New insights in correlating greenhouse gas emissions and microbial carbon and nitrogen transformations in wetland sediments based on genomic and functional analysis

Effects of substrate type on enhancing pollutant removal performance and reducing greenhouse gas emission in vertical subsurface flow constructed wetland

Rivers play an important role in greenhouse gas emissions. Over the past decade, because of global urbanization trends, rapid land use changes have led to changes in river ecosystems that have had a stimulating effect on the greenhouse gas production and emissions. Presently, there is an urgent need for assessments of the greenhouse gas concentrations and emissions in watersheds. Therefore, this study was designed to evaluate river-based greenhouse gas emissions and their spatial-temporal features as well as possible impact factors in a rapidly urbanizing area. The specific objectives were to investigate how river greenhouse gas concentrations and emission fluxes are responding to urbanization in the Liangtan River, which is not only the largest sub-basin but also the most polluted one in Chongqing City. The thin layer diffusion model method was used to monitor year-round concentrations of pCO2, CH4, and N2O in September and December 2014, and March and June 2015. The pCO2 range was (23.38±34.89)-(1395.33±55.45) Pa, and the concentration ranges of CH4 and N2O were (65.09±28.09)-(6021.36±94.36) nmol·L-1 and (29.47±5.16)-(510.28±18.34) nmol·L-1, respectively. The emission fluxes of CO2, CH4, and N2O, which were calculated based on the method of wind speed model estimations, were -6.1-786.9, 0.31-27.62, and 0.06-1.08 mmol·(m2·d)-1, respectively. Moreover, the CO2 and CH4 emissions displayed significant spatial differences, and these were roughly consistent with the pollution load gradient. The greenhouse gas concentrations and fluxes of trunk streams increased and then decreased from upstream to downstream, and the highest value was detected at the middle reaches where the urbanization rate is higher than in other areas and the river is seriously polluted. As for branches, the greenhouse gas concentrations and fluxes increased significantly from the upstream agricultural areas to the downstream urban areas. The CO2 fluxes followed a seasonal pattern, with the highest CO2 emission values observed in autumn, then successively winter, summer, and spring. The CH4 fluxes were the highest in spring and the lowest in summer, while N2O flux seasonal patterns were not significant. Because of the high carbon and nitrogen loads in the basin, the CO2 products and emissions were not restricted by biogenic elements, but levels were found to be related to important biological metabolic factors such as the water temperature, pH, DO, and chlorophyll a. The carbon, nitrogen, and phosphorus content of the water combined with sewage input influenced the CH4 products and emissions. Meanwhile, N2O production and emissions were mainly found to be driven by urban sewage discharge with high N2O concentrations. Rapid urbanization accelerated greenhouse gas emissions from the urban rivers, so that in the urban reaches, CO2/CH4 fluxes were twice those of the non-urban reaches, and all over the basin N2O fluxes were at a high level. These findings illustrate how river basin urbanization can change aquatic environments and aggravate allochthonous pollution inputs such as carbon, nitrogen, and phosphorus, which in turn can dramatically stimulate river-based greenhouse gas production and emissions; meanwhile, spatial and temporal differences in greenhouse gas emissions in rivers can lead to the formation of emission hotspots.

Abstract. The removal efficiency of carbon (C) and nitrogen (N) in constructed wetlands (CWs) is very inconsistent and frequently does not reveal whether the removal processes are due to physical attenuation or whether the different species have been transformed to other reactive forms. Previous research on nutrient removal in CWs did not consider the dynamics of pollution swapping (the increase of one pollutant as a result of a measure introduced to reduce a different pollutant) driven by transformational processes within and around the system. This paper aims to address this knowledge gap by reviewing the biogeochemical dynamics and fate of C and N in CWs and their potential impact on the environment, and by presenting novel ways in which these knowledge gaps may be eliminated. Nutrient removal in CWs varies with the type of CW, vegetation, climate, season, geographical region, and management practices. Horizontal flow CWs tend to have good nitrate (NO3−) removal, as they provide good conditions for denitrification, but cannot remove ammonium (NH4+) due to limited ability to nitrify NH4+. Vertical flow CWs have good NH4+ removal, but their denitrification ability is low. Surface flow CWs decrease nitrous oxide (N2O) emissions but increase methane (CH4) emissions; subsurface flow CWs increase N2O and carbon dioxide (CO2) emissions, but decrease CH4 emissions. Mixed species of vegetation perform better than monocultures in increasing C and N removal and decreasing greenhouse gas (GHG) emissions, but empirical evidence is still scarce. Lower hydraulic loadings with higher hydraulic retention times enhance nutrient removal, but more empirical evidence is required to determine an optimum design. A conceptual model highlighting the current state of knowledge is presented and experimental work that should be undertaken to address knowledge gaps across CWs, vegetation and wastewater types, hydraulic loading rates and regimes, and retention times, is suggested. We recommend that further research on process-based C and N removal and on the balancing of end products into reactive and benign forms is critical to the assessment of the environmental performance of CWs.

