Direct and Indirect Effects of Dissolved Organic Matter Source and Concentration on Denitrification in Northern Florida Rivers

Unveiling Microbial Nitrogen Metabolism in Rivers using a Machine Learning Approach

Microbial nitrogen metabolism is a complicated and key process in mediating environmental pollution and greenhouse gas emissions in rivers. However, the interactive drivers of microbial nitrogen metabolism in rivers have not been identified. Here, we analyze the microbial nitrogen metabolism patterns in 105 rivers in China driven by 26 environmental and socioeconomic factors using an interpretable causal machine learning (ICML) framework. ICML better recognizes the complex relationships between factors and microbial nitrogen metabolism than traditional linear regression models. Furthermore, tipping points and concentration windows were proposed to precisely regulate microbial nitrogen metabolism. For example, concentrations of dissolved organic carbon (DOC) below tipping points of 6.2 and 4.2 mg/L easily reduce bacterial denitrification and nitrification, respectively. The concentration windows for NO3--N (15.9-18.0 mg/L) and DOC (9.1-10.8 mg/L) enabled the highest abundance of denitrifying bacteria on a national scale. The integration of ICML models and field data clarifies the important drivers of microbial nitrogen metabolism, supporting the precise regulation of nitrogen pollution and river ecological management.

WIDESPREAD CAPACITY FOR DENITRIFICATION ACROSS A BOREAL FOREST LANDSCAPE

A warming climate combined with frequent and severe fires cause permafrost to thaw, especially in the region of discontinuous permafrost, where soil temperatures may only be a few degrees below 0 °C. Soil thaw releases carbon (C) and nitrogen (N) into the actively cycling pools, and whereas C emissions following permafrost thaw are well documented, the fates of N remain unclear. Denitrification could release N from ecosystems as nitrous oxide (N₂O) or nitrogen gas (N₂), but the contributions of these processes to the high-latitude N cycle remain uncertain. We quantified microbial capacity for denitrification and N₂O production in boreal soils, lakes, and streams using anoxic C- and N-amended assays, and assessed correlates of denitrifying enzyme activity (DEA) in Interior Alaska. Riparian soils and stream sediments supported the highest potential rates of denitrification, upland soils were intermediate, and lakes supported lower rates, whereas deep permafrost soils supported little denitrification. Time since fire had no effect on denitrification potential in upland soils. Across all landscape positions, DEA was negatively correlated with ammonium pools. Within each landscape position, potential rate of denitrification increased with soil or sediment organic matter content. Widespread N loss to denitrification in boreal forests could constrain the capacity for N-limited primary producers to maintain C stocks in soils following permafrost thaw.

Spatiotemporal Dynamics of Dissolved Organic Carbon and Freshwater Browning in the Zoige Alpine Wetland, Northeastern Qinghai-Tibetan Plateau

The concentrations of dissolved organic carbon (DOC) and its light-absorbing fraction (chromophoric dissolved organic matter; CDOM) in surface waters, particularly those draining organic-rich peatlands, have dramatically increased over the past decade due to climate change and human disturbance. To explore the spatiotemporal dynamics of DOC and CDOM in surface waters of the northeastern Qinghai-Tibetan Plateau, we collected water samples from two rivers in the Zoige alpine wetland and from two rivers in its adjacent alpine-gorge region, during wet and dry seasons. DOC concentration ranged from 4.82 mg·L−1 to 47.83 mg·L−1, with a mean value of 15.04 mg·L−1, 2.84 times higher than the global average. The Zoige rivers had higher DOC concentration and highly terrigenous CDOM. Significantly higher DOC concentration was observed for the Zoige rivers in the wet season compared to the dry season. In contrast, the alpine-gorge rivers had higher DOC levels in the dry season. No significant correlations were observed between DOC and CDOM at all rivers due to the influence of autochthonous sources on the alpine-gorge rivers and intensive photochemical degradation of terrigenous DOM in the Zoige rivers. Significant relationships between CDOM and specific ultraviolet absorbance at 254 nm (SUVA254) and between CDOM/DOC and SUVA254 were observed, indicating that the aromaticity of DOM in the rivers was mainly determined by CDOM. Moreover, the DOC/CDOM properties of the Hei River indicate critical human-induced water quality degradation. High DOC level and high browning degree were found in rivers in the Zoige alpine wetland, indicating that large amounts of terrigenous DOC were released to the aquatic systems of the region.

