Denitrification and patterns of electron donors and acceptors in eight riparian zones with contrasting hydrogeology

A better understanding of nitrate removal mechanisms is important for managing the water quality function of stream riparian zones. We examined the linkages between hydrologic flow paths, patterns of electron donors and acceptors and the importance of denitrification as a nitrate removal mechanism in eight riparian zones on glacial till and outwash landscapes in southern Ontario, Canada. Nitrate-N concentrations in shallow groundwater from adjacent cropland declined from levels that were often 10–30 mg L^–1 near the field-riparian edge to < 1 mg L^–1 in the riparian zones throughout the year. Chloride data suggest that dilution cannot account for most of this nitrate decline. Despite contrasting hydrogeologic settings, these riparian zones displayed a well-organized pattern of electron donors and acceptors that resulted from the transport of oxic nitrate-rich groundwater to portions of the riparian zones where low DO concentrations and an increase in DOC concentrations were encountered. The natural abundances of d15N and in situ acetylene injection to piezometers indicate that denitrification is the primary mechanism of nitrate removal in all of the riparian zones. Our data indicate that effective nitrate removal by denitrification occurs in riparian zones with hydric soils as well as in non-hydric riparian zones and that a shallow water table is not always necessary for efficient nitrate removal by denitrification. The location of hot spots of denitrification within riparian areas can be explained by the influence of key landscape variables such as slope, sediment texture and depth of confining layers on hydrologic pathways that link supplies of electron donors and acceptors.     

Impact of a first-order riparian zone on nitrogen removal and export from an agricultural ecosystem

Riparian zones are reputed to be effective at preventing export of agricultural groundwater nitrogen (N) from local ecosystems. This is one impetus behind riparian zone regulations and initiatives. However, riparian zone function can vary under different conditions, with varying impacts on the regional (and ultimately global) environment. Rates of groundwater delivery to the surface appear to have significant effects on the N-removing capabilities of a riparian zone. Research conducted at a first-order agricultural watershed with a well-defined riparian zone in the Maryland coastal plain indicates that more than 2.5 kg/day of nitrate-N can be exported under moderate-to-high stream baseflow conditions. The total nitrate-N load that exits the system increases with increasing flow not simply because of the greater volume of water export. Stream water nitrate-N concentrations also increase by more than an order of magnitude as flow increases, at least during baseflow. This appears to be largely the result of changes in dominant groundwater delivery mechanisms. Higher rates of groundwater exfiltration lessen the contact time between nitrate-carrying groundwater and potentially reducing riparian soils. Subsurface preferential flow paths, in the wetland and adjacent field, also strongly influence N removal. Simple assumptions regarding riparian zone function may be inadequate because of complexities observed in response to changing hydrologic conditions.     

Long-term nitrate removal in a stream riparian zone

The long-term capacity of riparian zones in regulating groundwater nitrate fluxes is not well understood. This study analyses patterns of nitrate removal for the period 1994–2012 at two sites in a river floodplain that have received high groundwater nitrate loading from a large upland aquifer for over 32 years. During the study, mean NO₃ ^?–N concentrations entering the riparian zone varied between 20–30 and 30–42 mg/L at the upstream and downstream sites respectively, but did not show any clear inter-annual trend. A permeable sand layer in the riparian zone is underlain by a regional aquitard at a depth of 5–6 m and 4 m at the upstream site and downstream site respectively. Denitrification resulted in a decline in nitrate concentrations as lateral groundwater flow in the sand layer interacted with buried peat and channel bar deposits that range up to 3 m in depth at both riparian sites. This interaction was greater at the downstream site where the organic deposits extend down to <1 m from the aquitard in some locations. At the upstream site nitrate removal efficiency in the sand layer, at depths of 3–4 m ~20 m from the river bank, declined from 68 % in 1996–1998 to 42 % in 2009–2012. A smaller decline from 92 to 82 % occurred in the sand layer 10 m from the river bank during the study. In contrast, no clear pattern of change was evident at the downstream site where a nitrate removal efficiency of 98–100 % occurred at the river bank in most years between 1994 and 2012. These data suggest that the long-term nitrate removal performance of some riparian zones may decline if carbon availability for denitrification becomes limited as a result of variations in the quantity, quality and location of subsurface organic deposits that interact with deeper groundwater flowpaths.     

