Estimating nitrogen fates and gross transformations in bioretention systems with applications of 15N labeling methods

Leaching sources and mechanisms of different nitrogen species from bioretention systems

At present, nitrogen (N) leaching from bioretention systems (BRSs) has become a key issue, imposing constraints on their application, a consequence of N dynamics of both inflow and legacy N at different time scales. In this study, the distinct sources (IL: immediate leaching, FL: fast leaching, SL: slow leaching) and the principal transformation processes of different N species (i.e., NH4+, NO3- and DON) leaching originating from inflow and legacy of BRSs were firstly unveiled by various 15N species labeling (i.e., 15N-NH4+, 15N-NO3- and 15N-DON). Results indicate that: NH4+ leaching was primarily caused by FL from influent organic N and SL from influent NH4+, with mineralization being the main transformation process influencing NH4+ leaching; NO3- leaching primarily originated from SL, with the major proportion attributed to the influent organic N in SL, autotrophic and heterotrophic nitrification were the main influencing factors; DON leaching primarily originated from SL, with similar proportions coming from influent organic N, NH4+, and NO3-, inorganic N assimilation was the principal transformation process affecting DON leaching. This study provides an effective framework for apportioning the leaching sources of different N species, providing valuable insights for the implementation of both inflow and legacy N leaching control measures.

Transformation mechanisms of ammonium and nitrate in subsurface wastewater infiltration system: Implication for reducing greenhouse gas emissions

Subsurface wastewater infiltration system (SWIS) has been recognized as a cost-effective and environmentally friendly tool for wastewater treatment. However, there is a lack of knowledge on the transformation processes of nitrogen (N), hindering the improvement of the N removal efficiency in SWIS. Here, the migration and transformation mechanisms of ammonium (NH4+-N) and nitrate (NO3+-N) over 10 days were explored by 15N labeling technique. Over the study period, 49% of the added 15NH4+-N remained in the soil, 29% was removed via gaseous N emissions, and 14% was leaked with the effluent in the SWIS. In contrast, only 11% of the added 15NO3--N remained in the soil while 65% of the added 15NO3--N was removed via gaseous N emissions, and 12% with the effluent in the SWIS. The main pathway for N2O emission was denitrification (52-70%) followed by nitrification (15-28%) and co-denitrification (9-20%). Denitrification was also the predominant pathway for N loss as N2, accounting for 88-96% of the N2 emission. The dominant biological transformation processes were different at divergent soil depths, corresponding to nitrification zone and denitrification zone along the longitudinal continuum in SWIS, which was confirmed by the expression patterns of microbial gene abundance. Overall, our findings reveal the mechanism of N transformation in SWIS and provide a theoretical basis for establishing a pollutant management strategy and reducing greenhouse gas emissions from domestic wastewater treatment.

Dynamics and control mechanisms of inorganic nitrogen removal during wetting-drying cycles: A simulated managed aquifer recharge experiment

Managed aquifer recharge (MAR) systems can be operated intermittently through wetting-drying cycles to simultaneously improve the water supply and quality. Although MAR can naturally attenuate considerable amounts of nitrogen, the dynamic processes and control mechanisms of nitrogen removal by intermittent MAR remain unclear. This study was conducted in laboratory sandy columns and lasted for 23 d, including four wetting periods and three drying periods. The hydraulic conductivity, oxidation reduction potential (ORP), and leaching concentrations of ammonia nitrogen and nitrate nitrogen of MAR systems were intensively measured to test the hypothesis that hydrological and biogeochemical controls play an essential role in regulating nitrogen dynamics at different stages of wetting-drying cycles. Intermittent MAR functioned as a sink for nitrogen while providing a carbon source to support nitrogen transformations; however, it occasionally became a source of nitrogen under intense flushes of preferential flow. Nitrogen dynamics were primarily controlled by hydrological processes in the initial wetting phase and were further regulated by biogeochemical processes during the subsequent wetting period, supporting our hypothesis. We also observed that a saturated zone could mediate nitrogen dynamics by creating anaerobic conditions for denitrification and buffering the flush effect of preferential flow. The drying duration can also affect the occurrence of preferential flow and nitrogen transformations, which should be balanced when determining the optimal drying duration for intermittent MAR systems.

Effects of reducing nitrogen application and adding straw on N2O emission and soil nitrogen leaching of tomato in greenhouse

Excessive input of nitrogen(N) fertilizer and improper selection of fertilizer types in the greenhouse vegetable production process will lead to a large amount of N loss. In order to relieve the environmental pollution caused by N loss, a planting experiment was carried out in a solar greenhouse in Shouguang, Shandong, China, to investigate the effects of N-reducing fertilizer and straw application on greenhouse vegetable yield and soil N loss, and to explore the fate of N after fertilizer application using the ¹⁵N isotope tracing technique. The experiment was planted for two seasons from July 2017 to June 2018 with four treatments: control (CK), conventional fertilizer (CN), reduced N topdressing (SN), and reduced N topdressing + straw (SNS). The results indicated that N reduction fertilizer and straw application resulted in a 35.25%-35.49% reduction in total N₂O emissions and 15.76%-41.77% reduction in mineral N leaching losses. ¹⁵N isotopes as tracers showed that the maximum abundance in N₂O was reduced by 58.5% and 55.68% for SN and SNS, respectively, and cumulative N₂O emissions were reduced by 80.44% and 81.67%, respectively, and mineral N leaching was reduced by 74.4% and 70.48%, respectively, after fertilization compared to CN treatment. There was no significant difference in tomato yield between the three fertilizer treatments in the two growing seasons. Therefore, in greenhouse vegetable production, the amount of N fertilizer was reduced by 40.7% and the addition of straw reduced N₂O emissions and N leaching without affecting tomato yields.

A review on plant-microbial interactions, functions, mechanisms and emerging trends in bioretention system to improve multi-contaminated stormwater treatment

Management and treatment of multi-polluted stormwater in bioretention system have gained significant attraction recently. Besides nutrients, recent source appointment studies found elevated levels of Potentially toxic metal(loid)s (PTMs) and contaminants of emerging concern (CECs) in stormwater that highlighted many limitations in conventional media adsorption-based pollutant removal bioretention strategies. The substantial new studies include biological treatment approaches to strengthen pollutants degradation and adsorption capacity of bioretention. The knowledge on characteristics of plants and their corresponding mechanisms in various functions, e.g., rainwater interception, retention, infiltration, media clogging prevention, evapotranspiration and phytoremediation, is scattered. The microorganisms' role in facilitating vegetation and media, plant-microorganism interactions and relative performance over different functions in bioretention is still unreviewed. To uncover the underneath, it was summarised plant and microbial studies and their functionality in hydrogeochemical cycles in the bioretention system in this review, contributing to finding their interconnections and developing a more efficient bioretention system. Additionally, source characteristics of stormwater and fate of associated pollutants in the environment, the potential of genetical engineered plants, algae and fungi in bioretention system as well as performance assessment of plants and microorganisms in non-bioretention studies to propose the possible solution of un-addressed problems in bioretention system have been put forward in this review. The present review can be used as an imperative reference to enlighten the advantages of adopting multidisciplinary approaches for the environment sustainability and pollution control.