsrB gene expression level resulted in sulfate reduction. These findings underscore the importance of salinity as a key environmental factor in shaping microbial community structure and function. The observed disruption of metabolic functions highlights the potential impact of saltwater intrusion due to sea-level rise or tidal changes on the soil microbial community, which in turn can have significant implications for ecosystem functioning and productivity by changing biogeochemical cycles.|