Analysis of urea nitrogen leaching under high-intensity rainfall using HYDRUS-1D

Reactive transport of different dissolved organic nitrogen components in an unconfined aquifer

Dissolved organic nitrogen (DON) is often an overlooked form of nitrogen that can leach from the soil into aquifers. The reactive transport and dispersion of DON in aquifers can contribute to regional nitrogen contamination. The current body of research has primarily focused on the vertical leaching process of DON through the vadose zone. However, these studies have largely ignored the broader reactive transport of DON within aquifers under the influence of groundwater flow. In this study, we investigate the reactive transport of DON under groundwater flow conditions. Utilizing molecular biological technologies, we aim to reveal DON's intrinsic role in the nitrogen cycle within aquifers. Our findings reveal that urea exhibits greater mobility compared to amino acids and proteins. The transport of amino acids and proteins reduces the NO3--N concentrations (44.6 % and 89.6 %) compared to the blank control, while urea leads to the accumulation of NO3--N in groundwater (10.1 %). Amino acid and protein columns show higher relative abundances of Pseudomonas (10.1 % and 7.3 %) and Thermomonas (3.9 % and 5.1 %) with denitrification functions, facilitating denitrification in groundwater. Conversely, the presence of urea increases the relative abundances of Nitrosomonadaceae and Nitrophilus (0.33 % and 0.67 %), posing a potential NO3--N contamination risk. Biotransformation has the greatest effect on protein transport (19.6 %), while adsorption mainly influences amino acid transport (12.4 %). The study provides fundamental insights into the reactive transport of different DON components in aquifers, which holds important implications for regional groundwater environment protection.

Microbial mechanisms of interactive climate-driven changes in soil N2O and CH4 fluxes: A global meta-analysis

Soils represent both a source of and sink for greenhouse gases (GHG). Elevated temperature (eT) affects both the physical and biological factors that drive GHG emissions from soil and thus understanding the effects of rising global temperatures on terrestrial GHG emission is needed to predict future GHG emissions, and to identify mitigation strategies. However, uncertainty remains about the interactive effects of multiple climate factors across different ecosystems, complicating our ability to develop robust climate change projections. Therefore, a global meta-analysis of 1337 pairwise observations from 150 peer-reviewed publications (1990-2023) was conducted to assess the individual effect of eT and its combined effects with eCO2 (eT + eCO2), drought (eT + drought) and increased precipitation (eT + ePPT) on soil N2O and CH4 fluxes, microbial functional genes, and soil extracellular enzyme activities across grassland, cropland, and forestland ecosystems. Across the dataset, eT significantly increased N2O emissions (21%) and CH4 uptake (36%). Nitrogen cycling was consistently stimulated by eT, with NO3- and NH4+ and the abundance of amoA-AOB gene increasing by 6%, 10%, and 18%, respectively. Soil water content (SWC) was reduced, whereas increases of 9% in soil organic carbon (SOC), 14% in microbial biomass carbon (MBC), and 10% in total plant biomass were found under eT. The stimulation of soil N2O emissions by eT was maintained for all ecosystems when combined with other global change factors (ie., eT + eCO2, eT + ePPT, and eT + drought). By contrast, effects of eT on CH4 uptake and emissions were more variable when combined with other factors; for instance, eT + eCO2 and eT + ePPT suppressed CH4 uptake in grasslands. This study highlights the urgent need to study the microbial mechanisms responsible for combined global change effects on N2O and especially CH4 fluxes.

What should we do for water security? A technical review on more yield per water drop

Water scarcity is a global challenge. A severe gap between water supply and demand will arise. Consequently, a large part of the world could face water shortage issues in the near future. Increasing cultivated areas and rapid population growth will intensify water consumption, which may lead to a "drink or grow" situation in many countries. Promoting more yield per water drop (MYWD) ideology for water secure development is critically important because the agriculture sector is the largest water consumer by 70%. In lower-middle and low-income countries, water use in agriculture ranges from 80-90%. Advanced water-saving technologies can reduce water consumption by 35-65%, but adoption is less than 20% of the total irrigated area in most countries. Mission 2050 in agricultural countries would be to cover at least 75% and 85% of land under water-saving technologies, which receive surface and groundwater, respectively. The water-saving technologies can decrease farm-scale water consumption, thus alleviating pressure on water resources. In the water scarcity mitigation agenda, the increasing cultivated area under the water-saving technologies should be mapped well since some researchers believe that the water-saving technologies are increasing cultivated area, thereby jeopardizing the "water-saving goal." This comprehensive review navigated the MYWD concept, discussed strategies, sketched a hydrological model to conserve resources, and highlighted economic feasibility, environmental benefits of water-saving technologies, and further improvements. This study can contribute significantly to the future water policy and measures worldwide.

