Fluorescence characteristics and biodegradability of dissolved organic matter in forest and wetland soils from coastal temperate watersheds in southeast Alaska

Understanding how the concentration and chemical quality of dissolved organic matter (DOM) varies in soils is critical because DOM influences an array of biological, chemical, and physical processes. We used PARAFAC modeling of excitation–emission fluorescence spectroscopy, specific UV absorbance (SUVA₂₅₄) and biodegradable dissolved organic carbon (BDOC) incubations to investigate the chemical quality of DOM in soil water collected from 25 cm piezometers in four different wetland and forest soils: bog, forested wetland, fen and upland forest. There were significant differences in soil solution concentrations of dissolved organic C, N, and P, DOC:DON ratios, SUVA₂₅₄ and BDOC among the four soil types. Throughout the sampling period, average DOC concentrations in the four soil types ranged from 9–32 mg C l⁻¹ and between 23–42% of the DOC was biodegradable. Seasonal patterns in dissolved nutrient concentrations and BDOC were observed in the three wetland types suggesting strong biotic controls over DOM concentrations in wetland soils. PARAFAC modeling of excitation–emission fluorescence spectroscopy showed that protein-like fluorescence was positively correlated (r ² = 0.82; P < 0.001) with BDOC for all soil types taken together. This finding indicates that PARAFAC modeling may substantially improve the ability to predict BDOC in natural environments. Coincident measurements of DOM concentrations, BDOC and PARAFAC modeling confirmed that the four soil types contain DOM with distinct chemical properties and have unique fluorescent fingerprints. DOM inputs to streams from the four soil types therefore have the potential to alter stream biogeochemical processes differently by influencing temporal patterns in stream heterotrophic productivity.     

Effects of Salmon-Borne Nutrients on Riparian Soils and Vegetation in Southwest Alaska

Spawning Pacific salmon (Oncorhynchus spp.) contribute marine-derived nutrients to riparian ecosystems, potentially affecting characteristics of the associated soils and vegetation. We quantified these effects by comparing soil and vegetative characteristics upstream and downstream of natural migratory barriers on ten spawning streams in southwest Alaska. Mean δ¹⁵N values—indicative of salmon-borne nutrients—were significantly higher in the O horizon and surface mineral soils downstream of barriers (near spawning reaches) than in soils upstream of barriers (near non-spawning reaches). However, the mean total N concentration in surface mineral soil was lower downstream than upstream. Mean foliar δ¹⁵N values were higher downstream for three plant species (Picea glauca, Salix alaxensis and Arctagrostis latifolia) with contrasting physiognomies. Mean overstory stem density was 100% higher downstream, primarily due to a fivefold difference in the density of large-diameter willows (Salix spp.). Mean understory stem density was 47% lower downstream, also driven by a difference in willow density. Mean ground layer non-vascular and dwarf shrub species covers were 28% and 73% lower downstream, respectively. Of the ten soil and vegetative characteristics that differed upstream to downstream, two (O horizon and Picea glauca δ¹⁵N) were correlated with the density of spawning salmon. Collectively, the data suggest that salmon-borne nutrients alter riparian soils and vegetation, while factors unrelated to salmon are responsible for the ultimate expression of many community characteristics.     

Uptake of Allochthonous Dissolved Organic Matter from Soil and Salmon in Coastal Temperate Rainforest Streams

