The combined effect of short-term hydrological and N-fertilization manipulation of wetlands on CO2, CH4, and N2O emissions

Reveling the micromolecular biological mechanism of acetate, thiosulfate and Fe0 in ecological floating beds for treating low C/N wastewater: Insight into nitrogen removals and greenhouse gases reductions

Selecting an appropriate electron donor to enhance nitrogen removal for treating low C/N wastewater in ecological floating beds (EFBs) is controversy. In this study, a systematic and comprehensive evaluation of sodium acetate (EFB-C), sodium thiosulfate (EFB-S) and iron scraps (EFB-Fe) was performed in a 2-year experiment on long-term viability including nitrogen removal and greenhouse gas emissions associated with key molecular biological mechanisms. The results showed that EFB-C (43-85 %) and EFB-S (40-88 %) exhibited superior total nitrogen (TN) removal. Temperature and hydraulic retention time (HRT) have significant impacts on TN removal of EFB-Fe, however, it could reach 86 % under high temperature (30-35 °C) and a long HRT (3 days), and it has lowest N2O (0-6.2 mg m-2 d-1) and CH4 (0-5.3 mg m-2 d-1) fluxes. Microbial network analysis revealed that the microbes changed from competing to cooperating after adding electron donors. A higher abundance of anammox genera was enriched in EFB-Fe. The Mantel's test and structural equation model provided proof of the differences, which showed that acetate and thiosulfate were similar, whereas Fe0 was different in the nitrogen removal mechanism. Molecular biology analyses further verified that heterotrophic, autotrophic, and mixotrophic coupled with anammox were the main TN removal pathways for EFB-C, EFB-S, and EFB-Fe, respectively. These findings provide a better understanding of the biological mechanisms for selecting appropriate electron donors for treating low C/N wastewater.

CH4 control and nitrogen removal from constructed wetlands by plant combination

Global warming trend is accelerating. This study proposes a green and economical methane (CH4) control strategy by plant combination in constructed wetlands (CWs). In this study, a single planting of Acorus calamus L. hybrid constructed wetland (HCW-A) and a mixed planting of Acorus calamus L. and Eichhornia crassipes (Mart.) Solms hybrid constructed wetland (HCW-EA) were constructed. The differences in nitrogen removal performance and CH4 emissions between HCW-A and HCW-EA were compared and analyzed. The findings indicated that HCW-EA demonstrated significant improvements over HCW-A, with NH4+-N and TN removal rates increasing by 21.61% and 16.38% respectively, and CH4 emissions decreased by 43.36%. The microbiological analysis results showed that plant combination promoted the enrichment of Proteobacteria, Alphaproteobacteria and Bacillus. More nitrifying bacteria carrying nxrA genes and denitrifying bacteria carrying nirK genes accelerated the nitrogen transformation process. In addition, the absolute abundance ratio of pmoA/mcrA increased, reducing the release of CH4.

Freshwater wetland restoration and conservation are long-term natural climate solutions

Freshwater wetlands have a disproportionately large influence on the global carbon cycle, with the potential to serve as long-term carbon sinks. Many of the world's freshwater wetlands have been destroyed or degraded, thereby affecting carbon-sink capacity. Ecological restoration of degraded wetlands is thus becoming an increasingly sought-after natural climate solution. Yet the time required to revert a degraded wetland from a carbon source to sink remains largely unknown. Moreover, increased methane (CH4) and nitrous oxide (N2O) emissions might complicate the climate benefit that wetland restoration may represent. We conducted a global meta-analysis to evaluate the benefits of wetland restoration in terms of net ecosystem carbon and greenhouse gas balance. Most studies (76 %) investigated the benefits of wetland restoration in peatlands (bogs, fens, and peat swamps) in the northern hemisphere, whereas the effects of restoration in non-peat wetlands (freshwater marshes, non-peat swamps, and riparian wetlands) remain largely unexplored. Despite higher CH4 emissions, most restored (77 %) and all natural peatlands were net carbon sinks, whereas most degraded peatlands (69 %) were carbon sources. Conversely, CH4 emissions from non-peat wetlands were similar across degraded, restored, and natural non-peat wetlands. When considering the radiative forcings and atmospheric lifetimes of the different greenhouse gases, the average time for restored wetlands to have a net cooling effect on the climate after restoration is 525 years for peatlands and 141 years for non-peat wetlands. The radiative benefit of wetland restoration does, therefore, not meet the timeframe set by the Paris Agreement to limit global warming by 2100. The conservation and protection of natural freshwater wetlands should be prioritised over wetland restoration as those ecosystems already play a key role in climate change mitigation.

