Review of soil dissolved organic nitrogen cycling: Implication for groundwater nitrogen contamination

Selective Solid Phase Extraction Using a Mixed-Mode Cation Exchange Cartridge Facilitates the Mass Spectrometry Analysis on Dissolved Organic Nitrogen

RATIONALE

Dissolved organic nitrogen (DON) is a crucial component in environment, which acts as the largest reservoir of nitrogen and plays a significant role in the nitrogen cycling, pollutant transport, and substrate utilization among various environmental systems. DON exhibits a relatively low concentration in environment, which presents a great challenge for DON detection, and an efficient separation and enrichment lab protocol is required to fully understand its structural and compositional characteristics. However, there is no standard method to extract DON from complex environmental samples efficiently.

METHODS

The DON was extracted utilizing solid-phase extraction (SPE), with a one-step elution by the Bond Elut PPL cartridge and the Waters HLB cartridge, and a three-step elution by the Waters MCX cartridge. UV-Vis, fluorescence, mass spectrometry (MS), and gas chromatography-nitrogen chemiluminescence detector (GC-NCD) techniques were utilized to investigate and compare the characteristics of DON from the three different SPE strategies.

RESULTS

Combined the fluorescence and MS results, it is found that the stepwise extraction using the MCX cartridge exhibits the best recovery of DON for both standard and environmental samples, and its performance is less affected by the chemical interferences such as surfactants during MS analysis. Furthermore, the MCX fraction exhibits the highest number of DON molecular formulas with a low O/C ratio and a high H/C ratio in environmental samples, and such a fraction also shows an enrichment of nitrosamine-type substances.

CONCLUSIONS

This work establishes an efficient MCX-SPE protocol to extract and analyze DON, which can be applied to environmental samples straightforward. The presented work provides a theoretical support for the analysis of DON, which facilitates a comprehensive understanding of the chemical composition and environmental effect of nitrogen-containing substances.

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.

Exploration of nitrogen sources and transformation processes in eutrophic estuarine zones based on DOM and stable isotope compositions

Our study examines nitrogen sources and transformations in Xiamen Bay, where eutrophication has increased due to higher nitrogen levels. By analyzing dissolved organic matter (DOM) and nitrate stable isotopes (δ15N-NO3-and δ18O-NO3-), the study finds that nitrate in low salinity areas is influenced by freshwater-seawater mixing and biogeochemical processes, while in high salinity areas, it is mainly affected by physical mixing. Bayesian mixing model (MixSIAR) results show that the primary nitrate sources are fecal matter and sewage, followed by atmospheric deposition. During the high flow period, DOM may facilitate nitrogen transformation and release through processes such as degradation or mineralization. In contrast, during the low flow period, the system is mainly influenced by the physical mixing of saline and freshwater. Studies have shown that DOM can indicate the biogeochemical intensity in water bodies, further identifying the main factors influencing the distribution and transformation processes of nitrate content, providing a basis for mitigating eutrophication in estuarine areas.

Constraints on vertical variability of geogenic ammonium in multi-layered aquifer systems

The elevated levels of geogenic (natural) ammonium in groundwater have been frequently documented in recent years. Although improving insights have been achieved in understanding the genesis of ammonium in the subsurface environment, the vertical variability of the geogenic ammonium in groundwater remains poorly understood. Here, we selected typical multi-layered aquifer systems in the central Yangtze River plain and characterized the vertical heterogeneity of geogenic ammonium through the hydrogeochemical analysis. Subsequently, the controlling factors were identified by examining the molecular composition of dissolved organic matter (DOM) and aquifer sediment features. The results indicated that the ammonium concentration in groundwater increased from the deep to shallow aquifers (2.13 to 9.88 mg/L as N), accompanied by a transition in organic matter (OM) degradation towards the methanogenic stage (δ13C-DIC: -23.07 to -0.34‰). Compared to the deeper aquifers, the DOM in the shallow aquifer was characterized by a higher abundance of the N-containing OM (15.1% > 13.13% > 12.76%) with a lower molecular lability index, corresponding to more thorough degradation extent. The characteristics of the soluble OM in depth-matched sediments were similar to those of the DOM in groundwater, suggesting the persistent water-rock interactions. Besides, the pumping tests revealed that the hydraulic conductivity decreased from deep to shallow aquifers (2.28 to 0.62 m/d), which further facilitated the more retention of geogenic ammonium in the shallow aquifer. That is, the combined effects of the abundant N-containing OM in sediments, strong degradation of the bioactive DOM, and long retention governed by hydrodynamics contributed to the increased ammonium enrichment in the shallow aquifer, thereby generating the vertical variability. The findings underscore the significance of the complex coupled factors in controlling the vertical distribution of geogenic ammonium in multi-layered aquifer systems, which was crucial for understanding the spatial heterogeneity of geogenic contaminated groundwater.

