Riparian Sites Research Articles

Decadal declines in bird abundance and diversity in urban riparian zones

Urbanization is frequently cited as a major driver of species losses worldwide; however, most studies in urban areas use a space-for-time substitution approach to document effects of urbanization through time. Ultimately, understanding the effects of urbanization on biodiversity requires long-term datasets. We examined long-term changes in bird assemblages at 12 riparian sites in the greater Phoenix metropolitan area and nearby Sonoran Desert region, featuring a range of human modifications and levels of water flow. Riparian areas in arid cities represent a key habitat type that is sensitive to human modification and supports high levels of species diversity. We used long-term data to: (1) explore variation in bird communities as a function of water permanence and degree of human-modification; (2) identify which environmental variables best describe differences found across riparian site types; and (3) assess how riparian bird communities, abundance, and species richness have changed through time. Engineered riparian sites supported more broadly distributed generalists; whereas, natural riparian sites supported more specialists. Sites with perennial flows had more vegetation and water compared to ephemeral sites and engineered sites had more impervious surface compared to natural sites. In nearly all comparisons, bird species richness, diversity, and abundance declined across riparian types during the period of study, even for common species. Bird communities in natural settings have changed more than communities at engineered sites. Overall, the riparian bird community is shifting toward urban dwelling, resident species that are characteristic of riparian sites with less water and more impervious surface.

Groundwater facilitated water-use efficiency along a gradient of groundwater depth in arid northwestern China

Groundwater strongly impacts ecosystem performance in arid regions by driving vegetation structure and species distribution. It is unknown how water use efficiency varies along a gradient of depth to groundwater (DWT). In this study, we developed a framework to estimate water use efficiency (WUE), groundwater use efficiency (GUE), and rain use efficiency (RUE), and to examine the contribution of rainfall to transpiration in groundwater-dependent ecosystems (GDEs). The method was applied to an arid region in northwest China with a gradient of groundwater depth from 0.5 to 12m. The results indicate that the above-ground primary production, evapotranspiration, plant transpiration, WUE, and GUE decreased significantly from riparian forest, wetland, oasis edge, desert-oasis ecotone, and to sandy desert along a gradient of increasing DWT. RUE is found to be 0.26gm−2mm−1 at the sandy desert without groundwater contribution where 21% of rainfall is used for transpiration. Water use efficiency increases to 0.85gm−2mm−1 at the riparian site where groundwater is about 0.5m depth. The fraction of rainfall consumed by plants increases with a decreasing DWT from a threshold of 6.3m, suggesting groundwater enhances rain use efficiency in GDEs.

Heavy metal concentrations in riparian soils along the Han River, China: The importance of soil properties, topography and upland land use

Riparian zones can act as an important sink for various contaminants, including heavy metals. The retention and accumulation of heavy metals in riparian soils varies spatially and is related to many local factors, such as soil properties, topography and land use. However, the relative importance of such factors in determining the heavy metal contamination of riparian soils remains unclear. In this study, we investigated the concentrations of 10 heavy metals (i.e., Ba, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sr and Zn) and their relationships to environmental features in 67 soil samples collected from 26 riparian sites along the Han River, China. Our results showed that there were significant and positive correlations among the concentrations of all heavy metals except for Ba, Mn and Sr. Heavy metal concentrations were, in general, positively associated with conductivity, fine substrate, carbon, nitrogen and phosphorus contents in soils but were negatively related to soil pH and density. Among the topographic conditions studied, elevation and distance from the river mouth were positively associated with the concentrations of Ba, Cd, Cr and Ni in riparian soils. Upland land use had a significant effect on the concentrations of Co, Cu and Zn. Generalized linear models revealed that soil properties, topography and land use together accounted for 11–69% of the variance in single metal concentrations, while redundancy analysis showed that such factors could explain 48% of the variance in overall metal levels. Our findings suggest that heavy metal concentrations in riparian soils are regulated not only by soil properties but also by physical characteristics such as topography and land use.