Concentrations and Loads of Dissolved and Particulate Organic Carbon in Urban Stormwater Runoff

Urban landscapes are significant contributors of organic carbon (OC) in receiving waters, where elevated levels of OC limit the light availability, increase the transport of pollutants, and result in high costs of potable water treatment. Our objective in this study was to investigate the concentrations, fractions (dissolved and particulate), and loads of OC in a residential catchment (3.89 ha drainage area) located in Florida, United States. The outlet of the stormwater pipe draining the residential catchment was instrumented with an automated sampler, a flowmeter, and a rain gauge. The rainfall and runoff samples collected over 25 storm events during the 2016 wet season (June to September) were analyzed for dissolved organic carbon (DOC) and total organic carbon (TOC), with particulate OC (POC) calculated as the difference between TOC and DOC. Mean concentration of DOC was 2.3 ± 1.7 mg L−1 and POC was 0.3 ± 0.3 mg L−1 in the rainfall, whereas DOC was 10.5 ± 6.20 mg L−1 and POC was 2.00 ± 4.05 mg L−1 in the stormwater runoff. Concentrations of DOC were higher during the rising limb of the hydrograph in 15 out of 25 storm events, suggesting flushing of DOC, with an increase in the amount of runoff, from the landscape sources in the residential catchment. The estimated total export of OC during the 2016 wet season was 66.0 kg ha−1, of which DOC was 56.9 kg ha−1 (86.2% of TOC), and POC was 9.1 kg ha−1 (13.8% of TOC). High concentrations and loads of OC, especially DOC, in the stormwater runoff imply that residential catchments in urban watersheds are hot-spots of DOC influx to water bodies. Reducing DOC transport in the urban landscapes is complex and require identifying the origin of DOC and then using site-specific targeted approaches to mitigate DOC loss.

A Mass Balance of Nitrogen in a Large Lowland River (Elbe, Germany)

Nitrogen (N) delivered by rivers causes severe eutrophication in many coastal waters, and its turnover and retention are therefore of major interest. We set up a mass balance along a 582 km river section of a large, N-rich lowland river to quantify N retention along this river segment and to identify the underlying processes. Our assessments are based on four Lagrangian sampling campaigns performed between 2011 and 2013. Water quality data served as a basis for calculations of N retention, while chlorophyll-a and zooplankton counts were used to quantify the respective primary and secondary transformations of dissolved inorganic N into biomass. The mass balance revealed an average N retention of 17 mg N m−2 h−1 for both nitrate N (NO3–N) and total N (TN). Stoichiometric estimates of the assimilative N uptake revealed that, although NO3–N retention was associated with high phytoplankton assimilation, only a maximum of 53% of NO3–N retention could be attributed to net algal assimilation. The high TN retention rates in turn were most probably caused by a combination of seston deposition and denitrification. The studied river segment acts as a TN sink by retaining almost 30% of the TN inputs, which shows that large rivers can contribute considerably to N retention during downstream transport.

Mulch-Derived Organic Carbon Stimulates High Denitrification Fluxes from Agricultural Ditch Sediments

Reactive N is an essential input for healthy, vibrant crop production, yet excess N is often transported off field via agricultural ditches to downstream receiving ecosystems, where it can cause negative impacts to human health, biodiversity loss, as well as eutrophication and resultant hypoxia. Denitrification, the transformation of reactive N to unreactive N gas, within agricultural ditches has potential to reduce impacts to downstream ecosystems but requires substantial organic C substrates. We used a flow-through intact core experiment to test the effects of low-cost management options including a common agricultural amendment, gypsum, and an overlying hardwood mulch layer on promoting denitrification within agricultural ditch sediments. We found significantly higher denitrification potentials in mulch (11.2 mg N-N m h) and mulch-gypsum cores (9.2 mg N-N m h) than in gypsum (1.3 mg N-N m h) or control cores (0.6 mg N-N m h). Higher denitrification rates corresponded with high dissolved organic C (DOC) fluxes within the mulch and mulch-gypsum treatments (72.8-115.2 mg m h) and were ultimately able to remove 65 to 69% of N loads. Results indicate DOC from overlying mulch additions to agricultural ditches significantly increase denitrification in intact cores and suggest that the addition of DOC sources in agricultural ditches may contribute a simple, low-cost option to reduce reactive N export and improve ecological outcomes within aquatic agroecosystems.