Spectral and molecular insights into the characteristics of dissolved organic matter in nitrate-contaminated groundwater

The characteristics of dissolved organic matter (DOM) serve as indicators of nitrate pollution in groundwater. However, the specific DOM components associated with nitrate in groundwater systems remain unclear. In this study, dual isotopes of nitrate, three-dimensional Excitation emission matrices (EEMs) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were utilized to uncover the sources of nitrate and their associations with DOM characteristics. The predominant nitrate in the targeted aquifer was derived from soil organic nitrogen (mean 46.0%) and manure &sewage (mean 34.3%). The DOM in nitrate-contaminated groundwater (nitrate-nitrogen >20 mg/L) exhibited evident exogenous characteristics, with a bioavailable content 2.58 times greater than that of uncontaminated groundwater. Regarding the molecular characteristics, DOM molecules characterized by CHO + 3N, featuring lower molecular weights and H/C ratios, indicated potential for mineralization, while CHONS formulas indicated the exogenous features, providing the potential for accurate traceability. These findings provided insights at the molecular level into the characterization of DOM in nitrate-contaminated groundwater and offer scientific guidance for decision-making regarding the remediation of groundwater nitrate pollution.

Microbial mechanisms for improved soil phosphorus mobilization in monoculture conifer plantations by mixing with broadleaved trees

Replanting broadleaved trees in monoculture conifer plantations has been shown to improve the ecological environment. However, not much is known about the distribution properties of soil phosphate-mobilizing bacteria (PMB) under different mixed plantings or how PMB affects biometabolism-driven phosphorus (P) bioavailability. The phoD and pqqC genes serve as molecular markers of PMB because they regulate the mobilization of organic (Po) and inorganic (Pi) P. Differences in soil bioavailable P concentration, phoD- and pqqC-harboring PMB communities, and their main regulators were analyzed using biologically-based P (BBP) and high-throughput sequencing approaches after combining coniferous trees (Pinus massoniana) and five individual broadleaved trees (Bretschneidera sinensis, Michelia maudiae, Cercidiphyllum japonicum, Manglietia conifera, and Camellia oleifera). The findings revealed that the contents of litter P, soil organic carbon (SOC), available Pi (CaCl2-P), and labile Po (Enzyme-P) were significantly higher in conifer-broadleaf mixed plantations than those in the monospecific Pinus massoniana plantations (PM), especially in the mixed stands with the introduction of Cercidiphyllum japonicum, Michelia maudiae, and Camellia oleifera. Conifer-broadleaf mixing had little effect on the abundance of phoD and pqqC genes but significantly altered species composition within the communities. Conifer-broadleaf mixing improved soil microbial habitat mainly by increasing the pH, increasing carbon source availability and nutrient content, decreasing exchangeable Fe3+ and Al3+ content, and decreasing the activation degrees of Fe and Al oxides in acidic soils. A small group of taxa (phoD: Bradyrhizobium, Tardiphaga, Nitratireductor, Mesorhizobium, Herbaspirillum, and Ralstonia; pqqC: Burkholderia, Variovorax, Bradyrhizobium, and Leptothrix) played a key role in the synthesis of P-related enzymes (e.g., alkaline phosphomonoesterase, ALP) and in lowering the levels of mineral-occluded (HCl-P) and chelated (Citrate-P) Pi. Overall, our findings highlight that mixing conifers and broadleaves could change the PMB communities that produce ALP and dissolve Pi to make P more bioavailable.

The dynamic features and microbial mechanism of nitrogen transformation for hydrothermal aqueous phase as fertilizer in dryland soil

Hydrothermal aqueous phase (HAP) contains abundant organics and nutrients, which have potential to partially replace chemical fertilizers for enhancing plant growth and soil quality. However, the underlying reasons for low available nitrogen (N) and high N loss in dryland soil remain unclear. A cultivation experiment was conducted using HAP or urea to supply 160 mg N kg-1 in dryland soil. The dynamic changes of soil organic matters (SOMs), pH, N forms, and N cycling genes were investigated. Results showed that SOMs from HAP stimulated urease activity and ureC, which enhanced ammonification in turn. The high-molecular-weight SOMs relatively increased during 5-30 d and then biodegraded during 30-90 d, which SUV254 changed from 0.51 to 1.47 to 0.29 L-1 m-1. This affected ureC that changed from 5.58 to 5.34 to 5.75 lg copies g-1. Relative to urea, addition HAP enhanced ON mineralization by 8.40 times during 30-90 d due to higher ureC. It decreased NO3-N by 65.35%-77.32% but increased AOB and AOA by 0.25 and 0.90 lg copies g-1 at 5 d and 90 d, respectively. It little affected nirK and increased nosZ by 0.41 lg copies g-1 at 90 d. It increased N loss by 4.59 times. The soil pH for HAP was higher than that for urea after 11 d. The comprehensive effects of high SOMs and pH, including ammonification enhancement and nitrification activity inhibition, were the primary causes of high N loss. The core idea for developing high-efficiency HAP fertilizer is to moderately inhibit ammonification and promote nitrification.