Shallow groundwater fluctuation: An ignored soil N loss pathway from cropland

Importance of isotope fractionation in SIAR model for quantifying NO3- sources in groundwater of China

Quantifying NO3- sources by the combination of dual nitrate isotopes (δ15N-NO3- and δ18O-NO3-) with Stable Isotope Analysis in R (SIAR) models is crucial for mitigating NO3- pollution in groundwater. However, isotope fractionation effects during denitrification lead to significant uncertainties when quantifying groundwater NO3- sources using the SIAR model. In this study, hydrochemical data, water isotopes (δD-H2O and δ18O-H2O), and dual nitrate isotopes of groundwater at the West Lake watershed, East China were measured to estimate the isotope fractionation effect of denitrification in groundwater and assess its impact on quantifying NO3- source contributions using the SIAR model. The significant spatial (εN: -6.9 ‰ and εO: -3.1 ‰ in G1; εN: -15.1 ‰ and εO: -10.0 ‰ in G2) and temporal (εN: -17.0 ‰ and εO: -4.1 ‰ in spring; εN: -4.9 ‰ and εO: -2.5 ‰ in summer; εN: -7.2 ‰ and εO: -6.0 ‰ in autumn) variations in isotope fractionation effects of denitrification in groundwater at the West Lake watershed were observed. By incorporating these respective isotope fractionation enrichment factors into the SIAR model, more accurate NO3- source apportionments for G1 and G2 were obtained, confirming that the isotope fractionation effect of denitrification is an important parameter for quantifying NO3- sources in groundwater using the SIAR model. Furthermore, the national δ15N-NO3- and δ18O-NO3- observations of groundwater were compiled and the SIAR model integrated with isotope fractionation effect of denitrification were used to quantify NO3- sources in groundwater of China. It was found that regional differences in human activities directly influenced spatial variations of δ15N-NO3- and δ18O-NO3- values. The SIAR model outputs on a national scale revealed that sewage/manure (22.9-42.1 %) and chemical fertilizers (23.0-42.7 %) were the main NO3- sources in groundwater of China, attributable to large populations and extensive agricultural cultivation areas. These results provide direct evidence for formulating suitable policies and measures to control and reduce groundwater NO3- in China.

Integrating stable isotopes and hydrological models to track nitrogen sources and transport pathways in plateau watersheds: a case study in Southwest China

Exogenous nitrogen inputs from agriculture and anthropogenic activities have dramatically altered the material cycling processes in the Plateau watershed, leading to a range of water pollution issues. Effective management of nitrogen pollution in water bodies is predicated on clarifying N export loads under different pathways in the watershed, as well as the contributions of different sources. Here, this study proposes an integrated framework that introduces multiple stable isotope techniques (δD-H2O, δ18O-H2O, δ15N-NO3- and δ18O-NO3-) based on the coupled Eckhardt's digital baseflow filter (ECK) and load estimation model (LOADEST). The integrated approach was applied for the first time in a typical plateau watershed in southwest China. Results showed that baseflow as the Fengyu River watershed (FRW) major hydrologic pathway, provides 69.6 % of the mean annual stream flow and 59.1 % of the mean annual NO3--N load. Furthermore, the FRW average annual TN and NO3--N export is 94.0 t and 55.1 t, respectively. The NO3--N was the primary form of N pollutant, with its average concentration in groundwater being 7 times that in river water. In groundwater, manure and sewage (M&S) and soil nitrogen (SN) contribution rates to NO3--N 53.6 %, and 37.2 %, respectively. While the river water shows low M&S (26.8 %) and high SN (61.5 %) characteristics. It can be seen that the baseflow is a key pathway for coupled water-nitrogen export from plateau agricultural watersheds. Blocking the migration of nitrogen-containing pollutants to groundwater is an important measure to control the degradation of the water environment in plateau watersheds from the root cause.