河岸带生态系统退化机制及其恢复研究进展

恢复和重建自然和人为干扰导致的退化河岸带生态系统是目前恢复生态学、流域生态学等学科研究的重要内容之一．对河岸带生态系统的干扰表现在河流水文特征改变、河岸带直接干扰和流域尺度干扰3个方面,分别具有不同的影响机制．河流水文特征改变通过改变河岸土壤湿度、氧化还原电位、生物生存环境以及沉积物传输规律对河岸带生态系统产生影响;河岸带直接干扰通过人类活动及外来物种入侵而直接影响河岸带植被多样性;流域尺度干扰则主要表现在河道刷深、河道淤积、河岸带地下水位降低和河流冲刷过程改变等．河岸带生态恢复评价对象包括河岸带生态系统各要素,评价指标已从单一的生态指标转向综合性指标．河岸带生态恢复应在景观或者流域尺度上进行考虑,识别对其影响的生物和物理过程以及导致其退化的干扰因子,通过植被重建与水文调控来进行．扩展研究尺度和研究对象及采用多学科的研究方法将是今后相关研究中的重要问题．     

DOC and DIC in Flowpaths of Amazonian Headwater Catchments with Hydrologically Contrasting Soils

Organic and inorganic carbon (C) fluxes transported by water were evaluated for dominant hydrologic flowpaths on two adjacent headwater catchments in the Brazilian Amazon with distinct soils and hydrologic responses from September 2003 through April 2005. The Ultisol-dominated catchment produced 30% greater volume of storm-related quickflow (overland flow and shallow subsurface flow) compared to the Oxisol-dominated catchment. Quickflow fluxes were equivalent to 3.2 ± 0.2% of event precipitation for the Ultisol catchment, compared to 2.5 ± 0.3% for the Oxisol-dominated watershed (mean response ±1 SE, n = 27 storms for each watershed). Hydrologic responses were also faster on the Ultisol watershed, with time to peak flow occurring 10 min earlier on average as compared to the runoff response on the Oxisol watershed. These different hydrologic responses are attributed primarily to large differences in saturated hydraulic conductivity (K _s). Overland flow was found to be an important feature on both watersheds. This was evidenced by the response rates of overland flow detectors (OFDs) during the rainy season, with overland flow intercepted by 54 ± 0.5% and 65 ± 0.5% of OFDs for the Oxisol and Ultisol watersheds respectively during biweekly periods. Small volumes of quickflow correspond to large fluxes of dissolved organic C (DOC); DOC concentrations of the hydrologic flowpaths that comprise quickflow are an order of magnitude higher than groundwater flowpaths fueling base flow (19.6 ± 1.7 mg l⁻¹ DOC for overland flow and 8.8 ± 0.7 mg l⁻¹ DOC for shallow subsurface flow versus 0.50 ± 0.04,mg l⁻¹ DOC in emergent groundwater). Concentrations of dissolved inorganic C (DIC, as dissolved CO₂–C plus HCO₃⁻–C) in groundwater were found to be an order of magnitude greater than quickflow DIC concentrations (21.5 mg l⁻¹ DIC in emergent groundwater versus 1.1 mg l⁻¹ DIC in overland flow). The importance of deeper flowpaths in the transport of inorganic C to streams is indicated by the 40:1 ratio of DIC:DOC for emergent groundwater. Dissolved CO₂–C represented 92% of DIC in emergent groundwater. Results from this study illustrate a highly dynamic and tightly coupled linkage between the C cycle and the hydrologic cycle for both Ultisol and Oxisol landscapes: organic C fluxes strongly tied to flowpaths associated with quickflow, and inorganic C (particularly dissolved CO₂) transported via deeper flowpaths.     

