Spatiotemporal distribution and controlling factors on ammonium in waters in the central Yangtze River Basin, China

High levels of ammonium in water can compromise the ecological environment and be harmful to human beings. It is of great significance to understand the source and controlling factors of ammonium in waters. However, the distribution and controlling factors on ammonium in the central Yangtze River Basin have been rarely reported. The results showed that 6.58% of the surface water (SW) exceeded the China national guideline of category III for NH4+-N (i.e., 1.0 mg/L) and 30.19% of the groundwater (GW) exceeded the China national guideline of category III for NH4+-N (i.e., 0.5 mg/L). Notably, the ammonium concentrations of the plain area in the middle were much higher, which reached to the highest value at the junction of the Yangtze River and Dongting Lake. Nitrogen in SW may originate from manure but more nitrogen sources in GW. The net anthropogenic nitrogen input (NANI) can provide enough organic nitrogen for the mineralization. NH4+-N in SW was more affected by fertilizer nitrogen and feed nitrogen input but more affected by agricultural nitrogen fixation in GW. Agricultural and industrial activities controlled NH4+-N in SW and GW by increasing nitrogen input and changing hydrological conditions. In general, this research exposed the controlling of different types of factors on ammonium in waters, providing a guidance for the water pollution prevention in study area.

Identification of nitrate sources of groundwater and rivers in complex urban environments based on isotopic and hydro-chemical evidence

Groundwater and rivers in Chinese cities suffer from severe nitrate pollution. The accurate identification of nitrate sources throughout aquatic systems is key to the water nitrate pollution management. This study investigated nitrogen components of groundwater for twelve years and analyzed the sources of nitrate in the aquatic system based on dual isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in the city of Nanjing, a core city of the Yangtze River Delta region, China. Our results showed that the ratio of nitrate to the sum of ammonia and nitrate in groundwater show an increasing trend during 2010-2021. The nitrate concentration was positively correlated with the proportion of cultivated land and negatively correlated with the proportion of forest land in the buffer zone. The relationship between Cl^- and NO₃^-/ Cl^- showed that agriculture and sewage sources increased during 2010-2015, sewage sources increased during 2016-2018, agriculture sources increased during 2019-2021. Manure and sewage were the primary sources of groundwater nitrate (72 %). There was no significant difference between the developed land (78 %), cultivated land (69 %), and aquaculture area (72 %). This indicates that dense population and intensive aquaculture in the suburbs have a significant impact on nitrate pollution. The contributions of manure and sewage to the fluvial nitrate sources in the lower reaches of the Qinhuai River Basin were 61 %. The non-point sources, including groundwater N (39 %) and soil N (35 %), were 74 % over the upper reaches. This study highlights the necessity of developing different N pollution management strategies for different parts of highly urbanized watersheds and considers groundwater restoration and soil nitrogen management as momentous, long-term tasks.

Assessing the changes in river water quality across a land-use change (forest to oil palm plantation) in peninsular Malaysia using the stable isotopes of water and nitrate

Land conversion from natural forests to plantations (e.g., oil palm) in Southeast Asia is one of the most intensive land-use changes occurring worldwide. To clarify the effects of oil palm plantations on water quality, we conducted multipoint river and stream water sampling in peninsular Malaysia at the end of the rainy season over a 3-year period (2013-2015). We measured the major dissolved ions and stable isotope ratios of water (δ²H-H₂O and δ¹⁸O-H₂O) and nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in water from the upper streams in mountainous forests to the midstream areas of two major rivers in peninsular Malaysia. The electrical conductivity increased, and the d-excess value (as an index of the degree of evaporation) decreased with increasing distance from the headwaters, suggesting the effect of evaporative enrichment and the addition of pollutants. We separated the sampling points into four groups (G1-G4) through cluster analysis of the water quality data. From the land use/land cover (LULC) classification maps developed from satellite images and local information, we found that G1 and G2 mainly consisted of sampling points in forested areas, while G3 and G4 were located in oil-palm-affected areas. The concentrations of major ions were higher in the oil palm areas, indicating the effects of fertilizer and limestone (i.e., pH adjustment) applications. The dissolved inorganic nitrogen concentration did not differ among the groups, but the dissolved organic carbon, total dissolved nitrogen, and δ¹⁵N-NO₃^- were higher in the oil palm area than in the forested area. Although the nitrogen concentration was low, even in the oil palm area, the significantly higher δ¹⁵N-NO₃^- in the oil palm area indicated substantial denitrification. This implies that denitrification contributed to the lowering of the NO₃^- concentration in rivers in the oil palm area, in addition to nutrient uptake by oil palm trees.