Use of rice straw nano-biochar to slow down water infiltration and reduce nitrogen leaching in a clayey soil

Biochar exhibits numerous advantages in enhancing the soil environment despite a few limitations due to its lower surface energy. Nanomodified biochar combines the advantages of biochar and nanoscale materials. However, its effects on water infiltration and N leaching in a clayey soil remain unclear. Therefore, this study prepared rice straw nano-biochar by a ball milling method, and investigated its physicochemical properties and effects of bulk biochar and nano-biochar at various addition rates (0 %, 0.5 %, 1 %, 2 %, 3 %, and 5 %) on wetting peak migration, cumulative infiltration, water absorption and retention, and N leaching. The results showed that, compared with bulk biochar, nano-biochar presented a more abundant pore structure with an increase in specific surface area of approximately 1.5 times, accompanied by a 20 % increase in acid functional groups. Compared with those for clayey soil without biochar addition, the wetting front migration time was increased by 10.2 %-123.9 % and 17.0 %-257.9 %, and the cumulative infiltration volume at 60 min was decreased by 26.0 %-48.4 % and 14.1 %-62.4 % for bulk biochar and nano-biochar, respectively. The parameter S of Philip model and the parameter a of Kostiakov model for nano-biochar were lower than those for bulk biochar, whereas the parameter b of Kostiakov model was greater, indicating that nano-biochar decreased initial soil infiltration rate and increased attenuation degree of the infiltration rate. Nano-biochar increased water absorption by 8.03 % and subsequently enhanced water retention capacity relative to bulk biochar. In addition, bulk biochar and nano-biochar reduced NH4+-N leaching by 3.0 %-13.1 % and 5.7 %-39.2 %, respectively, and NO3--N leaching by 2.7 %-3.6 % and 9.0 %-43.3 %, respectively, by decreasing N concentration and leachate volume relative to those with no biochar addition. This study provides new knowledge for nano-biochar application in a clayey soil.

High-frequency monitoring during rainstorm events reveals nitrogen sources and transport in a rural catchment

Rural areas lacking essential sewage treatment facilities and collection systems often experience eutrophication due to elevated nutrient loads. Understanding nitrogen (N) sources and transport mechanisms in rural catchments is crucial for improving water quality and mitigating downstream export loads, particularly during storm events. To further elucidate the sources, pathways, and transport mechanisms of N from a rural catchment with intensive agricultural activities during storm events, we conducted an analysis of 21 events through continuous sampling over two rainy seasons in a small rural catchment from the lower reaches of the Yangtze River. The results revealed that ammonia-N (NH4+-N) and nitrate-N (NO3--N) exhibited distinct behaviors during rainstorm events, with NO3--N accounting for the primary nitrogen loss, its load being approximately forty times greater than that of NH4+-N. Through examinations of the concentration-discharge (c-Q) relationships, the findings revealed that, particularly in prolonged rainstorms, NH4+-N exhibited source limited pattern (b = -0.13, P < 0.01), while NO3--N displayed transport limited pattern (b = -0.21, P < 0.01). The figure-eight hysteresis pattern was prevalent for both NH4+-N and NO3--N (38.1% and 52.0%, respectively), arising from intricate interactions among diverse sources and pathways. For NO3--N, the hysteresis pattern shifted from clockwise under short-duration rainstorms to counter-clockwise under long-duration rainstorms, whereas hysteresis remained consistently clockwise for NH4+-N. The hysteresis analysis further suggests that the duration of rainstorms modifies hydrological connectivity, thereby influencing the transport processes of N. These insights provide valuable information for the development of targeted management strategies to reduce storm nutrient export in rural catchments.

