Freshwater wetlands are natural sinks of carbon; yet, wetland conversion for agricultural uses can shift these carbon sinks into large sources of greenhouse gases. We know that the anthropogenic alteration of wetland hydrology and the broad use of N-fertilizers can modify biogeochemical cycling, however, the extent of their combined effect on greenhouse gases exchange still needs further research. Moreover, there has been recent interest in wetlands rehabilitation and preservation by improving natural water flow and by seeking alternative solutions to nutrient inputs. In a microcosm setting, we experimentally exposed soils to three inundation treatments (Inundated, Moist, Drained) and a nutrient treatment by adding high nitrogen load (300 kg ha⁻¹) to simulate physical and chemical disturbances. After, we measured the depth microprofiles of N₂O and O₂ concentration and CO₂ and CH₄ emission rates to determine how hydrological alteration and nitrogen input affect carbon and nitrogen cycling processes in inland wetland soils. Compared to the Control soils, N-fertilizer increased CO₂ emissions by 40% in Drained conditions and increased CH₄ emissions in Inundated soils over 90%. N₂O emissions from Moist and Inundated soils enriched with nitrogen increased by 17.4 and 18-fold, respectively. Overall, the combination of physical and chemical disturbances increased the Global Warming Potential (GWP) by 7.5-fold. The first response of hydrological rehabilitation, while typically valuable for CO₂ emission reduction, amplified CH₄ and N₂O emissions when combined with high nitrogen inputs. Therefore, this research highlights the importance of evaluating the potential interactive effects of various disturbances on biogeochemical processes when devising rehabilitation plans to rehabilitate degraded wetlands.

Dissolved inorganic nitrogen (DIN) is considered the main factor that induces eutrophication in water, and is readily influenced by hydrodynamic activities. In this study, a 4-year field investigation of nitrogen dynamics in a turbulent river was conducted, and a laboratory study was performed in the approximately homogeneous turbulence simulation system to investigate potential mechanisms involved in DIN transformation under turbulence. The field investigation revealed that, contrary to NO⁻₃ dynamics, the NH⁺₄ concentrations in water were lower in flood seasons than in drought seasons. Further laboratory results demonstrated that limitation of dissolved oxygen (DO) caused inactive nitrification and active denitrification in static river sediment. In contrast, the increased DO levels in turbulent river intensified the mineralization of organic nitrogen in sediment; moreover, ammonification and nitrification were activated, while denitrification was first activated and then depressed. Turbulence therefore decreased NH⁺₄ and NO⁻₂ concentrations, but increased NO⁻₃ and total DIN concentrations in the overlying water, causing the total DIN to increase from 0.4 mg/L to maximum of 1.0 and 1.7 mg/L at low and high turbulence, respectively. The DIN was maintained at 0.7 and 1.0 mg/L after the 30-day incubation under low and high turbulence intensities (ε) of 3.4 × 10⁻⁴ and 7.4 × 10⁻² m²/s³, respectively. These results highlight the critical role of DO in DIN budgets under hydrodynamic turbulence, and provide new insights into the DIN transport and transformation mechanisms in turbulent rivers.

Natural processes and human activities play a crucial role in changing the nitrogen cycle and increasing nitrous oxide (N₂O) emissions, which are accelerating at an unprecedented rate. N₂O has serious global warming potential (GWP), about 310 times higher than that of carbon dioxide. The food production, transportation, and energy required to sustain a world population of seven billion have required dramatic increases in the consumption of synthetic nitrogen (N) fertilizers and fossil fuels, leading to increased N₂O in air and water. These changes have radically disturbed the nitrogen cycle and reactive nitrogen species, such as nitrous oxide (N₂O), and have impacted the climatic system. Yet, systematic and comprehensive studies on various underlying processes and parameters in the altered nitrogen cycle, and their implications for the climatic system are still lacking. This paper reviews how the nitrogen cycle has been disturbed and altered by anthropogenic activities, with a central focus on potential pathways of N₂O generation. The authors also estimate the N₂O–N emission mainly due to anthropogenic activities will be around 8.316 Tg N₂O–N yr⁻¹ in 2050. In order to minimize and tackle the N₂O emissions and its consequences on the global ecosystem and climate change, holistic mitigation strategies and diverse adaptations, policy reforms, and public awareness are suggested as vital considerations. This study concludes that rapidly increasing anthropogenic perturbations, the identification of new microbial communities, and their role in mediating biogeochemical processes now shape the modern nitrogen cycle.