Modeling the effects of water diversion projects on surface water and groundwater interactions in the central Yangtze River basin

Due to the lack of the quantification of surface water (SW) and groundwater (GW) interaction, the chemicals transport and fate and wetland evolution are hard to predict under impact of both the natural condition and water diversion projects. To address this issue, a 3D regional numerical model is proposed in this study to analyze the effects of the South-to-North Water Diversion (SNWD) and Yangtze-Hanjiang Water Diversion (YHWD) projects on groundwater flow regimes and SW-GW interactions of Jianghan Plain in the central Yangtze River basin. The model results show that the Yangtze River and groundwater interactive pattern varied little, whereas the exchange capacity has been significantly affected by the SNWD but little affected by the YHWD. If only implemented SNWD project, the Hanjiang River and groundwater interactive pattern varied and the net exchange rate between the Hanjiang River and groundwater decreased by 69.3% compared to natural condition. Since YHWD was introduced to complement SNWD, the net exchange rate has been reduced by 25.3% compared with that under the only SNWD. SNWD and YHWD projects implementation caused the decrease of the groundwater level along the Yangtze River with the maximum value of 0.19 m but the increase of groundwater level along the Hanjiang River with the maximum rise reaching up to 0.78 m. This study provides the insights for quantification of GW-SW interaction at regional scale, which will benefiting for integrated water resource management and understanding contaminant reactive transport and wetland evolution in the central Yangtze River basin.

Isotopic evidence for nitrate sources and controls on denitrification in groundwater beneath an irrigated agricultural district

The application of N fertilisers to enhance crop yield is common throughout the world. Many crops have historically been, or are still, fertilised with N in excess of the crop requirements. A portion of the excess N is transported into underlying aquifers in the form of NO₃^-, which is potentially discharged to surface waters. Denitrification can reduce the severity of NO₃^- export from groundwater. We sought to understand the occurrence and hydrogeochemical controls on denitrification in NO₃^--rich aquifers beneath the Emerald Irrigation Area (EIA), Queensland, Australia, a region of extensive cotton and cereal production. Multiple stable isotope (in H₂O, NO₃^-, DIC, DOC and SO₄^2-) and radioactive isotope (³H and ³⁶Cl) tracers were used to develop a conceptual N process model. Fertiliser-derived N is likely incorporated and retained in the soil organic N pool prior to its mineralisation, nitrification, and migration into aquifers. This process, alongside the near absence of other anthropogenic N sources, results in a homogenised groundwater NO₃^- isotopic signature that allows for denitrification trends to be distinguished. Regional-scale denitrification manifests as groundwater becomes increasingly anaerobic during flow from an upgradient basalt aquifer to a downgradient alluvial aquifer. Dilution and denitrification occurs in localised electron donor-rich suboxic hyporheic zones beneath leaking irrigation channels. Using approximated isotope enrichment factors, estimates of regional-scale NO₃^- removal ranges from 22 to 93% (average: 63%), and from 57 to 91% (average: 79%) beneath leaking irrigation channels. In the predominantly oxic upgradient basalt aquifer, raised groundwater tables create pathways for NO₃^- to be transported to adjacent surface waters. In the alluvial aquifer, the transfer of NO₃^- is limited both physically (through groundwater-surface water disconnection) and chemically (through denitrification). These observations underscore the need to understand regional- and local-scale hydrogeological processes when assessing the impacts of groundwater NO₃^- on adjacent and end of system ecosystems.

Hydrogeological controls on ammonium enrichment in shallow groundwater in the central Yangtze River Basin

