Influences of shallow groundwater depth on N2O diffusion along the soil profile of summer maize fields in North China Plain.

The emissions of nitrous oxide (N2O) from agricultural fields are a significant contribution to global warming. Understanding the mechanisms of N2O emissions from agricultural fields is essential for the development of N2O emission mitigation strategies. Currently, there are extensive studies on N2O emissions on the surface of agricultural soils, while studies on N2O fluxes at the interface between the saturated and unsaturated zones (ISU) are limited. Uncertainties exist regarding N2O emissions from the soil-shallow groundwater systems in agricultural fields. In this study, a three-year lysimeter experiment (2019-2020, 2022) was conducted to simulate the soil-shallow groundwater systems under four controlled shallow groundwater depth (SGD) (i.e., SGD = 40, 70, 110, and 150 cm) conditions in North China Plain (NCP). Weekly continuous monitoring of N2O emissions from soil surface, N2O concentration in the shallow groundwater and the upper 10 cm of pores at the ISU, and nitrogen cycling-related parameters in the soil and groundwater was conducted. The results showed that soil surface N2O emissions increased with decreased shallow groundwater depth, and the highest emissions of 96.44 kg ha-1 and 104.32 kg ha-1 were observed at G2 (SGD = 40 cm) in 2020 and 2022. During the observation period of one maize growing season, shallow groundwater acted as a sink for the unsaturated zone when the groundwater depth was 40 cm, 70 cm, and 110 cm. However, when SGD was 150 cm, shallow groundwater became a source for the unsaturated zone. After fertilization, the groundwater in all treatment plots behaved as a sink for the unsaturated zone, and the diffusion intensity decreased with increasing SGD. The results would provide a theoretical basis for cropland water management to reduce N2O emissions.

Non-negligible N2O emission hotspots: Rivers impacted by ion-adsorption rare earth mining.

Rare earth mining causes severe riverine nitrogen pollution, but its effect on nitrous oxide (N2O) emissions and the associated nitrogen transformation processes remain unclear. Here, we characterized N2O fluxes from China's largest ion-adsorption rare earth mining watershed and elucidated the mechanisms that drove N2O production and consumption using advanced isotope mapping and molecular biology techniques. Compared to the undisturbed river, the mining-affected river exhibited higher N2O fluxes (7.96 ± 10.18 mmol m-2d-1 vs. 2.88 ± 8.27 mmol m-2d-1, P = 0.002), confirming that mining-affected rivers are N2O emission hotspots. Flux variations scaled with high nitrogen supply (resulting from mining activities), and were mainly attributed to changes in water chemistry (i.e., pH, and metal concentrations), sediment property (i.e., particle size), and hydrogeomorphic factors (e.g., river order and slope). Coupled nitrification-denitrification and N2O reduction were the dominant processes controlling the N2O dynamics. Of these, the contribution of incomplete denitrification to N2O production was greater than that of nitrification, especially in the heavily mining-affected reaches. Co-occurrence network analysis identified Thiomonas and Rhodanobacter as the key genus closely associated with N2O production, suggesting their potential roles for denitrification. This is the first study to elucidate N2O emission and influential mechanisms in mining-affected rivers using combined isotopic and molecular techniques. The discovery of this study enhances our understanding of the distinctive processes driving N2O production and consumption in highly anthropogenically disturbed aquatic systems, and also provides the foundation for accurate assessment of N2O emissions from mining-affected rivers on regional and global scales.

Metabolism and carbonate buffering drive seasonal dynamics of CO2 emissions from two German reservoirs.

Biological metabolism drives much of the variation in CO2 in terrestrial ecosystems but does not explain CO2 oversaturation and emission in net autotrophic lakes and reservoirs. The unexplained CO2 could be attributed to the equilibria between CO2 and the carbonate buffering system, which is seldom integrated into CO2 budgets, let alone its interplay with metabolism on CO2 emissions. Here, we perform a process-based mass balance modeling analysis based on an 8-year dataset from two adjacent reservoirs with similar catchment sizes but contrasting trophic states and alkalinity. We find that in addition to the well-acknowledged driver of net metabolic CO2 production, carbonate buffering also determines the total amount and seasonal dynamics of CO2 emissions from the reservoirs. Carbonate buffering can contribute up to nearly 50% of whole-reservoir CO2 emissions, by converting the ionic forms of carbonate to CO2. This results in similar seasonal CO2 emissions from reservoirs with differing trophic state, even in low alkalinity system. We therefore suggest that catchment alkalinity, instead of trophic state, may be more relevant in predicting CO2 emissions from reservoirs. Our model approach highlights the important role of carbonate buffering and metabolism that generate and remove CO2 throughout the reservoirs on a seasonal scale. The inclusion of carbonate buffering could diminish a major uncertainty in the estimation of reservoir CO2 emissions and increase the robustness of aquatic CO2 emission estimates.