Vegetable soils with high nitrogen input are hotspots of nitrous oxide (N₂O) and nitric oxide (NO), and biochar amended to soil has been documented to effectively decrease N₂O and NO emissions. However, the aging effects of biochar on soil N₂O and NO production and the relevant mechanisms are not thoroughly understood. A¹⁵N tracing microcosm study was conducted to clarify the responses of N₂O and NO production pathways to the biochar aging process in vegetable soil. The results showed that autotrophic nitrification was the predominant source of N₂O production. Biochar aging increased the O-containing functional groups while lowering the aromaticity and pore size. Fresh biochar enhanced the AOB-amoA gene abundance and obviously stimulated N₂O production by 15.5% via autotrophic nitrification and denitrification. In contrast, field-aged biochar markedly weakened autotrophic nitrification and denitrification and thus decreased N₂O production by 17.0%, as evidenced by the change in AOB-amoA and nosZI gene abundances. However, the amendment with artificially lab-aged biochar had no effect on N₂O production. With the extension of aging time, biochar application reduced the soil NO production dominated by nitrification. Changes in the N₂O and NO fluxes were closely associated with soil NH₄⁺-N and NO₂⁻-N contents, indicating that autotrophic nitrification played a critical role in NO production. Overall, our study demonstrated that field-aged biochar suppressed N₂O production via autotrophic nitrification and denitrification by regulating associated functional genes, but not for lab-aged biochar or fresh biochar. These findings improved our insights regarding the implications of biochar aging on N₂O and NO mitigation in vegetable soils.

Estuaries are considered as important sources of the global emission of greenhouse gases (GHGs). Urbanized estuaries often experience eutrophication under strong anthropogenic activities. Eutrophication can enhance phytoplankton abundance, leading to carbon dioxide (CO₂) consumption in the water column. Only a few studies have evaluated the relationship between GHGs and eutrophication in estuaries. In this study, we assessed the concentrations and fluxes of CO₂, methane (CH₄) and nitrous oxide (N₂O) in combination with a suite of biogeochemical variables in four sampling campaigns over two years in a highly urbanized tropical estuary in Southeast Asia (the Saigon River Estuary, Vietnam). The impact of eutrophication on GHGs was evaluated through several statistical methods and interpreted by biological processes. The average concentrations of CO₂, CH₄ and N₂O at the Saigon River in 2019–2020 were 3174 ± 1725 μgC-CO₂ L⁻¹, 5.9 ± 16.8 μgC-CH₄ L⁻¹ and 3.0 ± 4.8 μgN-N₂O L⁻¹, respectively. Their concentrations were 13–18 times, 52–332 times, and 9–37 times higher than the global mean concentrations of GHGs, respectively. While CO₂ concentration had no clear seasonal pattern, N₂O and CH₄ concentrations significantly differed between the dry and the rainy seasons. The increase in eutrophication status along the dense urban area was linearly correlated with the increase in GHGs concentrations. The presence of both nitrification and denitrification resulted in elevated N₂O concentrations in this urban area of the estuary. The high concentration of CO₂ was contributed by the high concentration of organic carbon and mineralization process. GHGs fluxes at the Saigon River Estuary were comparable to other urbanized estuaries regardless of climatic condition. Control of eutrophication in urbanized estuaries through the implantation of efficient wastewater treatment facilities will be an effective solution in mitigating the global warming potential caused by estuarine emissions.

