Do wetlands enhance downstream denitrification in agricultural landscapes?

Decreasing denitrification rates poses a challenge to further decline of nitrogen concentration in Lake Taihu, China

Nitrogen (N) concentrations in many lakes have decreased substantially in recent years due to external load reduction to mitigate harmful algal blooms. However, little attention has been paid to the linkage between the lakes' nitrogen removal efficiency and improved water quality in lakes, especially the variation of denitrification rate (DNR) under decreasing N concentrations. To understand the efficiency of N removal under improving water quality and its influence on the N control targets in Lake Taihu, a denitrification model based on in situ experimental results was developed and long-term (from 2007 to 2022) water quality and meteorological observations were used to estimate DNR and relate it to the amount of N removal (ANR) from the lake. The concentration of total nitrogen (TN) in Lake Taihu decreased from 3.28 mg L-1 to 1.41 mg L-1 from 2007 to 2022 but the reduction showed spatial heterogeneity. The annual mean DNR decreased from 45.6 μmol m-2 h-1 to 4.2 μmol m-2 h-1, and ANR decreased from 11.85×103 t yr-1 to 1.17×103 t yr-1 during the study years. N budget analysis suggested that the amount of N removed by denitrification accounted for 23.3 % of the external load in 2007, but decreased to only 4.0 % in 2022. Thus, the contribution of N removal by internal N cycling decreased significantly as water quality improved. Notably, the proportion of ANR in winter to total ANR increased from 14 % in 2007 to 23 % in 2022 due to warming. This could potentially lead to N deficiencies in spring and summer, thus limiting the availability of N to phytoplankton. A TN concentration of less than 1.0 mg L-1 in the lake and 1.5 mg L-1 in the inflowing lake zones in spring contribute to local N-limitation in Lake Taihu for cyanobacteria control. Our study revealed a general pattern that N removal efficiency decreases with improved water quality, which is instructive for eutrophic lakes in nitrogen management.

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.

Consistent stoichiometric long-term relationships between nutrients and chlorophyll-a across shallow lakes

Aquatic ecosystems are threatened by eutrophication from nutrient pollution. In lakes, eutrophication causes a plethora of deleterious effects, such as harmful algal blooms, fish kills and increased methane emissions. However, lake-specific responses to nutrient changes are highly variable, complicating eutrophication management. These lake-specific responses could result from short-term stochastic drivers overshadowing lake-independent, long-term relationships between phytoplankton and nutrients. Here, we show that strong stoichiometric long-term relationships exist between nutrients and chlorophyll a (Chla) for 5-year simple moving averages (SMA, median R² = 0.87) along a gradient of total nitrogen to total phosphorus (TN:TP) ratios. These stoichiometric relationships are consistent across 159 shallow lakes (defined as average depth < 6 m) from a cross-continental, open-access database. We calculate 5-year SMA residuals to assess short-term variability and find substantial short-term Chla variation which is weakly related to nutrient concentrations (median R² = 0.12). With shallow lakes representing 89% of the world's lakes, the identified stoichiometric long-term relationships can globally improve quantitative nutrient management in both lakes and their catchments through a nutrient-ratio-based strategy.

Optical and molecular indices of dissolved organic matter for estimating biodegradability and resulting carbon dioxide production in inland waters: A review