Dissolved organic matter (DOM) is an important component of aquatic food webs. We compare the uptake kinetics for NH₄–N and different fractions of DOM during soil and salmon leachate additions by evaluating the uptake of organic forms of carbon (DOC) and nitrogen (DON), and proteinaceous DOM, as measured by parallel factor (PARAFAC) modeling of DOM fluorescence. Seasonal DOM slug additions were conducted in three headwater streams draining a bog, forested wetland, and upland forest using DOM collected by leaching watershed soils. We also used DOM collected from bog soil and salmon carcasses to perform additions in the upland forest stream. DOC uptake velocity ranged from 0.010 to 0.063 mm s⁻¹ and DON uptake velocity ranged from 0.015 to 0.086 mm s⁻¹, which provides evidence for the whole-stream uptake of allochthonous DOM. These findings imply that wetlands could potentially be an important source of DOM to support stream heterotrophic production. There was no significant difference in the uptake of DOC and DON across the soil leachate additions (P > 0.05), although differential uptake of DOM fractions was observed as protein-like fluorescence was removed from the water column more efficiently than bulk DOC and DON (P < 0.05). Moreover, PARAFAC analysis of DOM fluorescence showed that protein-like fluorescence decreased downstream during all DOM additions, whereas humic-like fluorescence did not change. This differential processing in added DOM suggests slow and fast turnover pools exist for aquatic DOM. Taken together, our findings argue that DON could potentially fill a larger role in satisfying biotic N demand in oligotrophic headwater streams than previously thought. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author contributions  J.B.F. conceived of or designed study, performed research, analyzed data, contributed new methods or models, and wrote the paper. E.H. conceived of or designed study and analyzed data. R.T.E. conceived of or designed study and analyzed data. J.B.J. contributed new methods or models and analyzed data.     

Stable isotope evidence indicates the incorporation into Japanese catchments of marine‐derived nutrients transported by spawning Pacific Salmon

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Seasonal changes in the chemical quality and biodegradability of dissolved organic matter exported from soils to streams in coastal temperate rainforest watersheds

The composition and biodegradability of streamwater dissolved organic matter (DOM) varies with source material and degree of transformation. We combined PARAFAC modeling of fluorescence excitation–emission spectroscopy and biodegradable dissolved organic carbon (BDOC) incubations to investigate seasonal changes in the lability of DOM along a soil-stream continuum in three soil types: bog, forested wetland and upland forest. The percent BDOC ranged from 7 to 38% across all sites, and was significantly greater in soil compared to streamwater in the bog and forested wetland, but not in the upland forest. The percent BDOC also varied significantly over the entire sampling period in soil and streamwater for the bog and forested wetland, as BDOC peaked during the spring runoff and was lowest during the summer months. Moreover, the chemical quality of DOM in wetland soil and streamwater was similar during the spring runoff and fall wet season, as demonstrated by the similar contribution of protein-like fluorescence (sum of tyrosine and tryptophan fluorescence) in soil water and in streams. These findings suggest that the tight coupling between terrestrial and aquatic ecosystems is responsible for the delivery of labile DOM from wetland soils to streams. The contribution of protein-like fluorescence was significantly correlated with BDOC (p < 0.001) over the entire sampling period indicating DOM is an important source of C and N for heterotrophic microbes. Taken together, our findings suggest that the production of protein-rich, labile DOM and subsequent loss in stream runoff might be an important loss of labile C and N from coastal temperate watersheds.     

Unusual seasonal patterns and inferred processes of nitrogen retention in forested headwaters of the Upper Susquehanna River

Atmospheric deposition contributes a large fraction of the annual nitrogen (N) input to the basin of the Susquehanna River, a river that provides two-thirds of the annual N load to the Chesapeake Bay. Yet, there are few measurements of the retention of atmospheric N in the Upper Susquehanna’s forested headwaters. We characterized the amount, form (nitrate, ammonium, and dissolved organic nitrogen), isotopic composition (δ¹⁵N- and δ¹⁸O-nitrate), and seasonality of stream N over 2 years for 7–13 catchments. We expected high rates of N retention and seasonal nitrate patterns typical of other seasonally snow-covered catchments: dormant season maxima and growing season minima. Coarse estimates of N export indicated high rates of inorganic N retention (>95%), yet streams had unexpected seasonal nitrate patterns, with summer peaks (14–96 μmol L⁻¹), October crashes (<1 μmol L⁻¹), and modest rebounds during the dormant season (<1–20 μmol L⁻¹). Stream δ¹⁸O-nitrate values indicated microbial nitrification as the primary source of stream nitrate, although snowmelt or other atmospheric source contributed up to 47% of stream nitrate in some March samples. The autumn nitrate crash coincided with leaffall, likely due to in-stream heterotrophic uptake of N. Hypothesized sources of the summer nitrate peaks include: delayed release of nitrate previously flushed to groundwater, weathering of geologic N, and summer increases in net nitrate production. Measurements of shale δ¹⁵N and soil-, well-, and streamwater nitrate within one catchment point toward a summer increase in soil net nitrification as the driver of this pattern. Rather than seasonal plant demand, processes governing the seasonal production, retention, and transport of nitrate in soils may drive nitrate seasonality in this and many other systems.     