Storm-induced nitrogen transport via surface runoff, interflow and groundwater in a pomelo agricultural watershed, southeast China

The storm-induced export of nitrogen (N) from agricultural watersheds significantly impacts aquatic ecosystems, yet the mechanisms of source supply and transport behind N species remain unclear. Here, we investigated the hydrological factors influencing the timing and magnitude of river N species export in a Chinese pomelo agricultural watershed. We conducted continuous observations of watershed hydrology, N species, and their isotopic ratios along a soil-groundwater-river continuum during two storm events in 2018-2019. We found the export flux of river NO3-N covers ∼80% of the total N flux during storms, and the rest for other N species. Our results further revealed distinct pathways and timing of N transport among different N species, especially between ammonium N (NH4-N) and nitrate N (NO3-N). NH4-N in stormflow predominantly originates from sewage and soil leachate, rapidly transported via surface runoff and interflow. Orchard fertilization (contributed 41-56% based on SIAR analysis) was the major source of river NO3-N, which underwent initial dilution via surface runoff and subsequently became enriched through delayed discharge of soil leachate and groundwater. The variations in timing and magnitude of N transport between storms can be explained by antecedent conditions such as precipitation, soil N pools, and storm size. These findings emphasize the hydrological controls on N export from agricultural watersheds, and highlight the variations in source supply and transport pathways among different N species. The insights gained from this study hold significance for managing agricultural pollution and restoring impaired aquatic systems.

Prioritizing ecological restoration in hydrologically sensitive areas to improve groundwater quality

Greening is the optimal way to mitigate climate change and water quality degradation caused by agricultural expansion and rapid urbanization. However, the ideal sites to plant trees or grass to achieve a win-win solution between the environment and the economy remain unknown. Here, we performed a nationwide survey on groundwater nutrients (nitrate nitrogen, ammonia nitrogen, dissolved reactive phosphorus) and heavy metals (vanadium, chromium, manganese, iron, cobalt, nickel, copper, arsenic, strontium, molybdenum, cadmium, and lead) in China, and combined it with the global/national soil property database and machine learning (random forest) methods to explore the linkages between land use within hydrologically sensitive areas (HSAs) and groundwater quality from the perspective of hydrological connectivity. We found that HSAs occupy approximately 20 % of the total land area and are hotspots for transferring nutrients and heavy metals from the land surface to the saturated zone. In particular, the proportion of natural lands within HSAs significantly contributes 8.0 % of the variability in groundwater nutrients and heavy metals in China (p < 0.01), which is equivalent to their contribution (8.8 %) at the regional scale (radius = 4 km, area = 50 km2). Increasing the proportion of natural lands within HSAs improves groundwater quality, as indicated by the significant reduction in the concentrations of nitrate nitrogen, manganese, arsenic, strontium, and molybdenum (p < 0.05). These new findings suggest that prioritizing ecological restoration in HSAs is conducive to achieving the harmony between the environment (improving groundwater quality) and economy (reducing investment in area management).

Nitrogen Application Timing and Levels Affect the Fate and Budget of Fertilizer Nitrogen in the Apple-Soil System

This study aimed to determine the effects of the nitrogen (N) application period and level on the fate of fertilizer N and the contribution of N absorption and translocation to apple organ N. Two N application periods (labeled by the 15N tracer technique in spring and summer, represented by SP and SU, respectively) and three N levels (N0, MN, and HN) were used to determine the physiological indexes and aboveground, root, and soil 15N content of 4-year-old dwarf ('Red Fuji'/M9T337) and arborized ('Red Fuji'/Malus hupehensis Rehd.) apple trees. The results showed that HN led to shoot overgrowth, which was not conducive to the growth of the apple root system (root length, root tips, root surface area, and root volume) or the improvement of root activity. The contribution of soil N to apple organ N accounted for more than 50%, and the contribution of N application in summer to fruit N was higher than that in spring. Under HN treatment, the proportion of soil N absorbed by trees decreased, while that of fertilizer N increased; however, the highest proportion was still less than 50%, so apple trees were highly dependent on soil N. Under MN treatment, fertilizer N residue was similar to soil N consumption, and soil N fertility maintained a basic balance. Under HN treatment, fertilizer N residue was significantly higher than soil N consumption, indicating that excessive N application increased fertilizer N residue in the soil. Overall, the 15N utilization rate of arborized trees (17.33-22.38%) was higher than that of dwarf trees (12.89-16.91%). A total of 12.89-22.38% of fertilizer 15N was absorbed by trees, 30.37-35.41% of fertilizer 15N remained in the soil, and 44.65-54.46% of fertilizer 15N was lost. The 15N utilization rate and 15N residual rate of summer N application were higher than those of spring N application, and the 15N loss rate was lower than that of spring N application. High microbial biomass N (MBN) may be one of the reasons for the high N utilization rate and the low loss rate of N application in summer.

