Effect of gradual increase of atmospheric CO2 concentration on nitrification potential and communities of ammonia oxidizers in paddy fields

Warming increased the promotion of atmospheric CO2 concentration on biological nitrogen fixation by changing the nifH gene community

Understanding the nitrogen (N) fixation process in rice paddies under anticipated climate change is vital for regulating the soil N cycle, solving N pollution, and improving agricultural productivity in the agricultural system. To clarify the changes in nitrogen fixation under elevated carbon dioxide (CO2) concentration and temperature in rice paddies, we constructed an automated platform consisting of Open-Top Chamber. We set up four treatments: CK (ambient CO2 concentration + ambient temperature), EC (increase in CO2 concentration by 200 μmol mol-1 + ambient temperature), ET (ambient CO2 concentration + increase in temperature by 2 °C), and ECT (increase in CO2 concentration by 200 μmol mol-1 + increase in temperature by 2 °C), to analyze nitrogen fixation potential (NFP) and the abundance, diversity, and composition of the nifH gene of nitrogen-fixing bacteria communities in paddy soils. The results showed that EC, ET, and ECT increased NFP and the abundance of nifH gene in paddy soils to different degrees. ET significantly increased NFP in the paddy soils at the tillering, elongation, and flowering stages. ECT significantly increased nifH gene abundance at the tillering, elongation, and maturity stages compared to CK. EC, ET, and ECT affected the community structures of nitrogen-fixing microorganisms to a certain extent, especially at maturity, where the community structure of EC, ET, and ECT treatments changed considerably. NFP increased with increasing nifH gene abundance and soil NH4+-N content and decreased with increasing soil pH and DOC content. In summary, ET promotes nitrogen cycling in paddy soils by directly promoting soil MBN and NH4+-N content, increasing soil temperature, and suppressing soil DOC content and pH, thus indirectly influencing the community structures and nifH gene abundance in paddy soils, which further contributes to the positive effect of EC on biological nitrogen fixation in rice fields.

Impact of elevated CO2 on microbial communities and functions in riparian sediments: Role of pollution levels in modulating effects

The impact of elevated CO2 levels on microorganisms is a focal point in studying the environmental effects of global climate change. A growing number of studies have demonstrated the importance of the direct effects of elevated CO2 on microorganisms, which are confounded by indirect effects that are not easily identified. Riparian zones have become key factor in identifying the environmental effects of global climate change because of their special location. However, the direct effects of elevated CO2 levels on microbial activity and function in riparian zone sediments remain unclear. In this study, three riparian sediments with different pollution risk levels of heavy metals and nutrients were selected to explore the direct response of microbial communities and functions to elevated CO2 excluding plants. The results showed that the short-term effects of elevated CO2 did not change the diversity of the bacterial and fungal communities, but altered the composition of their communities. Additionally, differences were observed in the responses of microbial functions to elevated CO2 levels among the three regions. Elevated CO2 promoted the activities of nitrification and denitrification enzymes and led to significant increases in N2O release in the three sediments, with the greatest increase of 76.09 % observed in the Yuyangshan Bay (YYS). Microbial carbon metabolism was promoted by elevated CO2 in YYS but was significantly inhibited by elevated CO2 in Gonghu Bay and Meiliang Bay. Moreover, TOC, TN, and Pb contents were identified as key factors contributing to the different microbial responses to elevated CO2 in sediments with different heavy metal and nutrient pollution. In conclusion, this study provides in-depth insights into the responses of bacteria and fungi in polluted riparian sediments to elevated CO2, which helps elucidate the complex interactions between microbial activity and environmental stressors.

Ammonia-oxidizing archaea adapted better to the dark, alkaline oligotrophic karst cave than their bacterial counterparts

Subsurface karst caves provide unique opportunities to study the deep biosphere, shedding light on microbial contribution to elemental cycling. Although ammonia oxidation driven by both ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) is well explored in soil and marine environments, our understanding in the subsurface biosphere still remained limited to date. To address this gap, weathered rock and sediment samples were collected from the Xincuntun Cave in Guilin City, an alkaline karst cave, and subjected to high-throughput sequencing and quantification of bacterial and archaeal amoA, along with determination of the potential nitrification rates (PNR). Results revealed that AOA dominated in ammonia oxidation, contributing 48-100% to the PNR, and AOA amoA gene copies outnumbered AOB by 2 to 6 orders. Nitrososphaera dominated in AOA communities, while Nitrosopira dominated AOB communities. AOA demonstrated significantly larger niche breadth than AOB. The development of AOA communities was influenced by deterministic processes (50.71%), while AOB communities were predominantly influenced by stochastic processes. TOC, NH4+, and Cl- played crucial roles in shaping the compositions of ammonia oxidizers at the OTU level. Cross-domain co-occurrence networks highlighted the dominance of AOA nodes in the networks and positive associations between AOA and AOB, especially in the inner zone, suggesting collaborative effort to thrive in extreme environments. Their high gene copies, dominance in the interaction with ammonia oxidizing bacteria, expansive niche breadth and substantial contribution to PNR collectively confirmed that AOA better adapted to alkaline, oligotrophic karst caves environments, and thus play a fundamental role in nitrogen cycling in subsurface biosphere.