Shallow groundwater table fluctuations: A driving force for accelerating the migration and transformation of phosphorus in cropland soil

The accumulation of phosphorus (P) in soil profiles of intensive cropland and the losses caused by runoff and leaching have been widely concerned. However, the loss of soil P due to shallow groundwater table (SGT) fluctuations driven by seasonal changes is often neglected, and the migration and transformation mechanisms of soil P are still unclear. On the basis of the long-term monitoring of cropland soil P accumulation and SGT fluctuations around Erhai Lake, the characteristics of soil P loss driven by SGT fluctuations and the corresponding mechanisms were investigated through a 260-day microcosm experiment. The results revealed that the fluctuations in SGT significantly changed the content and form of P in the soil profile. The soil P loss mainly occurred in dissolved form, mainly involving inorganic P, accounting for 75 %. Compared with those under continuous saturated conditions, soil total P (TP) release during SGT fluctuations significantly increased by 9.5 %, and soil TP storage was reduced by 2 %. SGT fluctuations increased the complexity of microbial networks in the soil profile, stimulated the expression of functional genes for soil P cycling, and promoted soil organic P mineralization. The SGT fluctuations caused an increase in the soil TP loss from cropland to 88.5 kg/ha, which was 70 and 25 times greater than that via leaching and runoff, respectively. These results indicated that SGT fluctuations accelerated the P loss from soil profile of cropland. Therefore, some measures should be comprehensively applied to prevent its loss, such as reducing external P input, improving surface soil P storage capacity and soil P utilization efficiency, reducing surface P leaching into deep soil, and reducing P accumulation in deep soil profiles.

Mitigating soil nitrification and greenhouse gas emissions in non-paddy cropping systems by micro-molar hydrogen peroxide

Non-paddy cropping systems play a significant role in food production. However, excessive nitrogen loss from non-paddy soils through nitrate leaching and ammonia volatilization poses a significant challenge to environmental sustainability. In this study, microcosm and field-scale experiments were conducted to explore the potential for using hydrogen peroxide (H2O2) to mitigate nitrogen loss and greenhouse gas emissions, aiming at filling gaps in knowledge regarding the underlying biochemical mechanisms. The results show that input of micromolar H2O2 from either artificial addition or natural rainwater into soils in the presence of magnetite (Fe3O4) could trigger Fenton-like reaction, which inhibited microbially mediated nitrification of soil-borne ammonium but did not affect the growth of the test crop plant (water spinach). In the absence of Fe3O4, input of rainwater-borne H2O2 into non-paddy soils caused reduction in the emissions of nitrous oxide (N2O) and carbon dioxide (CO2). There was a trend showing that the degree of reduction in N2O and CO2 fluxes increased with increasing concentration of rainwater-borne H2O2. It was likely that microbially mediated reduction of iron oxides took place during rainfall events, providing Fe(II) that is needed for reaction with rainwater-borne H2O2, triggering Fenton-like reaction to inhibit the soil microbes that mediate production of N2O and CO2 in the soils. The findings obtained from this study have implications for developing strategies to manage soil‑nitrogen to minimize its environmental impacts.

Shallow groundwater table fluctuations weaken nitrogen accumulation in the thin layer vadose zone of cropland around plateau lakes, Southwest China

Excessive accumulation of nitrogen (N) in the soil profile in the intensive agricultural region will seriously threaten groundwater quality and safety. However, the impact of shallow groundwater table (SGWT) fluctuations driven by seasonal variations on the N accumulation characterizations in the soil profiles has not been well quantified, particularly in the regions with thin layer vadose zone. Through in-situ monitoring and simulation experiments, the changes in the SGWT and N accumulation of soil profile in intensive cropland around 7 plateau lakes in Yunnan were studied during the rainy season (RS) and dry season (DS), and the N loss in soil profile of cropland driven by SGWT fluctuations was estimated. The results showed that the SGWT and N accumulation in soil profile of cropland around the plateau lakes had obvious seasonal variation characteristics. The proportion of N storage in different forms in 60-100 cm soil layer in the RS was greater than that in the DS, particularly the proportion of NH4+-N storage was as high as 55 %, while N accumulation in surface soil was obvious in the DS. Compared with the DS, due to the rising SGWT in the RS, the maximum storages of TN and NO3--N in the 0-100 cm soil layer decreased by17% and 36 %, respectively. The TN loss intensities from the 0-100 cm soil profiles of cropland around Fuxian Lake, Yilong Lake, Qilu Lake, Dianchi Lake, Yangzong Lake, Erhai Lake, and Xingyun Lake were 74, 54, 127, 105, 93, 72 and 207 kg/ha, respectively. Moreover, if the SGWT was <30 cm, the average TN loss intensity and amount could reach 177 kg/ha and 1250 t, respectively. Therefore, the SGWT regulation was one of the key measures to reducing soil N loss from the thin layer vadose zone of cropland around plateau lakes and improving groundwater quality.