The controlling processes of excessive ammonium in surface water and groundwater in the central Yangtze River Basin remain unclear. In this study, monitoring of water levels and temporal-spatial distributions of major N compounds were implemented at the large Jiangshan plain and at the local site scale in the central Yangtze River Basin. The results indicate that the recharge, movement and transformation of ammonium were controlled by hydrogeological conditions. Manure and sewage from anthropogenic activities were identified as the main source of nitrogen compounds. The nitrogen loading into aquifers were governed by water table and groundwater flow. After entering subsurface soils, nitrification and dissimilatory nitrate reduction to ammonium (DNRA) were proposed as the ammonium consumption and production mechanisms, respectively, by combining the concentrations of ammonium‑nitrogen and nitrate‑nitrogen with the corresponding isotopic compositions. These microbially mediated processes controlling transport and transformation of nitrogen compounds were influenced by the seasonally varying groundwater flow regime that changed the redox conditions in the aquifers. In the subsurface environments, ammonium was converted to nitrate when sufficient oxygen supply was available, and this process was reversed under anoxic conditions along the groundwater flow path. A conceptual model for the reactive transport of nitrogen compounds jointly controlled by the vertical groundwater flows and biogeochemical processes was proposed, which provides new insights into the genesis of high ammonium groundwater.

Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi-site dual nitrate isotopes

Increasing nitrogen (N) deposition in subtropical forests in south China causes N saturation, associated with significant nitrate (NO₃^- ) leaching. Strong N attenuation may occur in groundwater discharge zones hydrologically connected to well-drained hillslopes, as has been shown for the subtropical headwater catchment "TieShanPing", where dual NO₃^- isotopes indicated that groundwater discharge zones act as an important N sink and hotspot for denitrification. Here, we present a regional study reporting inorganic N fluxes over two years together with dual NO₃^- isotope signatures obtained in two summer campaigns from seven forested catchments in China, representing a gradient in climate and atmospheric N input. In all catchments, fluxes of dissolved inorganic N indicated efficient conversion of NH₄⁺ to NO₃^- on well-drained hillslopes, and subsequent interflow of NO₃^- over the argic B-horizons to groundwater discharge zones. Depletion of ¹⁵ N- and ¹⁸ O-NO₃^- on hillslopes suggested nitrification as the main source of NO₃^- . In all catchments, except one of the northern sites, which had low N deposition rates, NO₃^- attenuation by denitrification occurred in groundwater discharge zones, as indicated by simultaneous ¹⁵ N and ¹⁸ O enrichment in residual NO₃^- . By contrast to the southern sites, the northern catchments lack continuous and well-developed groundwater discharge zones, explaining less efficient N removal. Using a model based on ¹⁵ NO₃^- signatures, we estimated denitrification fluxes from 2.4 to 21.7 kg N ha^-1 year^-1 for the southern sites, accounting for more than half of the observed N removal. Across the southern catchments, estimated denitrification scaled proportionally with N deposition. Together, this indicates that N removal by denitrification is an important component of the N budget of southern Chinese forests and that natural NO₃^- attenuation may increase with increasing N input, thus partly counteracting further aggravation of N contamination of surface waters in the region.

Unprocessed Atmospheric Nitrate in Waters of the Northern Forest Region in the U.S. and Canada

Little is known about the regional extent and variability of nitrate from atmospheric deposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers (δ¹⁸O and δ¹⁵N) of nitrate sources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitrate was transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrate during stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric deposition variation among storms and a near complete lack of monitoring for these events. A general lack of observance may bias perceptions of occurrence; sustained monitoring of chronic nitrogen pollution effects on forests with nitrate source apportionments may offer insights needed to advance the science as well as assess regulatory and management schemes.

Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification

Measurements of the stable isotope ratios of nitrogen (¹⁵N/¹⁴N) and oxygen (¹⁸O/¹⁶O) in nitrate (NO₃^-) enable identification of sources, dispersal, and fate of natural and contaminant NO₃^- in aquatic environments. The ¹⁸O/¹⁶O of NO₃^- produced by nitrification is often assumed to reflect the proportional contribution of oxygen atom sources, water, and molecular oxygen, in a 2:1 ratio. Culture and seawater incubations, however, indicate oxygen isotopic equilibration between nitrite (NO₂^-) and water, and kinetic isotope effects for oxygen atom incorporation, which modulate the NO₃^{- 18}O/¹⁶O produced during nitrification. To investigate the influence of kinetic and equilibrium effects on the isotopic composition of NO₃^- produced from the nitrification of ammonia (NH₃), we incubated streamwater supplemented with ammonium (NH₄⁺) and increments of ¹⁸O-enriched water. Resulting NO₃^{- 18}O/¹⁶O ratios showed (1) a disproportionate sensitivity to the ¹⁸O/¹⁶O ratio of water, mediated by isotopic equilibration between water and NO₂^-, as well as (2) kinetic isotope discrimination during O atom incorporation from molecular oxygen and water. Empirically, the NO₃^{- 18}O/¹⁶O ratios thus produced fortuitously converge near the ¹⁸O/¹⁶O ratio of water. More elevated NO₃^{- 18}O/¹⁶O values commonly reported in soils and oxic groundwater may thus derive from processes additional to nitrification, including NO₃^- reduction.