A comparative estimate of life cycle greenhouse gas emissions from two types of constructed wetlands in Tianjin, China

Greenhouse gas emission in constructed wetlands for wastewater treatment: A review

Greenhouse gas emissions from constructed wetlands: A bibliometric analysis and mini-review

New insights for simultaneous nutrient removal enhancement and greenhouse gas emissions reduction of constructed wetland by optimizing its redox environment through manganese oxide addition

Life cycle assessment of vertical and horizontal flow constructed wetlands for wastewater treatment considering nitrogen and carbon greenhouse gas emissions

Influence of seasonal water-level fluctuations on depth-dependent microbial nitrogen transformation and greenhouse gas fluxes in the riparian zone

Major US electric utility climate pledges have the potential to collectively reduce power sector emissions by one-third

Constructed wetlands (CWs) are increasingly considered for secondary wastewater treatment, removing both conventional contaminants and emerging pollutants, notably pharmaceutical and personal care products (PPCPs). However, the CW design and operational conditions to biodegrade PPCPs as micropollutants may promote greenhouse gas (GHG) emissions, raising sustainability concerns. This meta-analysis investigates the relationship between PPCP removal (caffeine, ibuprofen, naproxen, diclofenac, ketoprofen, carbamazepine, sulfonamide compounds) and GHG emissions (methane, carbon dioxide, nitrous oxide) in CWs. We uniquely integrate two sets of studies, as prior research has not linked PPCP biodegradation with GHG emissions. Data from 26 papers identify factors driving PPCP removal and 26 publications inform GHG emission factors. Spearman's correlation coefficient and multiple linear regression assess parameter effects and interlinkages. Results highlight biological processes, particularly secondary metabolism or co-metabolism, as pivotal for PPCP removal and GHG emissions, with inlet PPCP concentration, carbon load, and temperature being significant influencers (p < 0.05). Challenges persist in optimizing operations to improve PPCP removal and abate GHG emissions simultaneously. Still, CW depth, influent chemical oxygen demand (COD), hydraulic retention time, and subsurface flow wetland configuration emerge as strategic parameters. This study underscores the need for integrated approaches to enhance PPCP removal and decrease GHG emissions in CWs, thereby advancing sustainable water management practices.

Salinity has a significant impact on the sewage treatment efficiency of constructed wetlands (CWs), as well as affecting the greenhouse gas emissions of CWs. A lab-scale CW simulation system was constructed to observe the treatment efficiency and greenhouse gas flux occurring in CWs at different influent salinities (0%, 0.5%, 1.0%, 1.5%, and 2.0%). The results show that (1) the removal rates of COD, TN, NH4+-N, NO3--N, and TP reach the highest at salinity of 0 or 0.5%. And the lowest removal rates are all at a salinity of 2.0%. (2) The emission flux of CO2, CH4, and N2O in CWs varies with an increase in salinity. The trends of CO2 and CH4 emission flux were consistent with those of COD reduction rate. However, it was opposite for N2O flux to that of TN, NH4+-N, and NO3--N removal rate. Affected by salinity, the greenhouse gas emission flux in this study is generally lower than what was reported in literature. (3) Correlation analysis showed that CO2 and CH4 emission fluxes were positively correlated with the COD reduction rate. N2O emission flux was negatively correlated with the removal rates of TN, NH4+-N, and NO3--N. The results suggest that different pollutants are inhibited by salinity to different degrees. COD is more affected by salinity than nitrogen and phosphorus, while nitrogen is more easily inhibited by salinity than phosphorus. CWs can have a high removal rate of pollutants in treating low-salinity wastewater. Although increased salinity reduces treatment efficiency of wastewater to some extent, it also inhibits the emission of CO2 and CH4.