Unveiling the dynamic of nitrogen through migration and transformation patterns in the groundwater level fluctuation zone of a different hyporheic zone sediment

This study investigates the impact of water levels and soil texture on the migration and transformation of nitrate (NO3--N) and ammonium (NH4+-N) within a soil column. The concentrations of NO3--N gradually decreased from an initial concentration of 34.19 ± 0.86 mg/L to 14.33 ± 0.77 mg/L on day 70, exhibiting fluctuations and migration influenced by water levels and soil texture. Higher water levels were associated with decreased NO3--N concentrations, while lower water levels resulted in increased concentrations. The retention and absorption capacity for NO3--N were highest in fine sand soil, followed by medium sand and coarse sand, highlighting the significance of soil texture in nitrate movement and retention. The analysis of variance (ANOVA) confirmed statistically significant variations in pH, dissolve oxygen and oxidation-reduction potential across the soil columns (p < 0.05). Fluctuating water levels influenced the migration and transformation of NO3--N, with distinct patterns observed in different soil textures. Water level fluctuations also impacted the migration and transformation of NH4+-N, with higher water levels associated with increased concentrations and lower water levels resulting in decreased concentrations. Among the soil types considered, medium sand exhibited the highest absorption capacity for NH4+-N. These findings underscore the significant roles of water levels, soil texture, and soil type in the migration, transformation, and absorption of nitrogen compounds within soil columns. The results contribute to a better understanding of nitrogen dynamics under varying water levels and environmental conditions, providing valuable insights into the patterns of nitrogen migration and transformation in small-scale soil column experiments.

Simulating arsenic discharge flux at a relic smelting site in Guangxi Zhuang Autonomous Region, China

Anthropogenic groundwater arsenic (As) pollution is common in many aquifers in Southwest China. It is concerned that long-term random disposal of As smelting slag could induce the transport of high-As groundwater into previously uncontaminated aquifers. Here, we used HELP-MODFLOW-MT3DMS model simulations to integrate the percolation, groundwater flow, and solute transport processes at an aquifer at site scale, constrained by weather, hydrogeology, and monitoring data. Our simulations provide a new method framework of the simulated percolation by HELP model and have induced As spatiotemporal distribution in the aquifer. According to the HELP model simulation results, percolation volume accounts for 24% of rainfall over 18 years. This work determined that the As discharge trend was fitted by double-constants kinetics based on the leaching experiment. And this work calculates total mass distribution of As in the aquifer over 18 years. We have found that the sustained As pollution relies on the rainfall that acts as the primary contributor of elevated As concentrations. Model simulation results suggest that 51.70% of the total As mass (1.96 × 104 kg) was fixed in low permeability solid media. The total As mass discharged into groundwater reached 9.3 × 103 kg, accounting for 24.68%. The accumulative outflow mass of arsenic was 8.0 × 103 kg, accounting for 21.62%.