Effect of metal-modified sewage sludge biochar tubule on immobilization of chromium in unsaturated soil: Groundwater table fluctuations induced by rainfall

Many studies have studied biochar immobilizing chromium (Cr) in soil. However, few studies were conducted to reduce the environmental risks due to biochar aging in soil. In this study, we adopt FeCl3, MgCl2, and AlCl3 to activate sewage sludge to form modified biochar and produce biochar tubules. Then, the column experiments were carried out to study the effect of fluctuating groundwater table on Cr leaching behavior, total Cr, and fractions distribution with the insertion of biochar tubule. Results showed that the Cr immobilization performance was improved by metal-modification biochar, the biochar tubules can significantly decrease the Cr leaching amounts, retard the Cr downward migration in the soil, and there was a better effect with slightly Cr-contaminated soil. In addition, the immobilization effect is also impacted by the biochar's application mode and the hydrodynamic conditions. Detailedly, the Cr leaching amounts maximally decreased by 33.39%, the residual amounts maximally increased by 10.05% in the soil column, and the exchangeable (EX) and carbonates-bound (CB) fractions were maximally increased by 85.18%, 151.78% at the equal depth of soil column, respectively. BET, SEM-EDS, XRD, and FTIR analyses revealed that biochars' immobilization mechanisms on Cr involved reduction(predominately), physisorption, precipitation, and complexation. Risk assessment demonstrated that the sewage sludge biochar has very low environmental risk. This study indicates that the biochar tubule applied to immobilize Cr in soil has potential and provides new insights into reducing environmental risks due to biochar aging.

A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

The groundwater table fluctuation (GTF) zone is an important medium for the hydrologic cycle between unsaturated soil and saturated aquifers, which accelerates the migration, transformation, and redistribution of contaminants and further poses a potential environmental risk to humans. In this review, we clarify the key processes in the generation of the GTF zone and examine its links with the variation of the hydrodynamic and hydrochemistry field, colloid mobilization, and contaminant migration and transformation. Driven by groundwater recharge and discharge, GTF regulates water flow and the movement of the capillary fringe, which further control the advection and dispersion of contaminants in soil and groundwater. In addition, the formation and variation of the reactive oxygen species (ROS) waterfall are impacted by GTF. The changing ROS components partially determine the characteristic transformation of solutes and the dynamic redistribution of the microbial population. GTF facilitates the migration and transformation of contaminants (such as nitrogen, heavy metals, non-aqueous phase liquids, and volatile organic compounds) through colloid mobilization, the co-migration effect, and variation of the hydrodynamic and hydrochemistry fields. In conclusion, this review illustrates the limitations of the current literature on GTF, and the significance of GTF zones in the underground environment is underscored by expounding on the future directions and prospects.

Unveiling the biogeochemical mechanism of nitrate in the vadose zone-groundwater system: Insights from integrated microbiology, isotope techniques, and hydrogeochemistry

Clarifying the biogeochemical mechanism of nitrate (NO3-) in the vadose zone-groundwater system, particularly in agricultural contexts, is crucial for mitigating groundwater NO3- pollution. However, comprehensive studies on the impacts of changes in chemical indicators and microbial communities on NO3- are still lacking. This paper aims to address this gap by employing hydrogeochemistry, stable isotopes, and microbial techniques to assess the NO3- biogeochemical processes in the vadose zone-groundwater system. The findings suggested that NO3- in upper soil layers was primarily influenced by plant root absorption, assimilation, and nitrification processes. The oxygen contents gradually decreased with the nitrification process, resulting in the occurrence of the denitrification. However, denitrification predominantly occurred in the 60-80 cm soil layer in the study area. The limited thickness of the denitrification layer results in less NO3- consumption, leading to increased NO3- leaching into groundwater. Hydrochemical and isotopic characteristics further indicated that groundwater NO3- concentrations were mainly controlled by nitrification, followed by denitrification and mixing processes. The 16S rRNA sequencing analysis revealed great influences of soil sampling depths and groundwater NO3- concentrations on the microbial community structure. Additionally, the PICRUSt2-based prediction results demonstrated a stronger potential for dissimilatory reduction of NO3- to ammonium (DNRA) in both soil and groundwater compared to the other processes, potentially due to the widespread presence of the nrfH functional genes. However, the chemical indicators and isotopes used in this study did not support the occurrence of DNRA process in the vadose zone-groundwater system. This finding highlights the importance of an integrated approach combining microbiological, isotopic, and hydrogeochemical data to comprehensive understanding biogeochemical processes. The study developed a conceptual model elucidating the NO3- biogeochemical processes in the vadose zone-groundwater system within an agricultural area, contributing to enhanced comprehension and advancement of sustainable management practices for groundwater nitrogen.