Seasonal and spatial variations of greenhouse gas (CO2, CH4 and N2O) emissions from urban ponds in Brussels

Freshwaters have been recognized as important sources of greenhouse gases (GHG) to the atmosphere. However, urban ponds have received little attention even though their number is increasing due to expanding urbanisation globally. Ponds are frequently associated to urban green spaces that provide several ecosystemic services such as cooling local climate, regulating the water cycle, and acting as small carbon sinks This study aims to identify and understand the processes producing GHGs (CO2, CH4, and N2O) in the urban ponds of the temperate European city of Brussels in Belgium. 22 relatively small ponds (0.1-4.6 ha) surrounded by contrasted landscape (strictly urban, bordered by cropland or by forest), were sampled during four seasons in 2021-2022. The mean ± standard deviation was 3,667 ± 2,904 ppm for the partial pressure of CO2 (pCO2), 2,833 ± 4,178 nmol L-1 for CH4, and 273 ± 662% for N2O saturation level (%N2O). Relationships of GHGs with oxygen and water temperature suggest that biological processes controlled pCO2, CH4 concentration and%N2O. However, pCO2 was also controlled by external inputs as indicated by the higher values of pCO2 in the smaller ponds, more subject to external inputs than larger ones. The opposite was observed for CH4 concentration that was higher in larger ponds, closer to the forest in the city periphery, and with higher macrophyte cover. N2O concentrations, as well as dissolved inorganic nitrogen, were higher closer to the city center, where atmospheric nitrogen deposition was potentially higher. The total GHG emissions from the Brussels ponds were estimated to 1kT CO2-eq per year and were equivalent to the carbon sink of urban green spaces.

Novel annual nitrogen management strategy improves crop yield and reduces greenhouse gas emissions in wheat-maize rotation systems under limited irrigation

Excessive irrigation and nitrogen application have long seriously undermined agricultural sustainability in the North China Plain (NCP), leading to declining groundwater tables and intensified greenhouse gas (GHG) emissions. Developing low-input management practices that meet the growing food demand while reducing environmental costs is urgently needed. Here, we developed a novel nitrogen management strategy for a typical winter wheat-summer maize rotation system in the NCP under limited irrigation (wheat sowing irrigation only (W0) or sowing and jointing irrigation (W1)) and low nitrogen input (360 kg N ha-1, about 70 % of traditional annual nitrogen input). Novel nitrogen management strategy promoted efficient nitrogen fertilizer uptake and utilization by both crops via optimization of nitrogen fertilizer allocation between the two crops, i.e., increasing nitrogen inputs to wheat (from 180 to 240 kg N ha-1) while reducing nitrogen inputs to maize (from 180 to 120 kg N ha-1). Three-year field study demonstrated that integrated management practices combining novel nitrogen management strategy with limited irrigation increased annual yields and PFPN by 1.9-5.7 %, and reduced TGE by 55-68 kg CO2-eq ha-1 and GHGI by 2.2-10.3 %, without any additional cost. Our results provide agricultural operators and policymakers with practical and easy-to-scalable integrated management strategy, and offer key initiative to promote grain production in the NCP towards agriculture sustainable intensification with high productivity and efficiency, water conservation and emission reduction.

The driving effects of nitrogen deposition on nitrous oxide and associated gene abundances at two water table levels in an alpine peatland

Alpine peatlands are recognized as a weak or negligible source of nitrous oxide (N2O). Anthropogenic activities and climate change resulted in the altered water table (WT) levels and increased nitrogen (N) deposition, which could potentially transition this habitat into a N2O emission hotspot. However, the underlying mechanism related with the effects is still uncertain. Hence, we conducted a mesocosm experiment to address the response of growing-season N2O emissions and the gene abundances of nitrification (bacterial amoA) and denitrification (narG, nirS, norB and nosZ) to the increased N deposition (20 kg N ha-1 yr-1) at two WT levels (WT-30, 30 cm below soil surface; WT10, 10 cm above soil surface) in the Zoige alpine peatland, Qinghai-Tibetan Plateau. The results showed that the WT did not affect N2O emissions, and this was attributed with the limitation of soil NO3-. The higher WT level increased denitrification (narG and nirS gene abundance) resulting in the depletion of soil NO3-, but the consequent NO3- deficiency further limited denitrification, while the WT did not affect nitrification (bacterial amoA gene abundance). Meanwhile, the N deposition increased N2O emissions, regardless of WT levels. This was associated with the N-deposition induced increase in denitrification-related gene abundances of narG, nirS, norB and nosZ at WT-30 and narG at WT10. Additionally, the N2O emission factor assigned to N deposition was 1.3 % at WT-30 and 0.9 % at WT10, respectively. Our study provided comprehensive understanding of the mechanisms referring N2O emissions in response to the interactions between climate change and human disturbance from this high-altitude peatland.