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.

Effects of no-tillage on greenhouse gas emissions in maize fields in a semi-humid temperate climate region.

Agricultural tillage practices have a significant impact on the generation and consumption of greenhouse gases (GHGs), the primary causes of global warming. Two tillage systems, conventional tillage (CT) and no-tillage (NT), were compared to evaluate their effects on GHG emissions in this study. Averaged from 2018 to 2020, significant decreases of CO2 and N2O emissions by 7.4% and 51.1% were observed in NT as compared to those of CT. NT was also found to inhibit the soil CH4 uptake. In this study, soil was a source of CO2 and N2O but a sink for CH4. The effect of soil temperature on the fluxes of CO2 was more pronounced than that of soil moisture. However, soil temperature and soil moisture had a weak correlation with CH4 and N2O flux variations. As compared to CT, NT did not affect maize yields but significantly reduced global warming potential (GWP) by 8.07%. For yield-scaled GWP, no significant difference was observed in NT (9.63) and CT (10.71). Taken together, NT was an environment-friendly tillage practice to mitigate GHG emissions in the soil under the tested conditions.

Virtual water transfers in Africa: Assessing topical condition of water scarcity, water savings, and policy implications.

Africa is facing an increasing challenge with respect to water scarcity (WS), which is driven by climate change, population growth, and socioeconomic growth combined with inadequate water resources management. In particular, there is significant concern of virtual water (VW) trade, which plays the key role in water resource management and food security sustainability. Using bilateral trade data, this study consistently evaluated the change and balanced trade of major grains, the VW flows, WS status, water dependency (WD), water self-sufficiency (WSS), and water savings/losses within5 African sub-regions and their partners from 2000 to 2020. The ratio of water use to water availability was used to estimate the WS. The WD was quantified by the ratio of the net VW import to the regional water appropriation and the regional water savings/losses were also quantified by multiplying the inter-regional trade by the virtual water content of the imported/exported grains. The overall average trade deficit of African regions was found to increase to -1364.22 × 106 tons and Africa imported 41,359.07 Bm3 of VW from grain products. Green water contributed 79.33% of the total VWI. The WS values for East African countries were >100, indicating overexploitation. Besides, the overall WD in Africa was 465.5% for the studied period. The trade of main grains between Africa and the rest of the planet corresponded to a global water loss of 2820.7 Bm3·yr-1. However, the inter-continental cereal VW trade pattern and high trend will continue in the future. In view of the rising tension of WS, some African countries need to revise international crop trade and water resources conservation policies to promote a more balanced ecosystem. This study exemplifies that decision makers would consider VW flows and water savings/losses for enhancing water use efficiency and fair trading, thus increasing food production in Africa.

Fluvial CO2 and CH4 in a lowland agriculturally impacted river network: Importance of local and longitudinal controls.

Despite streams and rivers play a critical role as conduits of terrestrially produced organic carbon to the atmosphere, fluvial CO2 and CH4 are seldom integrated into regional carbon budgets. High spatial variability hinders our ability to understand how local and longitudinal controls affect underlying processes of riverine CO2 and CH4 and challenge the prediction and upscaling across large areas. Here, we conducted a survey of fluvial CO2 and CH4 concentrations spanning multiple stream orders within an agriculturally impacted region, the North China Plain. We explored the spatial patterns of fluvial CO2 and CH4 concentrations, and then examined whether catchment and network properties and water chemical parameters can explain the variations in both carbon gases. Streams and rivers were systematically supersaturated with CO2 and CH4 with the mean concentrations being 111 and 0.63 µmol L-1, respectively. Spatial variability of both gases was regulated by network properties and catchment features. Fluvial CO2 and CH4 declined longitudinally and could be modeled as functions of stream order, dissolved oxygen, and water temperature. Both models explained about half of the variability and reflected longitudinal and local drivers simultaneously, albeit CO2 was more local-influenced and CH4 more longitudinal-influenced. Our empirical models in this work contribute to the upscaling and prediction of CO2 and CH4 emissions from streams and rivers and the understanding of proximal and remote controls on spatial patterns of both gases in agriculturally impacted regions.

Evaluation of no-tillage impacts on soil respiration by 13C-isotopic signature in North China Plain.