Nitrogen removal in subsurface water by narrow buffer strips in the intensive farming landscape of the Po River watershed, Italy

In many countries buffer strips have become an important management tool widely accepted for controlling the diffuse pollution and supporting the development of more sustainable agriculture. However, there is the need to investigate their role in intensive farming systems where a realistic and shareable proposal to realize buffer strips can only foresee the use of a limited space. We evaluated the nitrogen buffering capacities of two narrow riparian strips (5-8 m) along irrigation ditches located in a typical flat agricultural watershed of the alluvial plain of the River Po (Northern Italy). Subsurface water level and nutrient concentrations were monitored along transects of piezometers installed from crop fields to ditches in two different areas. Spatial and temporal variation in water chemistry and hydrology were investigated to individuate the main processes (biological or physical) leading to groundwater nitrate depletion related to fertilization, pluviometric regime and seasonal variation. The results obtained indicate an elevated nitrate removal efficiency in both riparian areas. Compared to the high mean concentrations measured at the exit of the crop fields (10-90 mg l⁻¹ N-NO₃⁻), nitrate levels within riparian sites can be very low, completely disappearing below the ditches. The patterns of some chemical species (O₂, SO₄²⁻ and HCO₃⁻) and the potential denitrification rates suggest that denitrification plays a predominant role in the N-NO₃⁻ depletion observed in the first few meters of the herbaceous strip. The key factors in the system are the elevated groundwater residence time and the effect of the evapotranspiration. The water uptake by woody vegetation affects the subsurface water to flow through the riparian zone and, at the same time, it contributes to completely remove the nitrate from the groundwater.Our findings also suggest the double role of riparian vegetation both in ecohydrological and biological terms. In fact the water uptake by trees affects the subsurface flow pattern and contributes to completely remove the nitrate in the riparian zone.     

Surface and subsurface nitrate flow pathways on a watershed scale

Determining the interaction and impact of surface runoff and subsurface flow processes on the environment has been hindered by our inability to characterize subsurface soil structures on a watershed scale. Ground penetrating radar (GPR) data were collected and evaluated in determining subsurface hydrology at four small watersheds in Beltsville, MD. The watersheds have similar textures, organic matter contents, and yield distributions. Although the surface slope was greater on one of the watersheds, slope alone could not explain why it also had a nitrate runoff flux that was 18 times greater than the other three watersheds. Only with knowledge of the subsurface hydrology could the surface runoff differences be explained. The subsurface hydrology was developed by combining GPR and surface topography in a geographic information system. Discrete subsurface flow pathways were identified and confirmed with color infrared imagery, real-time soil moisture monitoring, and yield monitoring. The discrete subsurface flow patterns were also useful in understanding observed nitrate levels entering the riparian wetland and first order stream. This study demonstrated the impact that subsurface stratigraphy can have on water and nitrate (NO3-N) fluxes exiting agricultural lands, even when soil properties, yield distributions, and climate are similar. Reliable protocols for measuring subsurface fluxes of water and chemicals need to be developed.     

Estimation of nitrate removal by riparian wetlands and streams in agricultural catchments: effect of discharge and stream order

1. Assessment of the role of landscape structures such as buffers is a necessary prerequisite for the sustainable management of water resources in an agricultural setting. 2. We monitored nitrate concentrations during interstorm periods at the outlet of 16 subcatchments of different orders within a catchment of 378 km². We characterised stream network, wetlands, agricultural practices and land cover and identified their relationships with nitrate fluxes and concentrations. 3. Two main factors controlled annual nitrate fluxes: the agricultural nitrogen surplus and the nature of the system comprising the wetland zone and adjoining watercourses. In the latter case, nitrate fluxes were reduced in proportion to the surface area of the riparian wetland and the flowpath distance of fluxes in the stream network. At the scale of the order‐6 stream, 53% of annual nitrate flux during interstorm periods was removed during transfer via the wetland and the river, corresponding to 21.1 kg N ha^?1 per year. 4. The influence of the riparian wetland zone/watercourse system increased during periods of low water level, explaining up to 64% of nitrate concentration variation among locations within the river network, but only 9% during periods of high water level. 5. The buffering role was stronger at higher stream orders, and the dependence on stream order was more apparent at low water level, when we observed mean nitrate concentrations in the order‐6 stream that were 47% lower than observed in order‐2 or order‐3 streams.     

