Urea-N is ubiquitous in soils, having both natural and anthropogenic sources. The enzyme urease catalyzes its hydrolysis to NH3 and is produced by plants and many soil microorganisms, but there are ...growing concerns related to possible urea-induced eutrophication of surface waters proximate to agricultural fields. Agronomic research has focused on the relationship between urea hydrolysis and soil physical or chemical properties, rather than on direct measurements of the microbial community and its population diversity, especially using quantification of genes that code for urease. We quantified bacterial and archaeal 16S rRNA, fungal ITS, and bacterial ureC gene copies as a function of physical and chemical soil properties. Soils were sampled from A and B horizons along a toposequence that comprised an agricultural field, a grassed field border, and a forested riparian zone in the Chesapeake Bay watershed of Maryland. The riparian zone soils contained the highest total number of genes among both A- and B-horizon soils. The soils were then experimentally altered in the laboratory to achieve a range of pH values between 3.1 and 7.1. Soil pH was chosen as a variable because it varies both naturally and due to agronomic practices, and it influences microbial community structure and function. Archaeal 16S rRNA extracted from the pH-adjusted soils did not show a consistent pattern of increase or decrease with changes in pH, while ITS was greatest at low pH and bacterial 16S and bacterial ureC were greatest at high pH. We measured higher urea hydrolysis rates and gene copy numbers in A-horizon soils than in B-horizon soils, and found that urea hydrolysis rate was significantly correlated with gene copies of bacterial 16S, ureC, and increased pH. This suggests that liming acid soils increases urea hydrolysis rates in part by encouraging the growth of microorganisms capable of producing urease.
•pH impacts urease gene numbers.•Bacterial urease gene numbers are correlated with urea hydrolysis rate.•1–20% of bacterial community was ureolytic in the studied soils.•No correlation found between archaeal 16S gene copy number and urea hydrolysis rate.•No correlation found between fungal ITS gene copy number and urea hydrolysis rate.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Regular impoundment of the Three Gorges Reservoir (TGR) with intensified human activities in the watershed imparts a significant effect on the environmental changes in the riparian zone. In this ...study, six heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) in the riparian sediments of the entire TGR mainstream were investigated in 2014 and 2016 to identify their contamination and risk characteristics and decipher the main factors for the variation of the metal contamination. The results showed that the concentrations of the heavy metals in the sediments did not vary significantly between 2014 and 2016, and their contamination degrees decreased in the order of Cd> > Cu ≈ Zn > Pb > Cr ≈ Ni in 2014 and Cd> > Zn > Cu ≈ Pb > Cr ≈ Ni in 2016. The potential eco-risk of Cd was extremely high in the two years, while the eco-risk of other metals was very low. The sediments showed a moderate to high contamination level, a high potential eco-risk but a low toxic risk to aquatic biota in the two years. Spatially, the contamination and risk levels of heavy metals were relatively higher in the downstream TGR region in 2014 except for the sites close to the urban areas but in the upper-middle TGR region in 2016. Increasing anthropogenic influence contributed to the high contamination and risk levels of Cd, Cu, Pb and Zn in the upper-middle region in 2016. The results indicated that the Cd contamination in the riparian sediments of the TGR was still a vital environmental issue, and the decreased sediment inputs from the upstream major tributaries, the periodic and anti-seasonal flow regulation, local geomorphological characteristics and anthropogenic activities determined the contamination distribution of heavy metals in the riparian sediments.
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•Spatial variation of metal contamination in the riparian sediments could occur after the TGR flow regulation.•Higher contamination and eco-risk of metals existed in the downstream TGR in 2014 but in the upper-middle regions in 2016.•Cd was the concerned metal with high contamination and potential eco-risk in both years.•Human activities increasingly contributed to metal contamination in the upper-middle TGR region.•Decreasing sediment inputs, flow regulation, geomorphological and anthropogenic factors determined the metal redistribution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Riparian zones can receive large amounts of nitrate, potentially contributing to water pollution. Denitrification is a major pathway to remove nitrate. Previous research on riparian denitrification ...focused on natural factors, but frequently neglected the roles of human activity, such as pesticide accumulations. Here, we combined field investigations and exposure experiments to reveal the responses of denitrification and N2O emission to chlorothalonil (CTN, a common pesticide) in column experiments with riparian sediments. In this study, CTN inhibited denitrification and led to nitrate accumulation in sediments. Furthermore, CTN significantly increased N2O emission by 208–377%, and this response was regulated by N2O reductase (NOS) activity rather than nosZ abundance. A mechanistic study indicated that the critical step (glyceraldehyde-3-phosphate to 3-phosphogylcerate) catalyzed by glyceraldehyde-3-phosphate dehydrogenase during microbial metabolism greatly influenced denitrification in CTN-polluted sediments. Our data also revealed that CTN declined electron donor NADH, electron transport system, and denitrifying enzyme activities during denitrification. Such responses suggested that CTN deteriorated sediment denitrification by inhibiting electron production, transport and consumption in denitrifiers. Additionally, structure equation modeling indicated that NOS was the key factor in predicting denitrification rate in CTN-polluted sediments. Overall, this is the first study to explore the effects of pesticide on denitrification and N2O emission in riparian zones at microbial metabolism level. Our results suggest that the safety threshold of CTN accumulation for inhibiting sediment denitrification is approximately 2 mg kg−1, and imply that the wide presence of pesticides in riparian zones could impact eutrophication control of aquatic ecosystems.