Improving runoff quality in vertical greenery systems: Substrate type outweighed the effect of plant growth promoting microbes

Due to limited urban green spaces and catchments, researchers are exploring the capacity of vertical greenery systems (VGSs) in stormwater management as complementary strategies. While the literature acknowledges the significant impacts of vegetated roof substrates on stormwater, comparing the stormwater management capacities of organic and non-organic substrates for VGSs remains largely unexplored. It is thus essential to gather empirical evidence to enhance the stormwater management capacity of VGSs. Here, we report on the impact of installation factors (substrate type and plant growth-promoting microbe (PGPM) inoculation) and environmental factors (simulated rainwater quantity and substrate moisture) of an innovative VGS on the concentrations and total loads of 15 elements (N, P, Al, V, Cr, Fe, Mn, Co, Ni, Cu, Zn, As, Se, Cd, and Pb) in the runoff. Results showed that substrate type was the most influential factor: concentrations and total loads were significantly higher from a reed-based substrate with high organic matter than from a sandy loam substrate. Substrate type also had profound interactive effects with other factors. For instance, PGPM inoculation significantly reduced the total loads of As, Cr, N, Ni, and Se, regardless of substrate type, and reduced the total loads of Cd, Co, Cu, Fe, Mn, and Pb in the reed-based substrate only. In addition, PGPM inoculation primarily reduced total loads, yet had little effect on concentrations. Substrate type also interacted with simulated rainwater quantity and substrate moisture: for example, in the reed-based substrate, a higher simulated rainwater quantity reduced concentrations but increased total loads, while concentrations and total loads remained constantly low from the sandy loam substrate under various simulated rainwater quantities. High antecedent substrate moisture increased both concentrations and total loads for most of the elements. We conclude that leaching from VGSs can be contained via substrate selection, maintenance of substrate moisture, and beneficial PGPM inoculation.

Impact of extreme rainfall on non-point source nitrogen loss in coastal basins of Laizhou Bay, China

Extreme rainfalls often lead to large amounts of nitrogen (N) loss from river basins. However, the composition and spatial variation of N loss caused by extreme events and the effects of control measures are not well understood. To shed light into this question, the Soil and Water Assessment Tool (SWAT) was used to evaluate the spatiotemporal characteristics of organic and inorganic nitrogen (ON and IN) losses in the coastal basins of Laizhou Bay during typhoons Rumbia and Lekima. The effects of best management practices on controlling N loss were also explored during such extreme rainfall events. Results showed that extreme rainfall promoted transport of ON more than IN. The mass of ON and IN transported by the two typhoons exceeded 57 % and 39 % of the average annual N flux, respectively, and the loads were positively correlated with streamflow. During the two typhoons, the loss of ON was mainly concentrated in areas with steep slopes (θ > 15°) and natural vegetation (forests, grasslands, and shrublands). The IN loss was higher in areas with a 5-10° slope. Furthermore, subsurface flow was the main IN transport mechanism in areas with steep slope (θ > 5°). Simulations showed that implementation of filter strips in areas with slopes exceeding 10° can reduce N loss, with much greater reductions in ON (>36 %) than IN (>0.3 %). This study provides important insights into N loss during extreme events and the key role filter strips can play in trapping them before they reach downstream waterbodies.

Experimental and Simulation Research on the Process of Nitrogen Migration and Transformation in the Fluctuation Zone of Groundwater Level

The fluctuation of groundwater causes a change in the groundwater environment and then affects the migration and transformation of pollutants. To study the influence of water level fluctuations on nitrogen migration and transformation, physical experiments on the nitrogen migration and transformation process in the groundwater level fluctuation zone were carried out. A numerical model of nitrogen migration in the Vadose zone and the saturated zone was constructed by using the software HydrUS-1D. The correlation coefficient and the root mean square error of the model show that the model fits well. The numerical model is used to predict nitrogen migration and transformation in different water level fluctuation scenarios. The results show that, compared with the fluctuating physical experiment scenario, when the fluctuation range of the water level increases by 5 cm, the fluctuation range of the nitrogen concentration in the coarse sand, medium sand and fine sand media increases by 37.52%, 31.40% and 21.14%, respectively. Additionally, when the fluctuation range of the water level decreases by 5 cm, the fluctuation range of the nitrogen concentration in the coarse sand, medium sand and fine sand media decreases by 36.74%, 14.70% and 9.39%, respectively. The fluctuation of nitrogen concentration varies most significantly with the amplitude of water level fluctuations in coarse sand; the change in water level has the most significant impact on the flux of nitrate nitrogen and has little effect on the change in nitrite nitrogen and ammonium nitrogen, and the difference in fine sand is the most obvious, followed by medium sand, and the difference in coarse sand is not great.