Nitrogen Removal by Riparian Buffers along a European Climatic Gradient: Patterns and Factors of Variation

We evaluated nitrogen (N) removal efficiency by riparian buffers at 14 sites scattered throughout seven European countries subject to a wide range of climatic conditions. The sites also had a wide range of nitrate inputs, soil characteristics, and vegetation types. Dissolved forms of N in groundwater and associated hydrological parameters were measured at all sites; these data were used to calculate nitrate removal by the riparian buffers. Nitrate removal rates (expressed as the difference between the input and output nitrate concentration in relation to the width of the riparian zone) were mainly positive, ranging from 5% m⁻¹ to 30% m⁻¹, except for a few sites where the values were close to zero. Average N removal rates were similar for herbaceous (4.43% m⁻¹) and forested (4.21% m⁻¹) sites. Nitrogen removal efficiency was not affected by climatic variation between sites, and no significant seasonal pattern was detected. When nitrate inputs were low, a very large range of nitrate removal efficiencies was found both in the forested and in the nonforested sites. However, sites receiving nitrate inputs above 5 mg N L⁻¹ showed an exponential negative decay of nitrate removal efficiency (nitrate removal efficiency = 33.6 e^{−0.11 NO3input}, r ² = 0.33, P < 0.001). Hydraulic gradient was also negatively related to nitrate removal (r = −0.27, P < 0.05) at these sites. On the basis of this intersite comparison, we conclude that the removal of nitrate by biological mechanisms (for example, denitrification, plant uptake) in the riparian areas is related more closely to nitrate load and hydraulic gradient than to climatic parameters. Received 15 August 2001; accepted 2 May 2002.     

Riparian Trees and Flow Paths between the Hyporheic Zone and Groundwater in the Oberer Seebach,Austria

We investigated lateral subsurface water exchange in a 2^nd order mountain stream with a piezometer method. At both banks the stream hyporheic zone lost water to the riparian groundwater zone. Independently, the hydraulic heads at three sites in the streambed and in the riparian zone exhibited periodic, diurnal fluctuations. We attributed them to water consumption by the riparian trees, as solar radiation explained part of this additional variation. Our results demonstrate that subsurface water exchanges take place between the hyporheic zone and lateral riparian groundwater in spatially defined small‐scale flow paths. These small‐scale interactions occur within the context of large‐scale patterns of loss and gain of channel water.     

Denitrification Potential, Root Biomass, and Organic Matter in Degraded and Restored Urban Riparian Zones

Hydrologic changes associated with urbanization often lead to lower water tables and drier, more aerobic soils in riparian zones. These changes reduce the potential for denitrification, an anaerobic microbial process that converts nitrate, a common water pollutant, into nitrogen gas. In addition to oxygen, denitrification is controlled by soil organic matter and nitrate. Geomorphic stream restorations are common in urban areas, but their effects on riparian soil conditions and denitrification have not been evaluated. We measured root biomass, soil organic matter, and denitrification potential (anaerobic slurry assay) at four depths in duplicate degraded, restored, and reference riparian zones in the Baltimore, Maryland, U.S.A., metropolitan area. There were three main findings in this study. First, although reference sites were wet and had high soil organic matter, they had low levels of nitrate relative to degraded and restored sites and therefore there were few differences in denitrification potential among sites. Evaluations of riparian restorations that have nitrate removal by denitrification as a goal should consider the complex controls of this process and how they vary between sites. Second, all variables declined markedly with depth in the soil. Restorations that increase riparian water tables will thus foster interaction of groundwater nitrate with near-surface soils with higher denitrification potential. Third, we observed strong positive relationships between root biomass and soil organic matter and between soil organic matter and denitrification potential, which suggest that establishment of deep-rooted vegetation may be particularly important for increasing the depth of the active denitrification zone in restored riparian zones.     

