Use of stable nitrogen isotopes to track plant uptake of nitrogen in a nature-based treatment system

Uptake patterns for nitrogen and sulfur source by aquatic plants and various nitrogen acquisition strategies: Affected by mining activities

Understanding the nitrogen and sulfur uptake strategies of mine plants, including sources and preferences for nitrogen forms (ammonium nitrogen (NH4+) vs nitrate nitrogen (NO3-)), is critical to improving understanding of the role of plants in participating in the biogeochemical cycles of nitrogen and sulfur in mining areas. In this study, the stable N and S isotopic compositions of two species of aquatic plants (calamus and reed) in Linhuan mining area were analyzed to determine their absorption strategies for different nitrogen and sulfur sources. The results showed that river water was the largest source of nitrogen and sulfur, contributing 54.6% and 53.9% respectively. NO3- is the main form of nitrogen uptake by reed and calamus, followed by NH4+. In order to adapt to the change of nitrogen form in the environment, reed and calamus tend to absorb and utilize NO3- to maintain their absorption of nitrogen. Mine effluents from mining activities provide at least 12.9% and 16.8% sulfate to reed and calamus respectively, and the effect of mine effluents on reed and calamus sulfur has been underestimated. This study reveals the key factors controlling plant isotope composition, and the use of nitrogen and sulfur isotope composition of aquatic plants can help quantify the level of influence of mining activities, and understand the biogeochemical cycle of nitrogen and sulfur in mining areas.

Persistent Trace Organic Contaminants Are Transformed Rapidly under Sulfate- and Fe(III)-Reducing Conditions in a Nature-Based Subsurface Water Treatment System

Subsurface treatment systems, such as constructed wetlands, riverbank filtration systems, and managed aquifer recharge systems, offer a low-cost means of removing trace organic contaminants from treated municipal wastewater. To assess the processes through which trace organic contaminants are removed in subsurface treatment systems, pharmaceuticals and several major metabolites were measured in porewater, sediment, and plants within a horizontal levee (i.e., a subsurface flow wetland that receives treated municipal wastewater). Concentrations of trace organic contaminants in each wetland compartment rapidly declined along the flow path. Mass balance calculations, analysis of transformation products, microcosm experiments, and one-dimensional transport modeling demonstrated that more than 60% of the contaminant removal could be attributed to transformation. Monitoring of the system with and without nitrate in the wetland inflow indicated that relatively biodegradable trace organic contaminants, such as acyclovir and metoprolol, were rapidly transformed under both operating conditions. Trace organic contaminants that are normally persistent in biological treatment systems (e.g., sulfamethoxazole and carbamazepine) were removed only when Fe(III)- and sulfate-reducing conditions were observed. Minor structural modifications to trace organic contaminants (e.g., hydroxylation) altered the pathways and extents of trace organic contaminant transformation under different redox conditions. These findings indicate that subsurface treatment systems can be designed to remove both labile and persistent trace organic contaminants via transformation if they are designed and operated in a manner that results in sulfate-and Fe(III)-reducing conditions.

Anthropogenic eutrophication of Lake Titicaca (Bolivia) revealed by carbon and nitrogen stable isotopes fingerprinting

Cultural eutrophication is the leading cause of water quality degradation worldwide. The traditional monitoring of eutrophication is time-consuming and not integrative in space and time. Here, we examined the use of carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic composition to track the degree of eutrophication in a bay of Lake Titicaca impacted by anthropogenic (urban, industrial and agricultural wastewater) discharges. Our results show increasing δ¹³C and decreasing δ¹⁵N signatures in macrophytes and suspended particulate matter with distance to the wastewater source. In contrast to δ¹⁵N and δ¹³C signatures, in-between aquatic plants distributed along the slope were not only affected by anthropogenic discharges but also by the pathway of carbon uptake, i.e., atmospheric (emerged) vs aquatic (submerged). A binary mixing model elaborated from pristine and anthropogenic isotope end-members allowed the assessment of anthropogenically derived C and N incorporation in macrophytes with distance to the source. Higher anthropogenic contribution was observed during the wet season, attributed to enhanced wastewater discharges and leaching of agricultural areas. For both seasons, eutrophication was however found naturally attenuated within 6 to 8 km from the wastewater source. Here, we confirm that carbon and nitrogen stable isotopes are simple, integrative and time-saving tools to evaluate the degree of eutrophication (seasonally or annually) in anthropogenically impacted aquatic ecosystems.

Fate of Dissolved Nitrogen in a Horizontal Levee: Seasonal Fluctuations in Nitrate Removal Processes

Horizontal levees are a nature-based approach for removing nitrogen from municipal wastewater effluent while simultaneously providing additional benefits, such as flood control. To assess nitrogen removal mechanisms and the efficacy of a horizontal levee, we monitored an experimental system receiving nitrified municipal wastewater effluent for 2 years. Based on mass balances and microbial gene abundance data, we determined that much of the applied nitrogen was most likely removed by heterotrophic denitrifiers that consumed labile organic carbon from decaying plants and added wood chips. Fe(III) and sulfate reduction driven by decay of labile organic carbon also produced Fe(II) sulfide minerals. During winter months, when heterotrophic activity was lower, strong correlations between sulfate release and nitrogen removal suggested that autotrophic denitrifiers oxidized Fe(II) sulfides using nitrate as an electron acceptor. These trends were seasonal, with Fe(II) sulfide minerals formed during summer fueling denitrification during the subsequent winter. Overall, around 30% of gaseous nitrogen losses in the winter were attributable to autotrophic denitrifiers. To predict long-term nitrogen removal, we developed an electron-transfer model that accounted for the production and consumption of electron donors. The model indicated that the labile organic carbon released from wood chips may be capable of supporting nitrogen removal from wastewater effluent for several decades with sulfide minerals, decaying vegetation, and root exudates likely sustaining nitrogen removal over a longer timescale.