Ratoon rice can improve rice yield by increasing the multiple cropping index in China. However, the greenhouse gas (CH4 and N2O) emission characteristics from ratoon rice fields and the cultivation methods to reduce CH4 and N2O emissions are rarely reported. This study first conducted the analysis of genotype differences in greenhouse gas emission fluxes using five strong ratoon ability rice varieties in 2020. Second, water management methods, including alternating the wet–dry irrigation (AWD) pattern and conventional flooding irrigation (CF) during the main season, were carried out in 2021. CH4 and N2O emission flux, agronomic traits, and rice yield during both main and ratoon seasons were investigated. The results showed that the CH4 emission flux during the main and ratoon seasons was 157.05–470.73 kg·ha–1 and 31.03–84.38 kg·ha–1, respectively, and the total N2O emission flux was 0.13–0.94 kg·ha–1 in the ratoon rice system over the two seasons (RRSTS). Compared with the main season, the CH4 emission flux during the ratoon season was significantly reduced, thus decreasing the greenhouse gas global warming potential (GWP) and greenhouse gas emission intensity (GHGI) in the ratoon rice system. Cliangyouhuazhan (CLYHZ) showed a high yield, and the lowest GWP and GHGI values among the five rice varieties in RRSTS. Compared with CF, the AWD pattern reduced the CH4 emission flux during the main and ratoon seasons by 67.4–95.3 kg·ha–1 and 1.7–5.1 kg·ha–1, respectively, but increased the N2O emission flux by 0.1–0.6 kg·ha–1 during the RRSTS. Further, compared with CF, the AWD pattern had a declined GWP by 14.3–19.4% and GHGI by 30.3–34.3% during the RRSTS, which was attributed to the significant reduction in GWP and GHGI during the main season. The AWD pattern significantly increased rice yield by 21.9–22.9% during the RRSTS, especially for YX203. Correlation analysis showed that CH4, GWP, and GHGI exhibited significant negative correlations with spikelet number per m2 and the harvest index during the main and ratoon seasons. Collectively, selecting the high-yield, low-emission variety CLYHZ could significantly reduce greenhouse gas emissions from ratoon rice while maintaining a high yield. The AWD pattern could reduce total CH4 emission during the main season, reducing the GWP and GHGI while increasing the ratoon rice system yield. It could be concluded that a variety of CLYHZ and AWD patterns are worthy of promotion and application to decrease greenhouse gas emissions in the ratoon rice area in the upper reaches of Yangtze River, China.

Response of grassland greenhouse gas emissions to different human disturbances – A global Meta-analysis

Nitrogen (N) fertilizer application can regulate the structure of soil microbial community and influence the abundance of functional genes involved in carbon (C) and N cycling, thereby affecting greenhouse gas (GHG) emissions. This study was conducted in 2023-2024, setting up six nitrogen application rates: N0 (0 kg·ha-1), N120 (0 kg·ha-1), N180 (0 kg·ha-1), N240 (0 kg·ha-1), N300 (0 kg·ha-1), and N360 (0 kg·ha-1). Using 16S amplicon sequencing technology and metagenomic sequencing, the study analyzed the abundance of carbon and nitrogen cycling functional genes. Combined with measurements of CH₄, N₂O, and CO₂ emission fluxes, the research elucidated the mechanism by which nitrogen fertilizer regulates microbial modulation of greenhouse gas emissions. The results indicated that nitrogen application significantly increased greenhouse gas (CH₄, N₂O, CO₂) emissions, with the highest emissions observed under the N300 treatment. Nitrogen application regulated soil nutrients, increasing soil total nitrogen, nitrate nitrogen, and microbial biomass carbon content. Reasonable nitrogen application (N240) increased bacterial α-diversity (Shannon index, Chao index, PD index) in the soil by 10.82, 14.65, and 1.92%, respectively, compared to N0. It also increased the abundance of dominant nitrogen-fixing bacterial phyla, including Actinobacteria, Proteobacteria, and Nitrospirota. Furthermore, it regulated the abundance of microbial-mediated functional genes involved in dissimilatory nitrate reduction (nirB), assimilatory nitrate reduction (nasA), denitrification (narG, narH, nirS), nitrification (norC, nxrA, nxrB, hao, amoC), as well as those in the carbon cycle related to methane metabolism (pmoA, pmoC, mttC), carbon fixation (por/nifj, rbcl/cbbl), and hydrogenotrophic methanogenesis (mch, hdrA, frdE). This regulation further modulated greenhouse gas emissions. Therefore, this study clarifies the microbe-associated mechanisms underlying the N fertilizer-driven coupling of C and N cycles with GHG emissions through an integrated analysis of microbial diversity and metagenomics. Furthermore, it offers new insights for sustainable N fertilizer management and emission mitigation strategies in agricultural systems.