Influence of base flow stream bank seepage on riparian zone nitrogen biogeochemistry

We examined the effect of sustained stream bank seepage during base flow conditions on the pore water nitrogen biogeochemistry of two riparian zones in lowland agricultural areas in southern Ontario, Canada. Nitrate, ammonium and dissolved oxygen concentrations in riparian subsurface water over a two-year period showed well-organized spatial patterns along stream bank seepage flow paths that extended seasonally up to 25 m inland. High levels of dissolved oxygen and NO₃⁻ in stream inflow were depleted rapidly at the stream bank interface suggesting the occurrence of aerobic microbial respiration followed by denitrification. A zone of NH₄⁺ accumulation persisted in more anaerobic sediments inland from the bank margin, although the magnitude and intensity of the pattern varied seasonally. A bromide tracer and NO₃⁻ co-injection at the stream bank interface indicated that bank seepage occurred along preferential flow paths in a poorly sorted gravel layer in the two riparian zones. Depletion of NO₃⁻ in relation to co-injected bromide confirmed that the bank margin was a hot spot of biogeochemical activity within the riparian zone. Conceptual models of humid temperate riparian zones have focused on nitrogen biogeochemistry in relation to hillslope to stream hydrologic flow paths. However, our results suggest that sustained stream bank inflow during low flow conditions can exert a dominant control on riparian nitrogen cycling in lowland landscapes where level riparian zones bounded by perennial streams receive limited subsurface inflows from adjacent slopes.     

Nitrogen retention in the riparian zone of catchments underlain by discontinuous permafrost

1. Riparian zones function as important ecotones that reduce nitrate concentration in groundwater and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high. 2. In this study, riparian zone nitrogen retention was examined in a high permafrost catchment (approximately 53% of land area underlain by permafrost) and a low permafrost catchment (approximately 3%). To estimate the contribution of the riparian zone to catchment nitrogen retention, we analysed groundwater chemistry using an end‐member mixing model. 3. Stream nitrate concentration was over twofold greater in the low permafrost catchment than the high permafrost catchment. Riparian groundwater was not significantly different between catchments, averaging 13 μm overall. Nitrogen retention, measured using the end‐member mixing model, averaged 0.75 and 0.22 mmol N m^?2 day^?1 in low and high permafrost catchments, respectively, over the summer. The retention rate of nitrogen in the riparian zone was 10–15% of the export in stream flow. 4. Our results indicate that the riparian zone functions as an important sink for groundwater nitrate and dissolved organic carbon (DOC). However, differences in stream nitrate and DOC concentrations between catchments cannot be explained by solute inputs from riparian groundwater to the stream and differences between streams are probably attributable to deeper groundwater inputs or flows from springs that bypass the riparian zone.     

Natural channeling in riverine forests determines variations in their floristic composition,structure, and land use in southern Brazil

Hydrological processes in riparian forests affect their geomorphology and drainage, and define their structure. Interactions among many environmental variables and the tree community in continuous riverine forests affect their formation and add to their complexity, making our understanding of these habitats particularly challenging. We evaluated the tree species diversity and richness in relation to relief and flooding regimes in the watershed of a riparian forest in southern Brazil. The environments of the topographic gradients were classified according to relief and soil. Canonical correspondence analysis and indicator species analysis were used for the environmental classification. A detailed soil survey showed that the organic matter content and pH are quite distinct among the environmental categories. Frequent flooding of low intensity on the floodplain was associated with incipient and shallow channels, greater frequency of livestock encroachment, and lower frequency of shade-tolerant species dominance in the deep relief of the valleys and the headwaters. We conclude that there is a dynamic flow of species migration between the flooded and non-flooded environments in small channels and that the narrow deep channels in the floodplain with less frequent and intense flooding favor an expansion of the upland tree species into riparian areas, with channel shape and depth important variables in the impact of cattle encroachment on vegetation. The greatest tree diversity was found in the ravine habitats in the intermediate landscapes.