Effects of Long-Term (17 Years) Nitrogen Input on Soil Bacterial Community in Sanjiang Plain: The Largest Marsh Wetland in China

Increased nitrogen (N) input from natural factors and human activities may negatively impact the health of marsh wetlands. However, the understanding of how exogenous N affects the ecosystem remains limited. We selected the soil bacterial community as the index of ecosystem health and performed a long-term N input experiment, including four N levels of 0, 6, 12, and 24 gN·m-2·a-1 (denoted as CK, C1, C2, and C3, respectively). The results showed that a high-level N (24 gN·m-2·a-1) input could significantly reduce the Chao index and ACE index for the bacterial community and inhibit some dominant microorganisms. The RDA results indicated that TN and NH4+ were the critical factors influencing the soil microbial community under the long-term N input. Moreover, the long-term N input was found to significantly reduce the abundance of Azospirillum and Desulfovibrio, which were typical N-fixing microorganisms. Conversely, the long-term N input was found to significantly increase the abundance of Nitrosospira and Clostridium_sensu_stricto_1, which were typical nitrifying and denitrifying microorganisms. Increased soil N content has been suggested to inhibit the N fixation function of the wetland and exert a positive effect on the processes of nitrification and denitrification in the wetland ecosystem. Our research can be used to improve strategies to protect wetland health.

The core mangrove microbiome reveals shared taxa potentially involved in nutrient cycling and promoting host survival

BACKGROUND

Microbes have fundamental roles underpinning the functioning of our planet, they are involved in global carbon and nutrient cycling, and support the existence of multicellular life. The mangrove ecosystem is nutrient limited and if not for microbial cycling of nutrients, life in this harsh environment would likely not exist. The mangroves of Southeast Asia are the oldest and most biodiverse on the planet, and serve vital roles helping to prevent shoreline erosion, act as nursery grounds for many marine species and sequester carbon. Despite these recognised benefits and the importance of microbes in these ecosystems, studies examining the mangrove microbiome in Southeast Asia are scarce.cxs RESULTS: Here we examine the microbiome of Avicenia alba and Sonneratia alba and identify a core microbiome of 81 taxa. A further eight taxa (Pleurocapsa, Tunicatimonas, Halomonas, Marinomonas, Rubrivirga, Altererythrobacte, Lewinella, and Erythrobacter) were found to be significantly enriched in mangrove tree compartments suggesting key roles in this microbiome. The majority of those identified are involved in nutrient cycling or have roles in the production of compounds that promote host survival.

CONCLUSION

The identification of a core microbiome furthers our understanding of mangrove microbial biodiversity, particularly in Southeast Asia where studies such as this are rare. The identification of significantly different microbial communities between sampling sites suggests environmental filtering is occurring, with hosts selecting for a microbial consortia most suitable for survival in their immediate environment. As climate change advances, many of these microbial communities are predicted to change, however, without knowing what is currently there, it is impossible to determine the magnitude of any deviations. This work provides an important baseline against which change in microbial community can be measured.

Effects of straw mulching and nitrogen application rates on crop yields, fertilizer use efficiency, and greenhouse gas emissions of summer maize

Although straw mulching and nitrogen applications are extensively practiced in the agriculture sector, large uncertainties remain about their impacts on crop yields and especially the environment. The responses of summer maize yields, fertilizer use efficiency, and greenhouse gas (GHG) emissions including carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) in the North China Plain (NCP) to two straw management practices (S0: no straw and S1: straw mulching) and two nitrogen application rates (N1: 180 and N2: 210 kg N ha^-1) were investigated in field tests in 2018, 2019, and 2020. The highest yields and partial factor productivity (PFP) were obtained by S1N1, followed by S1N2, S0N1, and S0N2. S1N2 had the highest CO₂ emissions and greatest CH₄ uptake, S0N1 had the lowest CO₂ emissions, and S0N2 had the smallest CH₄ uptake. The highest and lowest N₂O emissions were found in S0N1 and S1N1, respectively. The S1N2 treatment, an extensively applied practice, had the greatest global warming potential (GWP), which was 70.3 % larger than S1N1 and two times more than S0N1 and S0N2. The largest GHG emission intensity (GHGI) of 19.4 was found in the S1N2 treatment, while the other three treatments, S0N1, S0N2, and S1N1, had a GHGI of 10.1, 10.7, and 10.7, respectively according to three tested results. In conclusion, S1N1 treatment achieved a better trade-off between crop yields and GHG emissions of summer maize in NCP.