SPATIOTEMPORAL DYNAMICS OF SURFACE WATER EXTENT FROM THREE DECADES OF SEASONALLY CONTINUOUS LANDSAT TIME SERIES AT SUBCONTINENTAL SCALE

Surface water is a critical resource in semi-arid areas. The Murray-Darling Basin (MDB) of Australia, one of the largest semi-arid basins in the world is aiming to set a worldwide example of how to balance multiple interests (i.e. environment, agriculture and urban use), but has suffered significant water shrinkages during the Millennium Drought (1999-2009), followed by extensive flooding. Baseline information and systematic quantification of surface water (SW) extent and flooding dynamics in space and time are needed for managing SW resources across the basin but are currently lacking. <br><br> To synoptically quantify changes in SW extent and flooding dynamics over MDB, we used seasonally continuous Landsat TM and ETM+ data (1986 – 2011) and generic machine learning algorithms. We further mapped flooded forest at a riparian forest site that experienced severe tree dieback due to changes in flooding regime. We used a stratified sampling design to assess the accuracy of the SW product across time. <br><br> Accuracy assessment yielded an overall classification accuracy of 99.94%, with producer’s and user’s accuracy of SW of 85.4% and 97.3%, respectively. Overall accuracy was the same for Landsat 5 and 7 data but user’s and producer’s accuracy of water were higher for Landsat 7 than 5 data and stable over time. <br><br> Our validated results document a rapid loss in SW bodies. The number, size, and total area of SW showed high seasonal variability with highest numbers in winter and lowest numbers in summer. SW extent per season per year showed high interannual and seasonal variability, with low seasonal variability during the Millennium Drought. <br><br> Examples of current uses of the new dataset will be presented and include (1) assessing ecosystem response to flooding with implications for environmental water releases, one of the largest investment in environment in Australia; (2) quantifying drivers of SW dynamics (e.g. climate, human activity); (3) quantifying changes in SW dynamics and connectivity for water dependent organisms; (4) assessing the impact of flooding on riparian vegetation health. The approach developed here is globally applicable, relevant to areas with competing water demands (e.g. Okavango River delta, Mekong River Basin). Future work should incorporate Landsat 8 and Sentinel-2 data for continued quantification of SW dynamics.

Importance of the 2014 Colorado River Delta pulse flow for migratory songbirds: Insights from foraging behavior

The Lower Colorado River provides critical riparian areas in an otherwise arid region and is an important stopover site for migrating landbirds. In order to reverse ongoing habitat degradation due to drought and human-altered hydrology, a pulse flow was released from Morelos Dam in spring of 2014, which brought surface flow to dry stretches of the Colorado River in Mexico. To assess the potential effects of habitat modification resulting from the pulse flow, we used foraging behavior of spring migrants from past and current studies to assess the relative importance of different riparian habitats. We observed foraging birds in 2000 and 2014 at five riparian sites along the Lower Colorado River in Mexico to quantify prey attack rates, prey attack maneuvers, vegetation use patterns, and degree of preference for fully leafed-out or flowering plants. Prey attack rate was highest in mesquite (Prosopis spp.) in 2000 and in willow (Salix gooddingii) in 2014; correspondingly, migrants predominantly used mesquite in 2000 and willow in 2014 and showed a preference for willows in flower or fruit in 2014. Wilson’s warbler (Cardellina pusilla) used relatively more low-energy foraging maneuvers in willow than in tamarisk (Tamarix spp.) or mesquite. Those patterns in foraging behavior suggest native riparian vegetation, and especially willow, are important resources for spring migrants along the lower Colorado River. Willow is a relatively short-lived tree dependent on spring floods for dispersal and establishment and thus spring migrants are likely to benefit from controlled pulse flows.

Contribution of pulses of soil nitrogen mineralization and nitrification to soil nitrogen availability in three Mediterranean forests

Pulses of microbial nitrogen (N) supply often occur during storms in Mediterranean regions, but their contribution to soil N availability and catchment N exports is still unknown. We investigated patterns and controls of pulses of net N mineralization (NNM) and nitrification (NN) at three forest sites (riparian, evergreen oak and beech) that coexist within a Mediterranean headwater catchment. In addition, we examined the effect of these pulses on soil N availability and stream N loads. For a year, we measuredNNM,NN, precipitation, moisture and temperature within each forest site. MeanNNMandNNrates varied widely among forest sites (NNM= 1.35, 0.62 and 0.50 µg N g soil−1day−1;NN= 1.18, 0.24 and 0.07 µg N g soil−1day−1for riparian, oak and beech, respectively). In general, pulses ofNNMandNNoccurred in spring, immediately after large rain events (> 20 mm). High soil temperatures (> 16°C) promoted microbial pulses in summer at the riparian site, but there were no pulses ofNNat the beech site. Although pulses of microbial activity were infrequent, they could account for account for 21–35% of the annual rates of the annual rates ofNNMandNN. However,NNpulses only at the riparian site led to disproportional increases in soil N availability and stream N loads. These results suggest that upland Mediterranean forests are sinks of N even after storms, whereas riparian soil can be a critical source of nitrate for the stream. Our study emphasizes the relevance of intensive monitoring in to evaluate the effect of microbial pulses on soil N biogeochemistry in Mediterranean catchments.HighlightsWe investigated the contribution of microbial pulses to soil N availability at three forest sitesMicrobial pulses occurred after rewetting in spring and also in summer at the riparian siteMicrobial pulses contributed > 20% of annual rates of mineralization and nitrificationRiparian soil can be a source of N for Mediterranean streams, especially after nitrification pulses