Seasonal variation of potential denitrification rate and enhanced denitrification performance via water-lifting aeration technology in a stratified reservoir-A case study of Zhoucun reservoir

Zhoucun reservoir is one of the major water resources in Zao Zhuang city, northern China. The seasonal distribution of surface sediment denitrification rate and enhanced performance of denitrification via water-lifting aeration technology were explored using the acetylene-inhibition technique. Surface sediment denitrification rates ranged from 2.57 ± 1.32 to 923.90 ± 86.81 nmol N₂/(g dw·h), with the highest rates in November (ANOVA, p < 0.05) and significantly low rates in June, July, and August (ANOVA, p < 0.05), mainly because of the seasonal differences in nitrate concentration, water temperature, and sediment total nitrogen (STN). Meanwhile, the N₂/(N₂+N₂O) ratio (83.44-91.70% for the highest sediment denitrification period) indicated that N₂ accounted for a majority of denitrification. Correlation analysis between various environmental factors and denitrification was conducted, and nitrate concentration, STN, low molecular weight organic carbon, the number of aerobic denitrifying bacteria, and the environmental parameters of oxidation-reduction potential (ORP), pH, electrical conductivity (EC), and chlorophyll a (Chl-a) presented significant relationships during the entire study period. On the basis of the multiple regression model, nitrate and low molecular weight organic carbon concentration were the most influential factors on denitrification variability. Moreover, the denitrification rates of the surface sediment clearly increased, from 5.28 to 13.22 nmol N₂/(g dw·h) to 1117.02-3129.47 nmol N₂/(g dw·h), which were higher than those in the non-operating year. This suggests that the denitrification in the sediment system could be enhanced in situ by water-lifting and aeration technology in the reservoir ecosystem.

Biological regeneration of manganese (IV) and iron (III) for anaerobic metal oxide-mediated removal of pharmaceuticals from water

Applying manganese(IV)- or iron(III)-(hydr)oxides to remove pharmaceuticals from water could be attractive, due to the capacity of these metal oxides to remove pharmaceuticals and be regenerated. As pharmaceutical removal under anaerobic conditions is foreseen, Mn(IV) or Fe(III) regeneration under anaerobic conditions, or with minimum oxygen dosage, is preferred. In this study, batch experiments are performed to investigate (1) Mn(IV) and Fe(III) regeneration from Mn(II) and Fe(II); (2) the pharmaceutical removal during biological Mn(IV) and Fe(III) regeneration; and (3) anaerobic abiotic pharmaceutical removal with different Mn(IV) or Fe(III) species. Results show that biological re-oxidation of reduced Mn(II) to Mn(IV) occurs under oxygen-limiting conditions. Biological re-oxidation of Fe(II) to Fe(III) is obtained with nitrate under anaerobic conditions. Both bio-regenerated Mn(IV)-oxides and Fe(III)-hydroxides are amorphous. The pharmaceutical removal is insignificant by Mn(II)- or Fe(II)-oxidizing bacteria during regeneration. Finally, pharmaceutical removal is investigated with various Mn(IV) and Fe(III) sources. Anaerobic abiotic removal using Mn(IV) produced from drinking water treatment plants results in 23% metoprolol and 44% propranolol removal, similar to chemically synthesized Mn(IV). In contrast, Fe(III) from drinking water treatment plants outperformed chemically or biologically synthesized Fe(III); Fe (III) from drinking water treatment can remove 31-43% of propranolol via anaerobic abiotic process. In addition, one of the Fe(III)-based sorbents tested, FerroSorp^®RW, can also remove propranolol (20-25%). Biological regeneration of Mn(IV) and Fe(III) from the reduced species Mn(II) and Fe(II) could be more effective in terms of cost and treatment efficiency.