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•First study to test the effects of pesticides on denitrification in riparian sediments.•CTN significantly inhibited denitrification but stimulated N2O emission.•The critical step catalyzed by GAPDH in glycolysis greatly impacted denitrification.•N2O reductase was the key factor for predicting denitrification rate in CTN-polluted sediments.•Safety threshold of CTN for inhibiting denitrification should be no more than 2 mg kg−1.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Riparian zones are important buffer zones for streams as they are hotspots of nitrate transformation and removal in agricultural catchments. However, mixing of water from different sources and ...various transformation processes can complicate the quantification of nitrate turnover in riparian zones. In this study, we analyzed nitrate concentration and isotope data in riparian groundwater along a 2‐km stream section in central Germany. We developed a mathematical model combining end‐member mixing and isotope modeling to account for mixing of river water and groundwater and quantify nitrate transformation in riparian groundwater. This enabled us to explicitly determine the extent of denitrification (as process leading to permanent nitrate removal from riparian groundwater) and transient nitrate removal by additional processes associated with negligible isotope fractionation (e.g., plant uptake and microbial assimilation) and to perform an extensive uncertainty analysis. Based on the nitrogen isotope data of nitrate, the simulations suggest a mean removal of up to 27% by additional processes and only about 12% by denitrification. Nitrate removal from riparian groundwater by additional processes exceeded denitrification particularly in winter and at larger distance from the river, underlining the role of the river as organic carbon source. This highlights that nitrate consumption by additional processes predominates at the field site, implying that a substantial fraction of agricultural nitrogen input is not permanently removed but rather retained in the riparian zone. Overall, our model represents a useful tool to better compare nitrogen retention to permanent nitrogen removal in riparian zones at various temporal and spatial scales.
Plain Language Summary
Nitrogen is an important nutrient for agricultural crops. However, excessive nitrogen input into surface water in the form of nitrate can lead to algae blooms and lack of oxygen. The riparian zones of rivers are important buffer zones where groundwater is connected to soils, which are rich in soil organisms and organic matter pools fueling reaction processes. Hence, plants and bacteria can remove nitrate from riparian groundwater before it reaches the river. Bacterial consumption of nitrate (denitrification) leads to complete removal of nitrogen via release of nitrogen gas into the atmosphere. In contrast, other biogeochemical processes such as nitrate uptake by plants merely result in nitrogen retention within riparian zones. To quantify the role of denitrification relative to other processes, we developed a novel model combining concentration and isotope data of nitrate and applied it to a groundwater study site in Central Germany. We found that nitrate removal from riparian groundwater by additional processes largely exceeded denitrification. Hence, a major fraction of nitrogen inputs was retained in the riparian zone and may eventually end up in the river. Such information is highly relevant for many river ecosystems at risk of eutrophication because of high nitrogen inputs from agriculture.
Key Points
We present a model using concentration and isotope data to distinguish riparian denitrification from additional nitrate removal processes
The model was applied to concentration and dual‐element isotope data of nitrate from riparian groundwater wells
Nitrate removal by additional processes greatly exceeded denitrification, particularly at larger distance from the river and in winter
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
•Long-term effects of selective thinning along two small streams were investigated.•One of the streams was bordered by broadleaf trees and the other by conifers.•Water chemistry and benthic ...macroinvertebrates were monitored.•A few differences in water chemistry were detected.•Benthic macroinvertebrates responded to selective thinning.