Biodegradable dissolved organic carbon (BDOC) constitutes the most labile fraction of dissolved organic matter (DOM), which also functions as a source of CO₂ emissions from inland waters. However, no systematic review is available on DOM indicators of BDOC and CO₂ production potential. Optical and molecular indices can be used to track small changes in DOM composition during biodegradation. In this review, we identified four different methods for measuring BDOC together with their strengths and limitations. In addition, we discuss the potential of using documented optical indices based on absorption and fluorescence spectroscopy and molecular indices based on Fourier transform ion cyclotron mass spectrometry as proxies for estimating BDOC and biodegradation-induced CO₂ production based on previously reported relationships in the literature. Many absorbance- and fluorescence-based indices showed inconsistent relationships with BDOC depending on watershed characteristics, hydrology, and anthropogenic impacts. Nevertheless, several indices, including specific UV absorbance at 254 nm (SUVA₂₅₄), humification index (HIX), and terrestrial humic-like fluorescent DOM (FDOM) components, tended to have negative relationships with BDOC in tropical and temperate watersheds under baseflow or drought periods. Protein-like FDOM exhibited the strongest correlation with BDOC in different systems, except during storms and flood events. Despite the limited number of studies, DOM molecular indices exhibited consistent relationships with BDOC, suggesting that the relative abundance of aliphatic formulas and the molecular lability index could act as reliable proxies. The DOM optical indices explain up to 96% and 78% variability in BDOC and CO₂, respectively; nonetheless, there were limited studies on molecular indices, which explain up to 74% variability in BDOC. Based on literature survey, we recommend several sensitive indices such as SUVA₂₅₄, HIX, and terrestrial humic- and protein-like FDOM, which could be useful indicators of BDOC and dissolved CO₂ in inland water. Future research should incorporate a wider range of geographic regions with various land use, hydrology, and anthropogenic disturbances to develop system- or condition-specific DOM optical or molecular proxies for better prediction of BDOC and CO₂ emissions.

Riparian Forest Cover Modulates Phosphorus Storage and Nitrogen Cycling in Agricultural Stream Sediments

Watershed land cover affects in-stream water quality and sediment nutrient dynamics. The presence of natural land cover in the riparian zone can reduce the negative effects of agricultural land use on water quality; however, literature evaluating the effects of natural riparian land cover on stream sediment nutrient dynamics is scarce. The objective of this study was to assess if stream sediment phosphorus retention and nitrogen removal varies with riparian forest cover in agricultural watersheds. Stream sediment nutrient dynamics from 28 sites with mixed land cover were sampled three times during the growing season. Phosphorus dynamics and nitrification rates did not change considerably throughout the study period. Sediment total phosphorus concentrations and nitrification rates decreased as riparian forest cover increased likely due to a decline in fine, organic material. Denitrification rates were strongly correlated to surface water nitrate concentrations. Denitrification rate and denitrification enzyme activity decreased with an increase in forest cover during the first sampling period only. The first sampling period coincided with the greatest connectivity between the watershed and in-stream processing, indicating that riparian forest cover indirectly decreased denitrification rates by reducing the concentrations of dissolved nutrients entering the stream. This reduction in load may allow the sediment to maintain greater nitrogen removal efficiency, because bacteria are not saturated with nitrogen. Riparian forest cover also appeared to lessen the effect of agriculture in the watershed by decreasing the amount of fine material in the stream, resulting in reduced phosphorus storage in the stream sediment.

Available Dissolved Organic Carbon Alters Uptake and Recycling of Phosphorus and Nitrogen from River Sediments

Concurrent with nutrient pollution, agriculture has significantly impacted the quantity, composition, and bioavailability of catchment-derived dissolved organic carbon (DOC) in stream ecosystems. Based on the stoichiometric theory, we tested the hypothesis that bioavailable DOC will stimulate the heterotrophic uptake of soluble reactive P (SRP) and inorganic nitrogen in stream sediments. In a simplified laboratory column flow-through study, we exposed stream sediments to additions of glucose, nitrate, and phosphate alone and in combination (+C, +NP, +CNP), and calculated gross and net changes in DOC and nutrients via a mass balance approach. Our results show that glucose-C increased nutrient uptake, but also that NP additions resulted in the enhanced consumption of both native and added organic C. The effects of C addition were stronger on N than P uptake, presumably because labile C stimulated both assimilation and denitrification, while part of the P uptake was due to adsorption. Internal cycling affected net nutrient uptake due to losses of dissolved organically-complexed P and N (DOP and DON). Overall, our study shows that increases in the stoichiometric availability of organic carbon can stimulate N and P sequestration in nutrient-polluted stream sediments. Future studies are required to assess the effects of complex organic carbon sources on nutrient uptake in stream sediments under different environmental conditions, and whether these stoichiometric relations are relevant for ecosystem management.