     

Microbial communities across a hillslope‐riparian transect shaped by proximity to the stream,groundwater table,and weathered bedrock

Watersheds are important suppliers of freshwater for human societies. Within mountainous watersheds, microbial communities impact water chemistry and element fluxes as water from precipitation events discharge through soils and underlying weathered rock, yet there is limited information regarding the structure and function of these communities. Within the East River, CO watershed, we conducted a depth‐resolved, hillslope to riparian zone transect study to identify factors that control how microorganisms are distributed and their functions. Metagenomic and geochemical analyses indicate that distance from the East River and proximity to groundwater and underlying weathered shale strongly impact microbial community structure and metabolic potential. Riparian zone microbial communities are compositionally distinct, from the phylum down to the species level, from all hillslope communities. Bacteria from phyla lacking isolated representatives consistently increase in abundance with increasing depth, but only in the riparian zone saturated sediments we found Candidate Phyla Radiation bacteria. Riparian zone microbial communities are functionally differentiated from hillslope communities based on their capacities for carbon and nitrogen fixation and sulfate reduction. Selenium reduction is prominent at depth in weathered shale and saturated riparian zone sediments and could impact water quality. We anticipate that the drivers of community composition and metabolic potential identified throughout the studied transect will predict patterns across the larger watershed hillslope system.     

Riparian nitrogen dynamics in two geomorphologically distinct tropical rain forest watersheds: nitrous oxide fluxes

Fluxes of N₂O at the soil surface, dissolved N₂O in near-surface groundwater, and potential N₂O production rates were measured across riparian catenas in two rain forest watersheds in Puerto Rico. In the Icacos watershed, mean N₂O fluxes were highest at topographic breaks in the landscape (≃ 40–300 μg N₂O-N m⁻² h⁻¹). At other locations in the riparian zone and hillslope, fluxes were lower (⩽ 2 μg N₂O-N m⁻² h⁻¹). This pattern of surface N₂O fluxes was persistent. In the Bisley watershed, mean suface N₂O fluxes were lower (<40 μg N₂O-N m⁻² h⁻¹) and no identifiable spatial or temporal pattern. Although the spatial patterns and intensities of N₂O emissions differed between the two watersheds, surface soils from both sites had a high potential to reduce NO₃ to N₂O (and perhaps N₂). This potential declined sharply with depth as did soil %C, %N, and potential N-mineralization. Simple controls on denitrification (i.e. aeration, nitrate, and carbon) explained characteristics of potential N₂O production in surface and deep soils from riparian and upslope locations. In the field, spatial patterns in these controlling variables were defined by geomorphological differences between the two watersheds, which then explained the spatial patterns of observed N₂O flux     

USDA Conservation Practices Increase Carbon Storage and Water Quality Improvement Functions: An Example from Ohio

We compared potential denitrification and phosphorus (P) sorption in restored depressional wetlands, restored riparian buffers, and natural riparian buffers of central Ohio to determine to what extent systems restored under the U.S. Department of Agriculture's Wetland Reserve Program (WRP) and Conservation Reserve Program (CRP) provide water quality improvement benefits, and to determine which practice is more effective at nutrient retention. We also measured soil nutrient pools (organic C, N, and P) to evaluate the potential for long‐term C sequestration and nutrient accumulation. Depressional wetland soils sorbed twice as much P as riparian soils, but had significantly lower denitrification rates. Phosphorus sorption and denitrification were similar between the restored and natural riparian buffers, although all Natural Resources Conservation Service (NRCS) practices had higher denitrification than agricultural soils. Pools of organic C (2570–3320 g/m²), total N (216–243 g/m²), and total P (60–71 g/m²) were comparable among all three NRCS practices but were greater than nearby agricultural fields and less than natural wetlands in the region. Overall, restored wetlands and restored and natural riparian buffers provide ecosystem services to the landscape that were lost during the conversion to agriculture, but the delivery of services differs among conservation practices, with greater N removal by riparian buffers and greater P removal by wetlands, attributed to differences in landscape position and mineral soil composition. At the landscape, and even global level, wetland and riparian restoration in agricultural landscapes will reintroduce multiple ecosystem services (e.g. C sequestration, water quality improvement, and others) and should be considered in management plans .     