Poplar plantations along regulated rivers may resemble riparian forests after abandonment: a comparison of passive restoration approaches

Abandoning hybrid poplar plantations may be an alternative strategy for enlarging natural riparian corridors along regulated rivers where forest regeneration no longer takes place. Despite the generally high local diversity of plants in poplar plantations, their capacity to converge towards riparian forests following abandonment remains largely untested and uncertain, because maintenance‐related disturbance of plantations favors ruderal, not strictly riparian specialists. We assessed the spontaneous recolonization of vegetation in abandoned hybrid poplar plantations following two management strategies: harvesting or simple abandonment of standing trees. The floristic composition in four chronosequences of 10 active (1–15 years), 17 harvested (1–15 years), and 10 abandoned (8–20 years) hybrid poplar plantations, as well as 10 riparian sites established at gravel bars that appeared following the cessation of in‐channel gravel mining (8–25 years) along the highly regulated Garonne River (SW France) was assessed in the framework of ecological disturbance theory. Both harvested and abandoned sites still resembled active plantations more than riparian forests. When poplar resprouting was low after harvesting, sites were dominated by light‐demanding, nitrophilous herbs, sub‐shrubs, and vines showing both competitive and ruderal traits, and vegetation composition remained stable over time. Abandoned and harvested sites with high poplar resprouting developed forest communities in which competitive species that tolerate and generate shade dominated, and tree species recruitment was higher. Riparian sites hosted the highest number of indicator species, mainly wetland and exotic. Simple passive restoration strategies like abandonment of plantations can help create valuable ecosystems, although ones that diverge from riparian forests colonizing new fluvial landforms.

Surface water extent dynamics from three decades of seasonally continuous Landsat time series at subcontinental scale in a semi-arid region

Abstract Seasonally continuous long-term information on surface water and flooding extent over subcontinental scales is critical for quantifying spatiotemporal changes in surface water dynamics. We used seasonally continuous Landsat TM/ETM + data and generic random forest-based models to synoptically map the extent and dynamics of surface water and flooding (1986–2011) over the Murray–Darling Basin (MDB). The MDB is a large semi-arid basin with competing demands for water that has recently experienced one of the most severe droughts in the southeast of Australia. We used a stratified random probability sampling design with 500 sample pixels each observed across time to assess the accuracy of the surface water maps. We further developed models to map flooded forest at a riparian site that experienced severe tree dieback. Water indices and bands 5 and 6 were among the top 10 explanatory variables most important for mapping surface water. Surface water extent per season per year showed high inter-annual and seasonal variability, with low extent and variability during the Millennium Drought (1999–2009). Accuracy assessment yielded an overall classification accuracy of 99.9% (± 0.02% standard error) with 87% (± 3%) and 96% (± 2%) producer's and user's accuracy of water, respectively. User's and producer's accuracies of water were higher for Landsat 7 than Landsat 5 data. Both producer's and user's accuracies of water were lower in wet years compared to dry years. The approach presented here can be further developed for global application and is relevant to areas with competing water demands. Quantifying the uncertainty of the accuracy assessment and providing an unbiased accuracy estimate are imperative steps when remotely sensed products are intended to be used for follow on applications.