Multi-scale characterisation of stream nutrient and carbon dynamics in sandy near coastal catchments of south-western Australia

Managers tasked with repairing degraded stream ecosystems require restoration strategies that are tailored to local and regional characteristics. Emerging evidence suggests that local reach-scale approaches may be as effective, if not more so, than catchment-scale actions in highly permeable coastal landscapes, particularly if there is hydraulic connectivity to shallow groundwater and where recharge is strongly seasonal. This study assessed the relative influence of catchment-scale land use and reach-scale vegetation structure on the distribution of carbon and nutrient concentrations of streams within urban and agricultural catchments of the Perth region of south-western Australia. We used linear mixed-effects models to evaluate the extent to which phosphorus, nitrogen and carbon concentrations in different stream zones (streamwater, and fluvial and parafluvial sediments) were explained by catchment and reach-scale attributes and moderated by high versus low-flow periods, i.e., in wet versus dry months. We found that reach-scale vegetation (woody plant cover, annual plant cover) was a better predictor of nutrient concentrations than catchment-scale land use, particularly total imperviousness, a common measure of urbanisation. Flow was also important, with carbon and nutrient concentrations better described by reach- or catchment-scale attributes during the low flow period. The extent to which individual catchment and reach attributes influenced the distribution of nutrients in different stream zones was complex. However, our results suggest that planting woody vegetation can reduce nitrogen concentrations in surface water and fluvial sediments. Reducing the abundance of weedy annual species and restoring woody perennial species may further reduce phosphorus concentrations in surface water. We conclude that local riparian restoration can be a cost-effective strategy for managing excess nutrients and carbon in flat and permeable urban landscapes, particularly during low flow periods.

Nitrate availability affects denitrification in Phragmites australis sediments

Understanding relationships between an increase in nitrate (NO₃ ^- ) loading and the corresponding effects of wetland vegetation on denitrification is essential to designing, restoring, and managing wetlands and canals to maximize their effectiveness as buffers against eutrophication. Although Phragmites australis (Cav.) Trin. ex Steud. is frequently used to remediate nitrogen (N) pollution, no information is available on how NO₃ ^- concentration may affect plant-mediated denitrification. In the present study, denitrification was measured in outdoor vegetated and unvegetated mesocosms incubated in both summer and winter. After spiking the mesocosms with NO₃ ^- concentrations typical of agricultural drainage water (0.7-11.2 mg N L^-1 ), denitrification was quantified by the simultaneous measurement of NO₃ ^- consumption and dinitrogen gas (N₂ ) production. Although denitrification rates varied with vegetation presence and season, NO₃ ^- availability exerted a significant positive effect on the process. Vegetated sediments were more efficient than bare sediments in adapting their mitigation potential to an increase in NO₃ ^- , by yielding a one-order-of-magnitude increase in NO₃ ^- removal rates, under both summer (743-6007 mg N m^-2 d^-1 ) and winter (43-302 mg N m^-2 d^-1 ) conditions along the NO₃ ^- gradient. Denitrification was the dominant sink for water NO₃ ^- in winter and only for vegetated sediments in summer. Nitrification likely contributed to fuel denitrification in summer unvegetated sediments. Since denitrification rates followed Michaelis-Menten kinetics, P. australis-mediated depuration may be considered optimal up to 5.0 mg N L^-1 . The present outcomes provide experimentally supported evidence that restoration with P. australis can work as a cost-effective means of improving water quality in agricultural watersheds.

Non-floodplain Wetlands Affect Watershed Nutrient Dynamics: A Critical Review

Wetlands have the capacity to retain nitrogen and phosphorus and are thereby often considered a viable option for improving water quality at local scales. However, little is known about the cumulative influence of wetlands outside of floodplains, i.e., non-floodplain wetlands (NFWs), on surface water quality at watershed scales. Such evidence is important to meet global, national, regional, and local water quality goals effectively and comprehensively. In this critical review, we synthesize the state of the science about the watershed-scale effects of NFWs on nutrient-based (nitrogen, phosphorus) water quality. We further highlight where knowledge is limited in this research area and the challenges of garnering this information. On the basis of previous wetland literature, we develop emerging concepts that assist in advancing the science linking NFWs to watershed-scale nutrient conditions. Finally, we ask, "Where do we go from here?" We address this question using a 2-fold approach. First, we demonstrate, via example model simulations, how explicitly considering NFWs in watershed nutrient modeling changes predicted nutrient yields to receiving waters-and how this may potentially affect future water quality management decisions. Second, we outline research recommendations that will improve our scientific understanding of how NFWs affect downstream water quality.