It is a challenge to characterize soil respiration of crop residue return systems in the North China Plain (NCP) under no-tillage (NT) and conventional tillage (CT) practices. In this study, we addressed the "hot spot" research challenge of impacts of tillage practices on soil carbon storage and soil CO2 emissions in the NCP by 13C-isotopic signature. A short-term (2018-2020) field experiment was conducted with two tillage practices: NT and CT. The results showed that in the tested area, NT had advantages of lower CO2 emissions compared to CT with average reduced CO2 emissions by 10.82%-19.14%. The results of this study suggested that the NT facilitated enhanced soil carbon storage by 2.80%, which was evidenced by the δ13C data. Based on the path analysis model, the main line of soil respiration reduced by NT was attributed to the increased of soil microbial carbon and nitrogen as well as soil moisture in NT, which further increased δ13C and eventually inhibited soil respiration. Overall, adopting NT in NCP is an effective means to improve soil carbon pool and decrease soil CO2 emissions in agriculture practices.

Influence of the shallow groundwater table on the groundwater N2O and direct N2O emissions in summer maize field in the North China Plain.

Agriculture is an important N2O emissions source. Water cycle and nitrogen cycles have important effects on N2O in farmland ecosystems. The changes in the groundwater table can lead to changes in farmland the water and nitrogen cycle processes. However, how this such changes will affect N2O emissions from farmland remains unclear. In this study, a two-year volume lysimeter experiment (2019-2020), including four controlled groundwater tables (i.e., 40, 70, 110, and 150 cm), was performed to monitor the variations in the NO3- and N2O concentrations in shallow groundwater as well as the direct N2O emissions due to surface soil and groundwater evaporation. Our results showed that N2O emissions during fertilization accounted for 80%-90% of the total N2O emissions throughout the maize growing period. Direct N2O emissions increase with the increase in the groundwater table. The total N2O emissions in 2020 were 96.44, 9.75, 6.46, and 6.22 kg ha-1 y-1 at a groundwater table of 40, 70, 110, and 150 cm, respectively. The high water-filled pore space (WFPS) value resulting from the elevated groundwater table increased the groundwater-atmosphere connectivity, leading to significantly increased N2O emissions after fertilization. Increased precipitation (454.90 mm in 2020 vs. 180.30 mm in 2019) accelerated the hydrological processes in agroecology, reducing the retention time of N2O (6 weeks in 2020 vs. 7.5 weeks in 2019) and NO3- (6.75 weeks in 2020 vs. 7.25 weeks in 2019) in shallow groundwater. Studying the influence of shallow groundwater tables on direct N2O emissions will provide insights into the interaction between the water and nitrogen cycles in agroecosystems. The results of this study suggest that direct N2O emissions can be effectively reduced by controlling the groundwater table in agricultural fields in the North China Plain.

Agricultural impacts drive longitudinal variations of riverine water quality of the Aral Sea basin (Amu Darya and Syr Darya Rivers), Central Asia.

River ecosystems are under increasing stress in the background of global change and ever-growing anthropogenic impacts in Central Asia. However, available water quality data in this region are insufficient for a reliable assessment of the current status, which come as no surprise that the limited knowledge of regulating processes for further prediction of solute variations hinders the development of sustainable management strategies. Here, we analyzed a dataset of various water quality variables from two sampling campaigns in 2019 in the catchments of two major rivers in Central Asia-the Amu Darya and Syr Darya Rivers. Our results suggested high spatial heterogeneity of salinity and major ion components along the longitudinal directions in both river catchments, pointing to an increasing influence of human activities toward downstream areas. We linked the modeling outputs from the global nutrient model (IMAGE-GNM) to riverine nutrients to elucidate the effect of different natural and anthropogenic sources in dictating the longitudinal variations of the riverine nutrient concentrations (N and P). Diffuse nutrient loadings dominated the export flux into the rivers, whereas leaching and surface runoff constituted the major fractions for N and P, respectively. Discharge of agricultural irrigation water into the rivers was the major cause of the increases in nutrients and salinity. Given that the conditions in Central Asia are highly susceptible to climate change, our findings call for more efforts to establish holistic management of water quality.

Influence of land use and change in the proportion of electron donors required for denitrification on N2O in groundwater.