The maintenance of narrow strips of trees (forest buffers) along the shorelines of surface water bodies during logging is a common measure to protect freshwater habitats. The functionality of forest buffers may be improved by actively managing the streamside forest early in the rotation for their eventual function as buffers, including by increasing the proportion of broadleaf trees in spruce-dominated stands. In this study, long-term effects of different selective thinning regimes along two small forest streams were investigated in south-central Sweden. In a young coniferous forest, a c. 10 m-wide band along the streams was selectively thinned in 1998 to create a band with purely broadleaf trees along one of the streams and purely conifers along the other. Forest stand characteristics, water chemistry and benthic macroinvertebrates data were collected during 1996–2003 (before and after selective thinning). The streams were re-investigated 20–22 years after thinning, together with three streams representing operational forest management. The forest adjacent to all five streams was inventoried and litterfall, stream water chemistry, and benthic macroinvertebrates composition were monitored between spring and late autumn during 2018–2020. Twenty years after thinning, the thinned bands beside the streams were still dominated by either broadleaf trees or conifers, depending on the stream. Over the longer term, the differences in water chemistry between the streams with selective thinning were mainly related to lower pH, ANC, Tot-P and Tot-N concentrations in the stream bordered by mainly broadleaf trees. Analysis of benthic macroinvertebrates was based on environmental quality indices (ASPT and EPT), diversity and abundance metrics, and relative abundances of functional feeding groups. Streams with higher broadleaf litter inputs tended to score better on the ASPT and EPT indices than those with lower broadleaf inputs, as well as supporting higher relative abundances of one or more groups of invertebrate detritivores (leaf shredders, collector-gatherers and/or passive filter feeders). This suggests that management of the density of broadleaf trees beside these streams might support higher ecological status and will support a greater proportion of detritivores in benthic food webs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The Earth is experiencing excessive nitrogen (N) input to its various ecosystems due to human activities. How to effectively and efficiently remove N from ecosystems has been, is and will be at the ...center of attention in N research. Hyporheic and riparian zones are widely acknowledged for their buffering capacity to reduce contaminants (especially N) transport downstream. However, these zones are usually misunderstood that they can remove N at all spots and at any moments. Here pathways of N removal from hyporheic and riparian zones are reviewed and summarized with an emphasize on their hot spots and hot moments. N is biogeochemically removed by denitrification, anammox, nitrifier denitrification, denitrifying anaerobic methane oxidation, Feammox and Sulfammox. Hot moments of N removal are mainly triggered by precipitation, fire and snowmelt. Finally, some research needs are outlined and discussed, such as developing approaches for multiscale sampling and monitoring, quantifying the effects of hot spots and hot moments at hyporheic and riparian zones and evaluating the impacts of human activities on hot spots and hot moments, to inspire more research on hot spots and hot moments of N removal. By this review, we hope to bring awareness of the heterogeneity of hyporheic and riparian zones to catchment managers and policy makers when tackling N pollution problems.
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•Riparian and hyporheic zones are misunderstood that they can remove nitrogen at all spots and at any moments.•There are hot spots and hot moments of nitrogen removal from riparian and hyporheic zones.•Hot spots of six nitrogen removal pathways are summarized.•Precipitation, fire and snowmelt can trigger hot moments.•Suggestions for further applications of hot spots and hot moments to nitrogen removal are provided.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Reactive oxygen species (ROS) are ubiquitous in the natural environment and play a pivotal role in biogeochemical processes. However, the spatiotemporal distribution and production mechanisms of ROS ...in riparian soil remain unknown. Herein, we performed uninterrupted monitoring to investigate the variation of ROS at different soil sites of the Weihe River riparian zone throughout the year. Fluorescence imaging and quantitative analysis clearly showed the production and spatiotemporal variation of ROS in riparian soils. The concentration of superoxide (O2 •–) was 300% higher in summer and autumn compared to that in other seasons, while the highest concentrations of 539.7 and 20.12 μmol kg–1 were observed in winter for hydrogen peroxide (H2O2) and hydroxyl radicals (•OH), respectively. Spatially, ROS production in riparian soils gradually decreased along with the stream. The results of the structural equation and random forest model indicated that meteorological conditions and soil physicochemical properties were primary drivers mediating the seasonal and spatial variations in ROS production, respectively. The generated •OH significantly induced the abiotic mineralization of organic carbon, contributing to 17.5–26.4% of CO2 efflux. The obtained information highlighted riparian zones as pervasive yet previously underestimated hotspots for ROS production, which may have non-negligible implications for carbon turnover and other elemental cycles in riparian soils.
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IJS, KILJ, NUK, PNG, UL, UM
Physicochemical and toxicological characterization of leather tanning wastewater has been widely documented. However, few reports have examined the response of denitrification N2 and N2O emissions in ...riparian sediments of tannery wastewater-receiving rivers. In this study, 15N-nitrate labeling was used to reveal the effects of tanning wastewater on denitrification N2 and N2O emission in a wastewater-receiving river (the old Mang River, OMR). OMR riparian sediments were highly polluted with total organic carbon (93.39 mg/kg), total nitrogen (5.00 g/kg) and heavy metals; specifically, Cr, Zn, Cd, and Pb were found at concentrations 47.3, 5.8, 1.6, 4.3, and 2.8 times that in a nearby parallel river without tanning wastewater input (the new Mang River, NMR), respectively. The denitrification N2 emission rates (0.0015 nmol N · g−1 h−1) of OMR riparian sediments were significantly reduced by 2.5 times compared with those from the NMR (p < 0.05), but the N2O emission rates (0.31 nmol N · g−1 h−1) were significantly increased (4.1 times, p < 0.05). Although the dominant nitrogen-transforming bacteria phylum was Proteobacteria in the riparian sediments of both rivers, 11 nitrogen-transforming bacteria genera in the OMR were found to be significantly enriched; five of these were related to pollutant degradation based on linear discriminant analysis (LDA >3). The average activity of the electron transport system in the OMR was 6.3 times lower than that of the NMR (p < 0.05). Among pollution factors, heavy metal complex pollution was the dominant factor driving variations in N2O emissions, microbial community structure, and electron transport system activity. These results provide a new understanding and reference for the treatment of tanning wastewater-receiving rivers.