Nitrate pollution in oxygenated karst aquifers is common due to nitrification and anthropogenic inputs. However, the shift of nitrogen sources influenced by enhanced rural tourism activities and land use changes are not well understood. In this study, hydrochemistry and dual nitrate isotopes of water samples from a rural karst basin in Chongqing, southwestern China were employed to investigate the nitrate fate and its decadal change during the periods from 2007–2008 and 2017–2019. The results showed that δ¹⁵N–NO₃ and δ¹⁸O–NO₃ values at the groundwater basin resurgence averaged 9 ± 3.4‰ and 2.5 ± 3.4‰, respectively, with a mean NO₃⁻ concentration of 19.7 ± 5.4 mg/L in 2017–2019, clearly exceeding natural background levels. The dual isotope results suggested that nitrification occurred at the sampled sites. From 2007–2008 to 2017–2019, the mean δ¹⁵N–NO₃ values from the primary sink point and the resurgence of the underground river water samples increased from −0.2 ± 2.1 to 11.2 ± 4.8‰, 4.2 ± 0.9 to 9.0 ± 3.4‰, respectively. A Bayesian mixing model in R (MixSIAR) based on the isotopes revealed that soil organic nitrogen, and manure and sewage proportions for the groundwater increased by 34% and 23%, respectively, while chemical fertilizer and atmospheric precipitation proportions decreased by 32% and 25%, respectively. These decadal changes resulted from reforestation practices and enhanced rural tourism activities in the basin, which were evidenced by the change of land use patterns. The elevated nitrogen load from the rapid development of rural tourism is likely to increase this contamination in the near future if the infrastructure cannot meet the demands. The results from this study could contribute to minimizing environmental health risks in drinking water when rural tourism activities are increasing.

Microplastic (MP) contamination is ubiquitous in agricultural soils. As a cost-effective soil amendment, biochar (BC) often coincides with MP exposure. However, little research has been conducted regarding the independent and combined effects of MPs and BC on the soil microbiome and N₂O/CH₄ emissions. Therefore, in this study, polyethylene terephthalate (PET) and wheat straw-derived BC were used, respectively, as representative MP and BC during an entire rice growth period. The high-throughput sequencing results showed that PET alone lowered bacterial diversity by 26.7%, while PET and BC co-existence did not induce apparent change. The relative abundances of some microbes (e.g., Cyanobacteria, Verrucomicrobia, and Bacteroidetes) that are associated with C and N cycling were changed at the phylum and class levels by all the treatments. In comparison with the control, the treatment of BC, PET, and their co-existence reduced the cumulative CH₄ emissions by 50%, 53%, and 61%, respectively. The higher mitigation by BC + PET may be the result of higher soil Eh and a consequently lower methanogenesis functional gene mcrA abundance in the treated soils. In addition, BC and PET alone, as well as their combined treatment, increased the abundance of nitrification genes, enhancing the soil nitrification process. However, the relative contribution of the nitrification process to N₂O emission was possibly lower than that of denitrification, in which the N₂O reductase gene nosZ was found to be the primary gene regulating N₂O emissions. BC alone increased nosZ abundance by 42.3%, thereby showing the potential in suppressing N₂O emission. In contrast, when BC was co-added with PET, the nosZ abundance lowered possibly because of increased soil aeration, and thus its cumulative N₂O emission was 38% higher than the BC treatment. Overall, these results demonstrated that BC and PET function differently in soil ecosystems when they coexisted.

The occurrence of excessive ammonium in groundwater threatens human and aquatic ecosystem health across many places worldwide. As the fate of ammonium in groundwater systems is often affected by a complex mixture of transport and biogeochemical transformation processes, identifying the sources of groundwater ammonium is an important prerequisite for planning effective mitigation strategies. Elevated ammonium was found in both a shallow and an underlying deep groundwater system in an alluvial aquifer system beneath an agricultural area in the central Yangtze River Basin, China. In this study we develop and apply a novel, indirect approach, which couples the random forest classification (RFC) of machine learning method and fluorescence excitation-emission matrices with parallel factor analysis (EEM-PARAFAC), to distinguish multiple sources of ammonium in a multi-layer aquifer. EEM-PARAFAC was applied to provide insights into potential ammonium sources as well as the carbon and nitrogen cycling processes affecting ammonium fate. Specifically, RFC was used to unravel the different key factors controlling the high levels of ammonium prevailing in the shallow and deep aquifer sections, respectively. Our results reveal that high concentrations of ammonium in the shallow groundwater system primarily originate from anthropogenic sources, before being modulated by intensive microbially mediated nitrogen transformation processes such as nitrification, denitrification and dissimilatory nitrate reduction to ammonium (DNRA). By contrast, the linkage between high concentrations of ammonium and decomposition of soil organic matter, which ubiquitously contained nitrogen, suggested that mineralization of soil organic nitrogen compounds is the primary mechanism for the enrichment of ammonium in deeper groundwaters.