Application of Nitrogen and Oxygen Isotopes for Source and Fate Identification of Nitrate Pollution in Surface Water: A Review

Nitrate pollution in surface water has become an environmental problem of global concern. The effective way for controlling the nitrate pollution of surface water is to identify the pollution source and reduce the input of nitrate. In recent decades, nitrogen (δ15N) and oxygen (δ18O) isotopes of nitrate has been used as an effective approach for identifying the source and fate of nitrate pollution in surface water. However, owing to the complexity of nitrate pollution source and the influence of isotopic fractionation, the application of this method has some limitations. In this work, we systematically discussed the fundamental principle of using nitrogen and oxygen isotopes to trace the nitrate source, the fate identification of nitrate, and the major testing techniques. Subsequently, the applications of nitrogen and oxygen isotopes for source identification of surface water were illustrated. However, there are still significant gaps in the application of the source identification and transformation mechanisms to nitrate and many research questions on these topics need to be addressed.

Multiyear dual nitrate isotope signatures suggest that N-saturated subtropical forested catchments can act as robust N sinks

In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH₄⁺ ), causes significant nitrate (NO₃^- ) leaching from well-drained acid forest soils on hill slopes (HS), whereas significant retention of NO₃^- occurs in near-stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ¹⁵ N and δ¹⁸ O) of nitrate (NO₃^- ) in soil pore water along a hydrological continuum in the N-saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH₄⁺ and production of NO₃^- in soils on HS were associated with a significant decrease in δ¹⁵ N-NO₃^- , suggesting efficient nitrification despite low soil pH. The concentration of NO₃^- declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ¹⁵ N and δ¹⁸ O of residual NO₃^- , providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent ¹⁵ N enrichment factor (ε) of NO₃^- of about -5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ¹⁵ N and δ¹⁸ O-NO₃^- in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ¹⁵ N-NO₃^- values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N-saturated, subtropical forests with important implications for regional N budgets.

Microbial denitrification dominates nitrate losses from forest ecosystems

Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N2) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO3 (-)) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6-30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO3 (-) leaching, pointing to widespread dominance of denitrification in removing NO3 (-) from forest ecosystems across a range of conditions. Further, we report that the loss of NO3 (-) to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.

Nitrate isotopic composition reveals nitrogen deposition and transformation dynamics along the canopy-soil continuum of a suburban forest in Japan

RATIONALE

Heavy nitrogen (N) deposition often causes high nitrate (NO3(-)) accumulation in soils in temperate forested ecosystems. To clarify the sources and production pathways of this NO3(-), we investigated NO3(-) isotope signatures in deposition processes along the canopy-soil continuum of a suburban forest in Japan.

METHODS

The stable isotopes of N and oxygen (O) were used to trace the source and transformation dynamics of nitrate (NO3(-)) in two forest stands: a plantation of Cryptomeria japonica (coniferous tree; CJ) and a natural secondary forest of Quercus acutissima (broadleaf, deciduous tree; QA). The NO3(-) and ammonium (NH4(+)) concentrations were measured, as well as the δ(15)N and δ(18)O values of NO3(-), in rainfall, throughfall, stem flow, litter layer water, and soil water (10, 30, and 70 cm depths).

RESULTS

Seasonal variations were observed in the δ(15)N values of throughfall and stem flow NO3(-) at both sites, and in the δ(18)O values of throughfall and stem flow NO3(-) at the QA site. The range in the δ(18)O values of rainfall and throughfall NO3(-) was large (65-70‰) but decreased dramatically to 2-5‰ in soil water at both sites. At the QA site, the δ(18)O values of stem flow NO3(-) decreased to 40‰ during several rain events, especially in the growing season.

CONCLUSIONS

NO3(-) from atmospheric deposition was replaced by microbially generated NO3(-) mainly in the organic horizon and surface portion of the mineral soil under excess N deposition in this suburban forest. Microbial activity, including both immobilization and nitrification in organic-rich horizons near the surface, contributed to incorporating atmospheric NO3(-) quickly into the internal microbial N cycle. We also found evidence of microbial nitrification in the canopy of the QA stand during the growing season.