Sources and hydrogeochemical processes of groundwater under multiple water source recharge condition

Ecological water replenishment (EWR) is an essential approach for improving the quantity and quality of regional water. The Chaobai River is a major river in Beijing that is replenished with water from multiple sources, including reclaimed water (RW), the South-North Water Transfer Project (SNTP), reservoir discharge (RD). The effects of multiple water source recharge (MWSR) on groundwater quality remain unclear. In this study, hydrochemical ions, isotopes (δ2H-H2O, δ18O-H2O, δ15N-NO3-, and δ18O-NO3-), mixing stable isotope analysis in R (MixSIAR), and hydrogeochemical modeling were used to quantify the contributions and impacts of different water sources on groundwater and to propose a conceptual model. The results showed that during the period before reservoir discharge, RW and SNTP accounted for 38 %-41 % and 54 % of the groundwater in their corresponding recharge areas, respectively. The groundwater in the RW recharge area contained high levels of Na+ and Cl- leading to the precipitation of halite, and was the main factor for the spatial variation in groundwater hydrochemical components. The surface water changed from Na·K - Cl·SO4 type to Ca·Mg - HCO3 type which was similar to groundwater after reservoir discharge. RD accounted for 30 % of the groundwater; however, it did not change the hydrochemical type of groundwater. Dual nitrate stable isotopes and MixSIAR demonstrated that RW was the primary source of NO3- in groundwater, contributing up to 76-89 %, and reservoir discharge effectively reduced the contribution of RW. δ15N-NO3- or δ18O-NO3- in relation to NO3-N suggests that denitrification is the main biogeochemical process of nitrogen in groundwater, whereas water recharge from the SNTP and RD reduces denitrification and dilutes NO3-. This study provides insights into the impact of anthropogenically controlled ecological water replenishment from different water sources on groundwater and guides the reasonable allocation of water resources.

Quantify the effects of groundwater level recovery on groundwater nitrate dynamics through a quasi-3D integrated model for the vadose zone-groundwater coupled system

Groundwater level (GWL) recovery in some semiarid regions, attributed to mitigation countermeasures for groundwater depletion, potentially causes nitrate accumulated in the vadose zone to be introduced into the aquifer. However, the extent to which GWL recovery affects interactions between the vadose zone and saturated aquifers, migration pathways of soil nitrogen and groundwater nitrate dynamics have not been explicitly determined. This study established a quasi-3D feedback model for the vadose zone-groundwater coupled system in a typical GWL recovery area and quantitatively evaluated the effects of GWL recovery on nitrate-N leaching fluxes via the vadose zone and groundwater nitrate-N dynamics. Within the framework of the integrated model, both the water/contaminant leaching fluxes and the depth to groundwater were exchanged at each flow time step. The obtained results reveal that the temporal changes in nitrate-N leaching fluxes depended on the behaviors of precipitation, farmland irrigation and lithology of the vadose zone, while its spatial patterns were determined by both the GWL undulation and the vertical profiles of nitrate-N content. Furthermore, the GWL recovery caused the magnitude of the nitrate-N leaching fluxes into the aquifer to increase by 44.4%. Along with the GWL recovery, the phreatic aquifer volume increased by 7.47%, and the nitrate-N mass herein increased by 40.06%, which was largely driven by the nitrate-N leaching flux. Consequently, the average groundwater nitrate-N concentration in the GWL recovery region increased by approximately 2.4 mg/L, apart from the artificial recharge route. This finding suggests that the intensified leaching of soil contaminants, given the circumstances of GWL recovery, has a negative effect on groundwater quality. An appropriate groundwater management scheme is therefore urgently required to achieve an optimal balance between GWL recovery and groundwater environment.