Space-for-time substitution leads to carbon emission overestimation in eutrophic lakes

Unexpected increase of sulfate concentrations and potential impact on CH4 budgets in freshwater lakes

The continuous increase in sulfate (SO42-) concentrations discharged by anthropogenic activities lacks insights into their dynamics and potential impact on CH4 budgets in freshwater lakes. Here we conducted a field investigation in the lakes along the highly developed Yangtze River basin, China, additionally, we analyzed long-term data (1950-2020) from Lake Taihu, a typical eutrophic lake worldwide. We observed a gradual increase in SO42- concentrations up to 100 mg/L, which showed a positive correlation with the trophic state of the lakes. The annual variations indicated that eutrophication intensified the fluctuation of SO42- concentrations. A random forest model was applied to assess the impact of SO42- concentrations on CH4 emissions, revealing a significant negative effect. Synchronously, a series of microcosms with added SO42- were established to simulate cyanobacteria decomposition processes and explore the coupling mechanism between sulfate reduction and CH4 production. The results showed a strong negative correlation between CH4 concentrations and initial SO42- levels (R2 = 0.83), indicating that higher initial SO42- concentrations led to lower final CH4 concentrations. This was attributed to the competition for cyanobacteria-supplied substrates between sulfate reduction bacteria (SRB) and methane production archaea (MPA). Our study highlights the importance of considering the unexpectedly increasing SO42- concentrations in eutrophic lakes when estimating global CH4 emission budgets.

Instability in a carbon pool driven by multiple dissolved organic matter sources in a eutrophic lake basin: Potential factors for increased greenhouse gas emissions

Lakes serve as vital reservoirs of dissolved organic matter (DOM) and play pivotal roles in biogeochemical carbon cycles. However, the sources and compositions of DOM in freshwater lakes and their potential effects on lake sediment carbon pools remain unclear. In this study, seven inflowing rivers in the Lake Taihu basin were selected to explore the potential effects of multi-source DOM inputs on the stability of the lake sediment carbon pool. The results showed the high concentrations of dissolved organic carbon in the Lake Taihu basin, accompanied by a high complexity level. Lignins constituted the majority of DOM compounds, surpassing 40% of the total, while the organic carbon content was predominantly composed of humic acids (1.02-3.01 g kg-1). The high amounts of lignin oxidative cleavage led to CHO being the main molecular structure in the DOM of the seven rivers. The carbon constituents within the sediment carbon reservoir exhibited a positive correlation with dissolved CH4 and CO2, with a notable emphasis on humic acid and dissolved CH4 (R2 = 0.86). The elevated concentration of DOM, coupled with its intricate composition, contributed to the increases in dissolved greenhouse gases (GHGs). Experiments showed that the mixing of multi-source DOM can accelerate the organic carbon mineralization processes. The unit carbon emission efficiency was highest in the mixed group, reaching reached 160.9 μmol∙Cg-1, which also exhibited a significantly different carbon pool. The mixed decomposition of DOM from different sources influenced the roles of the lake carbon pool as source and sink, indicating that the multi-source DOM of this lake basin was a potential driving factor for increased carbon emissions. These findings have improved our understanding of the sources and compositions of DOM in lake basins and revealed their impacts on carbon emissions, thereby providing a theoretical basis for improving assessments of lake carbon emissions.

Interactions between Cyanobacteria and Methane Processing Microbes Mitigate Methane Emissions from Rice Soils

Cyanobacteria play a relevant role in rice soils due to their contribution to soil fertility through nitrogen (N2) fixation and as a promising strategy to mitigate methane (CH4) emissions from these systems. However, information is still limited regarding the mechanisms of cyanobacterial modulation of CH4 cycling in rice soils. Here, we focused on the response of methane cycling microbial communities to inoculation with cyanobacteria in rice soils. We performed a microcosm study comprising rice soil inoculated with either of two cyanobacterial isolates (Calothrix sp. and Nostoc sp.) obtained from a rice paddy. Our results demonstrate that cyanobacterial inoculation reduced CH4 emissions by 20 times. Yet, the effect on CH4 cycling microbes differed for the cyanobacterial strains. Type Ia methanotrophs were stimulated by Calothrix sp. in the surface layer, while Nostoc sp. had the opposite effect. The overall pmoA transcripts of Type Ib methanotrophs were stimulated by Nostoc. Methanogens were not affected in the surface layer, while their abundance was reduced in the sub surface layer by the presence of Nostoc sp. Our results indicate that mitigation of methane emission from rice soils based on cyanobacterial inoculants depends on the proper pairing of cyanobacteria-methanotrophs and their respective traits.