Seasonal Differences in Relationships between Nitrate Concentration and Denitrification Rates in Ditch Sediments Vegetated with Rice Cutgrass

Increased application of nitrogen (N) fertilizers in agricultural systems contributes to significant environmental impacts, including eutrophication of fresh and coastal waters. Rice cutgrass [ (L.) Sw.] can significantly enhance denitrification potential in agricultural ditch sediments and potentially reduce N export from agricultural watersheds, but relationships with known drivers are not well understood. To address this, we examined effects of nitrate (NO) availability on dinitrogen gas (N) and NO fluxes seasonally. Net denitrification rates were measured as positive N fluxes from vegetated intact sediment cores using membrane inlet mass spectrometry (MIMS). We developed Michaelis-Menten models for N fluxes across NO gradients in the spring, summer, and fall seasons. Summer N models exhibited the highest (maximum amount of net N flux) and (concentration of NO in the overlying water at which the net N flux is half of ), with a maximum production of N of ∼20 mg N m h. Maximum percentage NO retention occurred at 1 mg NO L in the overlying water in all seasons, except summer where maximum retention persisted from 1 to 5 mg NO L. Denitrification rates were strongly correlated with NO uptake by vegetated sediments in spring ( = 0.94, < 0.0001) and summer ( = 0.97, < 0.0001), but low NO uptake in fall and winter resulted in virtually no net denitrification during these seasons. Our results indicate that vegetated ditch sediments may act as effective NO sinks during the growing season. Ditch sediments vegetated with cutgrass not only immobilized a significant fraction of NO entering them but also permanently removed as much as 30 to 40% of the immobilized NO through microbial denitrification.

The effects of temperature and resource availability on denitrification and relative N2O production in boreal lake sediments

Anthropogenic environmental stressors (like atmospheric deposition, land use change, and climate warming) are predicted to increase inorganic nitrogen and organic carbon loading to northern boreal lakes, with potential consequences for denitrification in lakes. However, our ability to predict effects of these changes is currently limited as northern boreal lakes have been largely neglected in denitrification studies. The aim of this study was therefore to assess how maximum potential denitrification and N2O production rates, and the relationship between the two (relative N2O production), is controlled by availability of nitrate (NO3(-)), carbon (C), phosphorus (P), and temperature. Experiments were performed using the acetylene inhibition technique on sediments from a small, nutrient poor boreal lake in northern Sweden in 2014. Maximum potential denitrification and N2O production rates at 4°C were reached already at NO3(-) additions of 106-120μg NO3(-)-N/L, and remained unchanged with higher NO3 amendments. Higher incubation temperatures increased maximum potential denitrification and N2O production rates, and Q10 was somewhat higher for N2O production (1.77) than for denitrification (1.69). The relative N2O production ranged between 13% and 64%, and was not related to NO3(-) concentration, but the ratio increased when incubations were amended with C and P (from a median of 16% to 27%). Combined, our results suggests that unproductive northern boreal lakes currently have low potential for denitrification but are susceptible to small changes in NO3 loading especially if these are accompanied by enhanced C and P availability, likely promoting higher N2O production relative to N2.

The role of C:N:P stoichiometry in affecting denitrification in sediments from agricultural surface and tile-water wetlands

Nutrient stoichiometry within a wetland is affected by the surrounding land use, and may play a significant role in the removal of nitrate (NO3-N). Tile-drained, agricultural watersheds experience high seasonal inputs of NO3-N, but low phosphorus (PO4-P) and dissolved organic carbon (DOC) loads relative to surface water dominated systems. This difference may present stoichiometric conditions that limit denitrification within receiving waterways. We investigated how C:N:P ratios affected denitrification rates of sediments from tile-drained mitigation wetlands incubated for: 0, 5, 10, and 20 days. We then tested whether denitrification rates of sediments from surface-water and tile-drained wetlands responded differently to C:N ratios of 2:1 versus 4:1. Ratios of C:N:P (P < 0.05) and incubation length (P < 0.05) had a significant effect on denitrification in tile-drained wetland sediments. Carbon limitation of denitrification became evident at elevated NO3-N concentrations (20 mg L(-1)). Denitrification measured from tile water and surface water wetland sediments increased significantly (P < 0.05) at the 2:1 and 4:1 C:N treatments. The results from both experiments suggest wetland sediments provide a limiting pool of labile DOC to maintain prolonged NO3-N removal. Also, DOC limitation became more evident at elevated NO3-N concentrations (20 mg L(-1)). Irrespective of NO3-N concentrations, P did not limit denitrification rates. In addition to wetting period, residence time, and maintenance of anaerobic conditions, the availability of labile DOC is playing an important limiting role in sediment denitrification within mitigation wetlands.