Factors affecting agricultural nitrogen removal in riparian strips: Examples from groundwater-dependent ecosystems of the Po Valley (Northern Italy)

The role of riparian ecosystems in mitigating the effects of diffuse agricultural sources is recognized in several regulatory measures and public policy initiatives in many parts of the world. This study aimed to evaluate the N buffering capacity of semi-natural riparian zones associated with spring-fed lowland streams, also known as “fontanili”, representing the most important groundwater-dependent ecosystems in Northern Italy. Monitoring parcels were set up in nine riparian sites selected to cover a range of different soil properties and hydrogeological settings, and to sustain the evaluation of the main drivers affecting their N removal efficiency. Subsurface water level, nutrient concentrations and the main hydro-chemical parameters were monitored along transects of piezometers installed from crop fields to the spring channels. On selected samples from two sites stable isotopes of the water molecule were also determined. Median NO3− input concentrations from adjacent cropland to the riparian sites ranged from 0.10 to 21mgNL−1, with maximum values exceeding the drinking water limit recorded during the summer and winter fertilization periods. Highly variable groundwater nitrate patterns were found in the riparian areas, including short nitrate plumes extending from the adjacent cropland into some riparian zones, or in others, small patches where NO3− declined at variable distance from the stream. Some chemical indicators (e.g., NO3−/Cl− ratio, O2, DOC) suggested that NO3− attenuation was mostly due to the denitrifying activity occurring in the subsurface aquifers in specific conditions (hot spots and moments), although, in some cases, physical processes such as dilution also contributed. The overall N removal efficiency was greater than 90% in four sites, 74%, 34% and 30% in three sites, and zero in the remaining two sites. Useful predictors of the nitrate removal capacity were factors linked to the water residence time, such as the hydraulic conductivity, the soil texture and the slope of the riparian profile, together with the water table depth and soil organic carbon. A combination by standardized averaging of these five factors supported a clear discrimination of sites with zero or low N removal effectiveness from those with high efficiency.

It’s not all about the creeks: protection of multiple habitats will improve biodiversity conservation in a eucalypt forest

Understanding patterns in the distribution and abundance of wildlife across the landscape can aid in identifying the relative importance of habitats for biodiversity conservation. We aimed to identify whether riparian habitats were more important than other areas in the landscape to small terrestrial vertebrates. The study site was surveyed using 450 pit traps distributed across riparian, midslope and ridge top habitat. Riparian sites had the greatest abundance of small vertebrates of the three habitats. During some months of the year, there was a significant difference in the composition of the faunal assemblage between habitats. Unsurprisingly, riparian habitats were particularly important for frog species and it was these species that accounted for the greater abundance in this habitat. Riparian habitat was less important for other taxonomic groups and the more floristically rich midslope and ridge habitats, which had a greater abundance of leaf litter, fallen logs and rock cover, were favoured by mammal and reptile species. The conservation of riparian sites, without the protection of other habitats, overlooks a substantial proportion of the biodiversity in the landscape. This study may help inform management decisions in the Upper Warren region and other similar forested landscapes, regarding the location and timing of fauna monitoring and the frequency of fuel reduction burns.

First report of Phytophthora hydropathica in river water associated with riparian alder in Spain

Phytophthora hydropathica has been commonly reported from riparian sites in southeastern USA, on watersheds and nursery sites in Tennessee (Hulvey et al ., 2010) and in nursery irrigation reservoirs in Virginia (Hong et al ., 2010). Recently…

Diversity and Community Composition of Vertebrates in Desert River Habitats.

Animal species are seldom distributed evenly at either local or larger spatial scales, and instead tend to aggregate in sites that meet their resource requirements and maximise fitness. This tendency is likely to be especially marked in arid regions where species could be expected to concentrate at resource-rich oases. In this study, we first test the hypothesis that productive riparian sites in arid Australia support higher vertebrate diversity than other desert habitats, and then elucidate the habitats selected by different species. We addressed the first aim by examining the diversity and composition of vertebrate assemblages inhabiting the Field River and adjacent sand dunes in the Simpson Desert, western Queensland, over a period of two and a half years. The second aim was addressed by examining species composition in riparian and sand dune habitats in dry and wet years. Vertebrate species richness was estimated to be highest (54 species) in the riverine habitats and lowest on the surrounding dune habitats (45 species). The riverine habitats had different species pools compared to the dune habitats. Several species, including the agamid Gowidon longirostris and tree frog Litoria rubella, inhabited the riverine habitats exclusively, while others such as the skinks Ctenotus ariadnae and C. dux were captured only in the dune habitats. The results suggest that, on a local scale, diversity is higher along riparian corridors and that riparian woodland is important for tree-dependent species. Further, the distribution of some species, such as Mus musculus, may be governed by environmental variables (e.g. soil moisture) associated with riparian corridors that are not available in the surrounding desert environment. We conclude that inland river systems may be often of high conservation value, and that management should be initiated where possible to alleviate threats to their continued functioning.