Nitrate (NO3-) and nitrous oxide (N2O) accumulate in groundwater in relation to human activities and pose multiple threats to the global environment (harming human health and atmospheric damage). This study focused on the evaluation of groundwater NO3-, N2O, and its indirect emission factor under different land use types (agricultural land, urban land, and forest) and response mechanism of major anions to dissolved N2O within groundwater in Dexing which has the largest copper mine in Asia. Specifically, this work used self-organizing maps (SOMs) to identify which anion conditions (NO3-, SO42-, F-, Cl-) and water quality parameters were suitable for the accumulation of groundwater N2O. Finally, we found that the shallow groundwater of agricultural land has a high concentration of NO3- and N2O and the agricultural activity has a significant effect on the temporal and spatial variation of N2O in groundwater. The result of SOMs combined with the positive correlation between N2O and NO3-/SO42- suggested that the electron donor required for denitrification has a significant effect on N2O accumulation. In this respect, when an increased proportion of reduced sulfur is available as an electron donor for autotrophic denitrification, this results in lower concentrations of N2O in groundwater. Through the comprehensive evaluation of the anion conditions and N2O in groundwater under different land use types, this study case can help to estimate the N2O indirect emission from groundwater, so as to constrain the global nitrogen budget.

Influence of no-tillage and precipitation pulse on continuous soil respiration of summer maize affected by soil water in the North China Plain.

Soil respiration (RS) from cropland in response to tillage practices contribute to global climate change. We quantified the effect of no-tillage (NT) and conventional tillage (CT) on RS and precipitation in the North China Plain (NCP). An in-situ automatic sampling and measurement method was applied during the maize (Zea mays L.) growth stages in 2018 and 2019. The continuous daily RS, soil water content and temperature were monitored during all the maize growth stages, whereas maize grain yield, aboveground biomass, and soil microbial biomass were measured after harvest. The mean RS across tillage practices on bright days was higher in 2018 (16.69 g CO2 m-2 d-1) than that in 2019 (12.99 g CO2 m-2 d-1). Compared with CT, NT increased RS on bright days by 31.44% in 2018 and 15.60% in 2019. However, mean RS on rain-affected days across tillage practices was lower in 2018 than that in 2019. NT increased mean RS after precipitation in 2018 (p < 0.05). The contribution of RS after precipitation to cumulative RS (across tillage practices) was higher in 2019 (51.90%) than that in 2018 (41.18%). Mean soil water content and temperature were higher in 2018 than that in 2019 (p < 0.05). NT increased soil water content on bright days in 2019. Furthermore, soil water content was more important in regulating RS in 2018, while soil temperature was more critical after precipitation in 2019. Crop productivity was lower in 2019 than in 2018 (p < 0.05). However, neither crop productivity nor soil microbial biomass varied with tillage practices (p > 0.05). Overall, influence of tillage practices and precipitation on RS were different according to soil water content. Therefore, it is necessary to decrease excessive irrigation to reduce RS in dry years and to conduct continuous observations on RS after precipitation in the NCP.

Spatial Distribution and Health Risk Assessment of Dissolved Trace Elements in Groundwater in southern China.

To understand the groundwater environmental quality and the impact of trace elements in the construction of urban agglomeration in China, this study collected 58 groundwater samples from the core area of the Chang-Zhu-Tan urban agglomeration (Changsha, Zhuzhou, Xiangtan) and quantitatively analyzed the content of 13 dissolved trace element and their spatial distribution characteristics. The health risk assessment model was further used to evaluate the human health risk caused by trace element pollution in groundwater. It was observed that Ba had the highest average concentration (0.28 mg·L-1), whereas Cd had the lowest (2.1 × 10-5 mg·L-1). Compared with China's groundwater environmental quality standard, the exceeding rates of Se, Mn, Zn, and Ni concentrations were 37.93%, 17.24%, 1.72% and 1.72%, respectively. Ba, Cd, Co, Cr, Cu, Fe, Mo, and Pb did not exceed the corresponding standards. The 13 trace elements were distributed in a scattered pattern in space and the trace elements in both banks of the Xiang River, Zhuzhou, Weishui River and surrounding areas were relatively high. Health risk assessments showed that the carcinogenic risk values of Cd, Cr, and Pb and the health risk values of 10 non-carcinogenic elements were less than the corresponding maximum acceptable risk level. The health risks associated with non-carcinogenic substances through ingestion were higher than those associated with dermal absorption. Among the non-carcinogenic substances, Ba and Mn posed the greatest health risks. With respect to drinking water exposure, Cr had the highest carcinogenic risk, followed by Pb. Furthermore, Cd had the lowest carcinogenic risk. This study recommended that continuous monitoring of Ba, Mn, and Cr in groundwater should be practiced by assessing the risk of these elements in the Chang-Zhu-Tan urban agglomeration.