Spatial variation in leaf nutrient traits of dominant desert riparian plant species in an arid inland river basin of China

Understanding how patterns of leaf nutrient traits respond to groundwater depth is crucial for modeling the nutrient cycling of desert riparian ecosystems and forecasting the responses of ecosystems to global changes. In this study, we measured leaf nutrients along a transect across a groundwater depth gradient in the downstream Heihe River to explore the response of leaf nutrient traits to groundwater depth and soil properties. We found that leaf nutrient traits of dominant species showed different responses to groundwater depth gradient. Leaf C, leaf N, leaf P, and leaf K decreased significantly with groundwater depth, whereas patterns of leaf C/N and leaf N/P followed quadratic relationships with groundwater depth. Meanwhile, leaf C/P did not vary significantly along the groundwater depth gradient. Variations in leaf nutrient traits were associated with soil properties (e.g., soil bulk density, soil pH). Groundwater depth and soil pH jointly regulated the variation of leaf nutrient traits; however, groundwater depth explained the variation of leaf nutrient traits better than did soil pH. At the local scale in the typical desert riparian ecosystem, the dominant species was characterized by low leaf C, leaf N, and leaf P, but high leaf N/P and leaf C/P, indicating that desert riparian plants might be more limited by P than N in the growing season. Our observations will help to reveal specific adaptation patterns in relation to the groundwater depth gradient for dominant desert riparian species, provide insights into adaptive trends of leaf nutrient traits, and add information relevant to understanding the adaptive strategies of desert riparian forest vegetation to moisture gradients.     

Urban riparian systems function as corridors for both native and invasive plant species

Riparian areas are often the only green areas left in urban and suburban landscapes, providing opportunities for conservation and connectivity of both aquatic and terrestrial organisms. While city planners and land managers often tout the importance of riparian networks for these uses, it is not well established if urban riparian plant communities are actually functioning as connected assemblages. Furthermore, urban riparian zones are well known to be highly invaded by non-native plant species and may be functioning to increase the spread of non-native species across the landscape. Here we examine connectivity of plant assemblages in riparian networks within an extensively urbanized landscape. We sampled riparian plant communities at 13 sites along three second-order streams of the Rahway River watershed, New Jersey. We also characterized propagule dispersal at each site by sampling litter packs on the river banks five times between March–October 2011 and identifying germinants from litter packs after cold stratification. Species turnover of both riparian and litter vegetation was more strongly associated with flow distance, particularly for native species, indicating that riverine systems are important for promoting connectivity of native plant assemblages in urban landscapes. However, non-native germinants significantly dominated propagule dispersal along the stream reaches, particularly early in the growing season, suggesting spread utilizing the river system and preemption may be an important mechanism for invasion success in this system. Our data show that management of invasive species should be planned and implemented at the watershed scale to reduce spread via the river system.     

Vertical pattern and its driving factors in soil extracellular enzyme activity and stoichiometry along mountain grassland belts

Headwater streams are foci for nutrient and energy loading from terrestrial landscapes, in situ nutrient transformations, and downstream transport. Despite the prominent role that headwater streams can have in regulating downstream water quality, the relative importance of processes that can influence nutrient uptake have not been fully compared in heterotrophic aquatic systems. To address this research need, we assessed the seasonality of dissolved organic carbon (DOC) and nitrate (NO₃^?) uptake, compared the relative influence of hydrologic and biogeochemical drivers on observed seasonal trends in nutrient uptake, and estimated the influence of these biological transformations on watershed scale nutrient retention and export. We determined that seasonal reductions in DOC and NO₃^? concentrations led to decreases in the potential for the biotic community to take up nutrients, and that seasonality of DOC and NO₃^? uptake was consistent with the seasonal dynamics of ecosystem metabolism. We calculated that that during the post-snowmelt period (June to August), biotic retention of both dissolved organic carbon and nitrate exceeded export fluxes from this headwater catchment, highlighting the potential for biological processes to regulate downstream water quality.