Evaluation of ammonia and nitrate distribution and reduction within stormwater green infrastructure with different woody plants under multiple influencing factors

The impact of stormwater green infrastructures (GIs) with different woody plants on nitrogen (N) distribution is still poorly understood. Laboratory experiments were conducted for GIs without or with Sophora japonica and Malus baccata to investigate the distribution of NH₃-N and NO₃-N. The test data was utilized to calibrate and validate the HYDRUS-2D. The validated model was subsequently used to analyze the distribution of NH₃-N and NO₃-N within the different GIs under three different rainfall conditions: inflow/runoff pollutant concentration, rainfall recurrence interval (runoff amount of a rainfall event), and number of dry days (during which no rainwater infiltrates into the soil). The average NH₃-N and NO₃-N concentrations in the upper soil (0-30 cm) of the GIs were about 4.8 and 2.4 times those of the lower layer (30-60 cm). Compared to the control (V_c), the average NH₃-N concentrations in soil with Sophora japonica (V_s) and Malus baccata (V_m) decreased by 15.8% and 35.1% while those of NO₃-N decreased by 15.5% and 27.2%, respectively. Degrees of influence by the three factors on the average soil NH₃-N and NO₃-N concentrations were inflow concentration > number of dry days > recurrence interval. The number of dry days was the smallest influence factor for the overflow N load while the inflow concentration was the most significant influence factor for the outflow, bio-utilization, and soil nitrogen loads. Compared to the control, outflow (groundwater recharge) loads of NO₃-N from the V_s and V_m increased by 14.0-16.6% and 3.7-6.8%, respectively under different conditions. The overflow (runoff) loads from V_s and V_m decreased by 16.8-36.3% and 6.6%-8.4%, respectively. A multiple regression equation was used to establish a quantitative coupling relationship between N pollutant load reduction rates and influence factors (R² ≥ 0.83). This relationship can be used to estimate the runoff treatment effectiveness of green infrastructure on target pollutants.

Enabling Water Reuse by Treatment of Reverse Osmosis Concentrate: The Promise of Constructed Wetlands

As more cities experience water stress, the use of reverse osmosis (RO) membranes for wastewater treatment and reuse will expand. The concentrated waste stream resulting from RO treatment can pose chronic ecotoxicity risks if discharged to surface waters or shallow coastal ecosystems. Most existing RO concentrate treatment technologies are cost prohibitive, but constructed wetlands hold promise as a viable multibenefit solution because they have the potential to provide simultaneous treatment of nutrients, metals, and trace organic contaminants at a relatively low cost. They also are popular with the public. A handful of water-stressed cities have already begun experimenting with constructed wetlands for RO concentrate treatment. However, further research is needed to reduce the land area needed for treatment and increase the reliability of constructed wetland systems.

Using stable isotope probing and fluorescence spectroscopy to examine the roles of substrate and soluble microbial products in extracellular polymeric substance formation in activated sludge process

In this study, we used stable isotope-labeled soluble microbial products (SMP) and substrates to explore their assimilation into the formation of new biological products (i.e., extracellular polymeric substances and biomass) in two adjacent sequencing batch reactors. The isotope labeling approach along with fluorescence spectroscopy allowed us to distinguish between refractory and labile portions of SMP constituents as well as their roles in the formation of extracellular polymeric substances (EPS). Comparison of SMP fluorescence and the specific UV absorbance values between the two reactors revealed the presence of humic-like aromatic substances in the non-consumable part of SMP, which can be ultimately released as effluent organic matter. Parallel factor analysis modeling of fluorescence spectra showed that the hydrolysis of EPS contents mostly resulted in humic-like components in SMP rather than protein-like components, which were initially abundant in EPS (>80%). From variations in carbon and nitrogen isotopic contents in EPS and biomass, it was found that carbon-containing substrates were enriched faster than their nitrogenous counterparts. The contributions to new EPS formation reached 87.5% for carbon and 60.5% for nitrogen. Meanwhile, the isotopic tracking of the labeled SMP revealed that only 11.0% and 11.9% of carbon and 13.3% and 11.6% of nitrogen from the influent SMP were finally assimilated into EPS and biomass, respectively. In contrast, the isotopic enrichment in SMP was higher (~50%) than that of EPS and biomass, indicating the low bioavailability and refractory nature of the feed SMP. This study proposed a promising approach for estimating the relative contributions of different forms of labile substrate and SMP to the formation of EPS in activated sludge processes. This approach could be suggested as a versatile method for establishing the kinetics, substrate element flow, mass balance on organic substrates and nutrients, as well as for tracking the consumption and uptake pathways of hazardous materials.