Observations of High Quality Eastern Deciduous Riparian Sites in Southeast Manitoba's Whitemouth River Watershed

Observations of High Quality Eastern Deciduous Riparian Sites in Southeast Manitoba's Whitemouth River Watershed

Water uptake by saltcedar (Tamarix ramosissima) in a desert riparian forest: responses to intra‐annual water table fluctuation

AbstractThere is considerable interest in naturalizing flow regime on managed rivers to slow the spread of saltcedar (Tamarix ramosissima) invasion in southwestern USA or to preserve riparian forests dominated by saltcedar and other species in northwestern China. However, little is known about the responses of established saltcedar in water sources to frequent intra‐annual fluctuation of water table resulting from this new, more dynamic flow regime. This study investigates how saltcedar at a riparian site in the middle reaches of the Heihe River, northwest China, responds in water sources use to intra‐annual water table fluctuations. Stable oxygen isotope was employed to determine accurate depth at which saltcedar obtains its water supply, and soil moisture monitoring was used to determine sources of plant‐available soil water. We found that the primary zone of water uptake by saltcedar were stable at 25–60 cm depth, but the water sources used by saltcedar switched between groundwater and soil moisture with the water table fluctuations. Saltcedar derived its water from groundwater when water table was at depth less than 60 cm but switched to soil moisture at 25–60 cm depth when water table declined. It is supposed that the well‐developed clay layer at 60–80 cm depth constrained lateral roots of saltcedar to the soil layers above 60 cm, while the fine‐textured soils at this site, which were periodically resaturated by rising groundwater before the stored soil moisture had become depleted, provided an important water reservoir for saltcedar when groundwater dropped below the primary zone of fine roots. The root distribution of saltcedar may also be related to local groundwater history. The quick decline in water table in the early 1980s when the riparian saltcedar had established may strand its roots in the shallow unsaturated zone. We suggested that raising the water table periodically instead of maintaining it invariably above the rooting depth could sustain desired facultative phreatophytes while maximizing water deliveries. Copyright © 2015 John Wiley & Sons, Ltd.

Importance of Native Understory for Bird and Butterfly Communities in a Riparian and Marsh Restoration Project on the Lower Colorado River, Arizona

Heidi Kloeppel Trathnigg Corresponding author, Principal Biologist, Fred Phillips Consulting, LLC, 401 South Leroux St., Flagstaff, AZ, 86001, htrathnigg{at}fredphillipsconsulting.com.

Catchment agriculture and local environment affecting the soil denitrification potential and nitrous oxide production of riparian zones in the Han River Basin, China

Riparian zones play an important role in reducing nitrogen (N) loading to rivers and streams primarily through soil denitrification which reduces nitrate (NO3–) to nitrous oxide (N2O) and dinitrogen (N2) gases. Although the relationships between local environments and soil denitrification are well understood, relatively little is known about the indirect effects of landscape factors (e.g., catchment agriculture) on the soil denitrification of riparian zones. In this study, we used the acetylene block technique to measure the denitrification potential and net N2O production of soils collected from 62 riparian sites in 15 subtropical rivers of varying catchment land uses. The results indicated that, among the local factors studied, the soil moisture, organic matter and NO3– concentrations were positively associated with both the denitrification potential and N2O production rate. Agricultural riparian zones had a denitrification potential (2.81±1.01ngNg−1h−1) significantly higher than forested riparian zones (0.66±0.24ngNg−1h−1). Additionally, the riparian denitrification potential increased with the percentage of agriculture in the catchments (R=0.53, P<0.05). Structural equation modeling revealed that the indirect effects of catchment agriculture on the riparian denitrification potential and N2O production rate were mediated primarily through soil NO3–. Our findings suggest that, compared to forested riparian zones, agricultural riparian zones have greater potential to remove N from polluted runoff. The conversion of original vegetation to agricultural lands in catchments may have a profound impact on the soil N cycles and NO3– removal capacity of riparian zones.

Private lands habitat programs benefit California's native birds

To address the loss of wetlands and riparian forests in California, private lands habitat programs are available through U.S. federal and state government agencies to help growers, ranchers and other private landowners create and enhance wildlife habitat. The programs provide financial and technical assistance for implementing conservation practices. To evaluate the benefits of these programs for wildlife, we examined bird use of private wetlands, postharvest flooded croplands and riparian forests enrolled in habitat programs in the Central Valley and North Coast regions of California. We found that private Central Valley wetlands supported 181 bird species during the breeding season. During fall migration, postharvest flooded croplands supported wetland-dependent species and a higher density of shorebirds than did semipermanent wetlands. At the riparian sites, bird species richness increased after restoration. These results demonstrated that the programs provided habitat for the species they were designed to protect; a variety of resident and migratory bird species used the habitats, and many special status species were recorded at the sites.