Spatial and temporal variations in the geochemistry of shallow groundwater contaminated with nitrate at a residential site

The concentrations of nitrate (NO₃^-), major ions, and dissolved inorganic carbon (DIC) and the stable carbon isotopes of DIC (δ¹³C_DIC) in shallow groundwater below a 45 × 60 m residential property was investigated over a period of 38 months. Our aim was to identify the processes which control the spatial and temporal distribution of NO₃^- in the shallow groundwater and assess water-rock interactions linked to denitrification. Groundwater sampled quarterly from eight locations showed an average NO₃^- concentration of 36.8 mg/L and a range between 0.1 and 214.9 mg/L compared to the US EPA maximum contaminant level of 10 mg/L. Heterogeneity in nitrate distribution was from residential application of N-based fertilizers offsite and from onsite application on flower beds and for lawn care. The temporal behavior of nitrate at all eight groundwater locations was markedly different and independent of seasonal hydrologic variations. Nitrate attenuation was spatially controlled by heterogeneous denitrification and rain dilution near roof drains. Groundwater locations with active denitrification were characterized by higher DIC concentrations and lower δ¹³C_DIC from organic carbon mineralization and by higher ionic concentrations from weathering of aquifer minerals. The variation in the relative standard deviations (RSD) of the measured parameters over space (RSD-s) and time (RSD-t) was highest for NO₃^- associated with variable spatiotemporal input and lowest for pH, pCO₂, and δ¹³C_DIC indirectly controlled by denitrification. Denitrification induced mineral weathering products such as DIC, Ca²⁺, Mg²⁺, and HCO₃^- showed medium to high RSD-s and RSD-t. The RSD-s and RSD-t were positively correlated (R² = 0.85) with the RSD-s showing approximately twofold higher magnitude than RSD-t due to greater variability between monitoring wells locations than variability at each groundwater location over time. Nitrate contamination and denitrification represent important long-term driver of aquifer weathering and changes in groundwater geochemistry below residential communities.

Balancing macronutrient stoichiometry to alleviate eutrophication

Reactive nitrogen (N) and phosphorus (P) inputs to surface waters modify aquatic environments, affect public health and recreation. Source controls dominate eutrophication management, whilst biological regulation of nutrients is largely neglected, although aquatic microbial organisms have huge potential to process nutrients. The stoichiometric ratio of organic carbon (OC) to N to P atoms should modulate heterotrophic pathways of aquatic nutrient processing, as high OC availability favours aquatic microbial processing. Heterotrophic microbial processing removes N by denitrification and captures N and P as organically-complexed, less eutrophying forms. With a global data synthesis, we show that the atomic ratios of bioavailable dissolved OC to either N or P in rivers with urban and agricultural land use are often distant from a "microbial optimum". This OC-deficiency relative to high availabilities of N and P likely overwhelms within-river heterotrophic processing. We propose that the capability of streams and rivers to retain N and P may be improved by active stoichiometric rebalancing. Although autotrophic OC production contributes to heterotrophic rates substantial control on nutrient processing from allochthonous OC is documented for N and an emerging field for P. Hence, rebalancing should be done by reconnecting appropriate OC sources such as wetlands and riparian forests that have become disconnected from rivers concurrent with agriculture and urbanisation. However, key knowledge gaps require research prior to the safe implementation of this approach in management: (i) to evaluate system responses to catchment inputs of dissolved OC forms and amounts relative to internal production of autotrophic dissolved OC and aquatic and terrestrial particulate OC and (ii) evaluate risk factors in anoxia-mediated P desorption with elevated OC scenarios. Still, we find stoichiometric rebalancing through reconnecting landscape beneficial OC sources has considerable potential for river management to alleviate eutrophication, improve water quality and aquatic ecosystem health, if augmenting nutrient source control.