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•Complex pollution risks were evaluated in a tanning wastewater receiving river.•Complex pollution led to denitrification N2 emission inhibited and N2O enriched.•Heavy metals were the main driving factor for the N2O excessive emission.•Bacterial compositions of nitrogen transformation changed under the pollution.•The electron transfer activities were significantly inhibited by the pollution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Forest managers use tree buffers to protect streams from clear-cutting operations.•Functional diversity measures are critical indicators of forest disturbance.•Riparian buffer size has little impact ...on spider or vascular plant functional diversity.•Biotic and abiotic buffer conditions are more important for system function.•Forest management should promote species richness to increase ecosystem function.
Retention of forested buffers around streams following forest cutting operations is a common management technique used to protect aquatic resources and conserve the surrounding ecosystem services. Species richness, or α-diversity, is commonly used as an indicator of the effects of forestry management although it provides very little information about those effects on ecosystem processes and function. Functional diversity links species traits and ecosystem function incorporating species diversity, community composition, and functional guild and is more suitable to investigate the direct and indirect effects of forestry on ecosystem function. We sampled spiders and vascular plants in buffered and unbuffered stream-forest systems in southern Sweden and used a trait-based approach to assess the effects of buffer size and environmental variables on functional diversity. We used structural equation modeling (SEM) to explore the effects of buffer size and condition on spider and vascular plant diversity. We found no effect of buffer size on the functional richness or functional redundancy for spiders or vascular plants. Buffer size had a slight effect on the α-diversity of spiders within small buffers and fully forested sites but the effect was small. Other buffer variables including canopy closure, buffer density, bare ground coverage, and soil fertility had direct effects on spider and vascular plant functional diversity. The main driver of functional richness was α-diversity, but our SEM analysis illustrated other environmental variables working jointly to drive functional diversity. Using a trait-based approach, we showed that forested buffers have a minimal overall impact on spider and vascular plant functional diversity. However, it is important to maintain high levels of α-diversity to preserve and promote both spider and plant functional richness in production forests and we suggest that forest management conserves and encourages high levels of α-diversity to increase overall functional diversity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The biota of European rivers are affected by a wide range of stressors impairing water quality and hydro‐morphology. Only about 40% of Europe's rivers reach ‘good ecological status’, a target set by ...the European Water Framework Directive (WFD) and indicated by the biota. It is yet unknown how the different stressors in concert impact ecological status and how the relationship between stressors and status differs between river types. We linked the intensity of seven stressors to recently measured ecological status data for more than 50,000 sub‐catchment units (covering almost 80% of Europe's surface area), which were distributed among 12 broad river types. Stressor data were either derived from remote sensing data (extent of urban and agricultural land use in the riparian zone) or modelled (alteration of mean annual flow and of base flow, total phosphorous load, total nitrogen load and mixture toxic pressure, a composite metric for toxic substances), while data on ecological status were taken from national statutory reporting of the second WFD River Basin Management Plans for the years 2010–2015. We used Boosted Regression Trees to link ecological status to stressor intensities. The stressors explained on average 61% of deviance in ecological status for the 12 individual river types, with all seven stressors contributing considerably to this explanation. On average, 39.4% of the deviance was explained by altered hydro‐morphology (morphology: 23.2%; hydrology: 16.2%), 34.4% by nutrient enrichment and 26.2% by toxic substances. More than half of the total deviance was explained by stressor interaction, with nutrient enrichment and toxic substances interacting most frequently and strongly. Our results underline that the biota of all European river types are determined by co‐occurring and interacting multiple stressors, lending support to the conclusion that fundamental management strategies at the catchment scale are required to reach the ambitious objective of good ecological status of surface waters.
We analyzed the effects of multiple stressors on ecological status of more than 50,000 European river catchments. Hydro‐morphological degradation showed the strongest effects, followed by nutrient enrichment and toxic substances. Interactive stressor effects were prominent. Our findings highlight the role of multiple stressors acting on Europe's rivers and call for fundamental restorative management strategies at the catchment‐scale.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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