Value of Riparian Vegetation Remnants for Leaf-Litter Ants (Hymenoptera: Formicidae) in a Human-Dominated Landscape in Central Veracruz, Mexico.

Riparian remnants are linear strips of vegetation immediately adjacent to rivers that may act as refuges for biodiversity, depending on their habitat quality. In this study, we evaluated the role of riparian remnants in contributing to the diversity of leaf-litter ants by determining the relationship between ant diversity and several riparian habitat characteristics within a human-dominated landscape in Veracruz, Mexico. Sampling was carried out in 2012 during both dry and rainy seasons at 12 transects 100 m in length, where 10 leaf-litter samples were collected along each transect and processed with Berlese-Tullgren funnels and Winkler sacks. A total of 8,684 individuals belonging to 53 species, 22 genera, and seven subfamilies were collected. The observed mean alpha diversity accounted for 34.4% of the total species recorded and beta diversity for 65.6%. Species richness and composition were significantly related to litter-layer depth and soil compaction, which could limit the distribution of ant species depending on their nesting, feeding, and foraging habits. Riparian remnants can contribute toward the conservation of ant assemblages and likely other invertebrate communities that are threatened by anthropogenic pressures. In human-dominated landscapes where remnants of riparian vegetation give refuge to a diverse array of myrmecofauna, the protection of the few remaining and well-preserved riparian sites is essential for the long-term maintenance of biodiversity.

Rates of decomposition and nutrient release of herbivore inputs are driven by habitat microsite characteristics

AbstractUngulates use habitat with differential microclimate characteristics; therefore, fecal inputs to a particular habitat may result in vastly different rates of decomposition and nutrient release. We tested this hypothesis and conducted a 1‐year decomposition experiment where we deployed fecal samples from Shiras moose (Alces alces shirasi) and subsequently measured loss of fecal mass, nutrient release captured with resin bags, and associated consequences for nitrogen (N) cycling in soils. The microhabitat type in which fecal samples were deployed influenced rates of decomposition observed; samples experienced faster rates of decomposition in a riparian habitat type than a conifer site. Cumulative nutrient losses as nitrate (NO3−) measured with anion and cation exchange resin bags were significantly higher in the conifer site (0.08 g N/feces) than the riparian site (0.02 g N/feces), whereas ammonium (NH4+) losses, though higher than nitrate losses, were not significantly different between the riparian site (0.40 g N/feces) and the conifer site (0.26 g N/feces) after 1‐year. Concentrations of soil NH4+ and NO3− beneath the fecal samples in the riparian site were significantly higher relative to control soils after 1 year, but no differences were detected in the conifer site. Cumulatively, our findings supported our hypothesis that fecal deposition by large herbivores can strongly influence nutrient releases to the ecosystem through the decomposition of fecal matter. Such nutrient additions may have direct effects on N cycling in soil and provide valuable inputs that are available for plant uptake and subsequent growth.

Hydrologic and Biogeochemical Drivers of Riparian Denitrification in an Agricultural Watershed

This study investigated drivers of denitrification and overall NO3 − removal in an agricultural riparian area in central New York. Denitrification was measured using an in situ “push-pull” method with 15N–NO3 − as a tracer during summer and fall 2011 at a pair of riparian sites characterized by different hydrologic regimes. Median denitrification rates were 1347 and 703 μg N kg soil−1 day−1 for the two study sites. These rates are higher than those reported for other riparian areas, emphasizing the role of some riparian areas as hotspots of NO3 − removal. N2O production was significantly higher at one site, demonstrating that riparian areas can be a greenhouse gas source under certain conditions. Denitrification was negatively correlated with groundwater flux, suggesting that slower flushing of water, and thus longer residence time, promotes denitrification. A mass balance of NO3 − loss revealed that denitrification only accounted for 5–12 % of total NO3 − loss, and production of NH4 + indicated that dissimilatory NO3 − reduction to NH4 + (DNRA) may be occurring at both sites. While both sites were characterized by high NO3 − removal, differences in denitrification rates and NO3 − removal processes demonstrate the need to improve our ability to capture spatial and process heterogeneity in landscape biogeochemical models.