Intense methane diffusive emissions in eutrophic urban lakes, Central China

Urban lakes are hotspots of methane (CH4) emissions. Yet, actual field measurements of CH4 in these lakes are rather limited and our understanding of CH4 response to urban lake eutrophication is still incomplete. In this study, we measured dissolved CH4 concentrations and quantified CH4 diffusion from four urban lakes in subtropical China during wet and dry seasons. We found that these lakes were constantly CH4-saturated, contributing the greenhouse gas (GHG) to the atmosphere. Nutrient enrichment significantly increased CH4 concentrations and diffusive fluxes. Average CH4 flux rate in the highly-eutrophic lake zones (4.18 ± 7.68 mmol m-2 d-1) was significantly higher than those in the mesotrophic (0.19 ± 0.18 mmol m-2 d-1) and lightly/moderately-eutrophic zones (0.72 ± 2.22 mmol m-2 d-1). Seasonally, CH4 concentrations and fluxes were significantly higher in the wet season than in the dry season in the mesotrophic and the lightly/moderately-eutrophic lake zones, but an inverse pattern existed in the highly-eutrophic lake zones. CH4 concentrations and fluxes increased with elevated levels of nitrogen, phosphorus and dissolved organic carbon (DOC). The accumulation of nutrients provided autochthonous substrate for methanogenesis, indicated by a negative correlation between CH4 and the C:N ratio. Ammonium-nitrogen (NH4+-N) was the best predictor for spatial fluctuation of CH4 concentrations and diffusive fluxes in the mesotrophic and the lightly/moderately-eutrophic lake zones, while total nitrogen (TN) and total phosphorus (TP) levels showed the highest predictability in the highly-eutrophic lake zones. Based on the findings, we conclude that nutrient enrichment in urban lakes can largely increase CH4 diffusion, and that urban sewage inflow is a key concern for eutrophication boosting CH4 production and diffusive emission. Furthermore, our study reveals that small urban lakes may be an important missing source of GHG emissions in the global C accounting, and that the ratio of littoral-to-pelagic zones can be important for predicting lake-scale estimation of CH4 emission.

Comparing the variation and influencing factors of CO2 emission from subsidence waterbodies under different restoration modes in coal mining area

Subsidence waterbodies play an important role in carbon cycle in coal mining area. However, little effort has been made to explore the carbon dioxide (CO2) release characteristics and influencing factors in subsidence waterbodies, especially under different restoration modes. Here, we measured CO2 release fluxes (F(CO2)) across Anguo wetland (AW), louts pond (LP), fishpond (FP), fishery-floating photovoltaic wetland (FFPV), floating photovoltaic wetland (FPV) in coal mining subsidence area, with unrestored subsidence waterbodies (SW) and unaffected normal Dasha river (DR) as the control area. We sampled each waterbody and tested which physical, chemical, and biological characteristics of water and sediment related to variability in CO2. The results indicated that F(CO2) exhibited the following patterns: FFPV > FPV > FP > SW > DR > LP > AW. Trophic lake index (TLI) and microbial biomass carbon content (MBC) in sediment had a positive impact on F(CO2). The dominant archaea Euryarchaeota and Thaumarchaeota, and dominant bacteria Proteobacteria promoted F(CO2). This study can help more accurately quantify CO2 emissions and guide CO2 future emission reduction and subsidence waterbodies estoration.

Variable promotion of algae and macrophyte organic matter on methanogenesis in anaerobic lake sediment

Shallow lakes are an important natural source of atmospheric methane (CH4), and the input of autochthonous organic matter (OM) into their sediments encourages methanogenesis. Although algal- and macrophytic-originated OM in these lakes are expected to have different impacts on methanogenesis and methanogenic archaeal communities in lake sediments owing to their various properties, their specific influence and role in sediment remain unclear. In this study, a 148-day incubation was carried out by adding algal- and macrophytic-OM to the sediments of shallow eutrophic Lake Chaohu and Lake Taihu in China. CH4 was periodically monitored, while the methanogens were examined via qPCR and high-throughput sequencing at the end of incubation. Algal-OM stimulated CH4 production more than macrophytic-OM in both sediments, with the rates initially increasing and then decreasing before reaching a relative constant. Macrophytic-OM promoted CH4 production to a comparable extent in both lakes, while algal-OM promoted greater CH4 in Lake Chaohu than in Lake Taihu. However, algal-OM did not significantly increase mcrA gene copies, while macrophytic-OM did by 17.0-20.1-fold. Algal-OM potentially promoted the methylotrophic pathway in Lake Taihu but did not change the methanogenic structure in Lake Chaohu. Comparatively, macrophytic-OM promoted CH4 production mainly by acetoclastic methanogen proliferation in both lakes. More CH4 release with algal-OM compared to macrophytic-OM deserves further attention owing to the prevailing increasing algal blooms and the declining macrophyte population in lakes.