Raw biochar with high pH possibly stimulated ammonia (NH₃) volatilization in the agricultural soil. We hypothesized that the modified biochar (MBC) with low pH can synchronically decrease the NH₃ and nitrous oxide (N₂O) losses. We performed a two-year experiment to clarify how citric acid MBC influence the NH₃ volatilization and N₂O emission as well as the underlying mechanisms. Two typical paddy soils, i.e., Hydragric Anthrosol and Haplic Acrisol, receiving equal urea N with 240 kg ha⁻¹ but varied rates of MBC with 0, 5, 10, and 20 t ha⁻¹ (named Urea, Urea + MBC5, Urea + MBC10, and Urea + MBC20, respectively) were studied. The results showed that MBC-amended treatments effectively mitigated the NH₃ volatilization from Hydragric Anthrosol and Haplic Acrisol by 29.6%–57.9% and 30.5%–62.4% in 2017, and by 16.5%–21.0% and 24.5%–35.0% in 2018, respectively, compared to Urea treatment. In addition, significantly lower N₂O emissions with averaged 38.3% and 43.1% in 2017, and 51.7% and 26.7% were recorded under Hydragric Anthrosol and Haplic Acrisol, respectively, following the MBC application (P < 0.05). Increased MBC addition performed higher efficacy on mitigating NH₃ volatilization, particularly in the first rice season, while this “dosage effect” was not found for N₂O reduction. Lowered pH in overlying water, enhanced adsorption of NH₄⁺-N and its nitrification rate likely contributed to the lower NH₃ volatilization as result of MBC addition. The nirS and nosZ gene copies were not changed by MBC, while the nirK gene copies were decreased as result of MBC amendment by 8.3%–25.2% under Hydragric Anthrosol and by 21.8%–24.9% under Haplic Acrisol. Consequent lower ratio of nirK/(nirS + nosZ) explained the mitigation effect of MBC on N₂O emission. In conclusion, the present two-year study recommends that MBC applied at a low dosage can perform positive effect on controlling the nitrogenous gas pollutants from paddy soil.

Nitrate (NO₃⁻) pollution in water bodies has received widespread attention, but studies on nitrogen transformation and pollution risk assessment are still limited, especially in rare earth mining areas. In this study, surface and groundwater samples were collected from the largest rare earth mining site in southern China, and analyzed for the hydrochemical and stable isotopic characteristics. The results showed that the NO₃⁻ concentrations ranged from 1.61 to 453.11 mg/L, with 35% of surface water and 53.3% of groundwater samples exceeding the WHO standard (i.e., 50 mg/L). Health risk assessment showed that 31.4% of the water samples had a moderate to high non-carcinogenic risk, and the high-risk areas were concentrated in rare earth mining regions. Additionally, adults were more vulnerable to the non-carcinogenic health risks than children. The high variability of δ¹⁵N–NO₃⁻ (from −6.43 to 17.09‰) and δ¹⁸O–NO₃⁻ (from −7.91 to 22.79‰) showed that NO₃⁻ was influenced by multiple nitrogen sources and transformation processes. Hydrochemistry and isotopic evidence further indicated that NO₃⁻ was primarily influenced by nitrification and hydraulic connection between surface and groundwater. The results of the Bayesian mixing model showed that about 70% of NO₃⁻ originated from mine drainage and soil N in the rare earth mining area, while more than 90% of NO₃⁻ originated from fertilizer, soil N, and manure and sewage in rural and urban areas in the middle and downstream. This study suggests reducing anthropogenic nitrogen discharge (e.g., leaching agents and fertilizer inputs) as the primary means of NO₃⁻ pollution control with biogeochemical processes (e.g., denitrification) to further reduce its pollution.