Salmon‐derived nutrient and organic matter fluxes from a coastal catchment in southeast Alaska

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Role of brown bears (Ursus arctos) in the flow of marine nitrogen into a terrestrial ecosystem

We quantified the amount, spatial distribution, and importance of salmon (Oncorhynchus spp.)-derived nitrogen (N) by brown bears (Ursus arctos) on the Kenai Peninsula, Alaska. We tested and confirmed the hypothesis that the stable isotope signature (δ¹⁵N) of N in foliage of white spruce (Picea glauca) was inversely proportional to the distance from salmon-spawning streams (r=–0.99 and P<0.05 in two separate watersheds). Locations of radio-collared brown bears, relative to their distance from a stream, were highly correlated with δ¹⁵N depletion of foliage across the same gradient (r=–0.98 and –0.96 and P<0.05 in the same two separate watersheds). Mean rates of redistribution of salmon-derived N by adult female brown bears were 37.2±2.9 kg/year per bear (range 23.1–56.3), of which 96% (35.7±2.7 kg/year per bear) was excreted in urine, 3% (1.1±0.1 kg/year per bear) was excreted in feces, and <1% (0.3± 0.1 kg/year per bear) was retained in the body. On an area basis, salmon-N redistribution rates were as high as 5.1±0.7 mg/m² per year per bear within 500 m of the stream but dropped off greatly with increasing distance. We estimated that 15.5–17.8% of the total N in spruce foliage within 500 m of the stream was derived from salmon. Of that, bears had distributed 83–84%. Thus, brown bears can be an important vector of salmon-derived N into riparian ecosystems, but their effects are highly variable spatially and a function of bear density. Received: 11 February 1999 / Accepted: 7 July 1999     

Isotopic signature of nitrate in two contrasting watersheds of Brush Brook,Vermont, USA

We used the dual isotope method to study differences in nitrate export in two subwatersheds in Vermont, USA. Precipitation, soil water and streamwater samples were collected from two watersheds in Camels Hump State Forest, located within the Green Mountains of Vermont. These samples were analyzed for the δ¹⁵N and δ¹⁸O of NO₃⁻. The range of δ¹⁵N–NO₃⁻ values overlapped, with precipitation −4.5‰ to +2.0‰ (n = 14), soil solution −10.3‰ to +6.2‰ (n = 12) and streamwater +0.3‰ to +3.1‰ (n = 69). The δ¹⁸O of precipitation NO₃⁻ (mean 46.8 ± 11.5‰) was significantly different (P < 0.001) from that of the stream (mean 13.2 ± 4.3‰) and soil waters (mean 14.5 ± 4.2‰) even during snowmelt periods. Extracted soil solution and streamwater δ¹⁸O of NO₃⁻ were similar and within the established range of microbially produced NO₃⁻, demonstrating that NO₃⁻ was formed by microbial processes. The δ¹⁵N and δ¹⁸O of NO₃⁻ suggests that although the two tributaries have different seasonal NO₃⁻ concentrations, they have a similar NO₃⁻ source.     

Evaluating the consequences of salmon nutrients for riparian organisms: Linking condition metrics to stable isotopes

Stable isotope ratios (δ¹³C and δ¹⁵N) have been used extensively to trace nutrients from Pacific salmon, but salmon transfer more than carbon and nitrogen to stream ecosystems, such as phosphorus, minerals, proteins, and lipids. To examine the importance of these nutrients, metrics other than isotopes need to be considered, particularly when so few studies have made direct links between these nutrients and how they affect riparian organisms. Our study specifically examined δ¹³C and δ¹⁵N of riparian organisms from salmon and non‐salmon streams in Idaho, USA, at different distances from the streams, and examined whether the quality of riparian plants and the body condition of invertebrates varied with access to these nutrients. Overall, quality and condition metrics did not mirror stable isotope patterns. Most notably, all riparian organisms exhibited elevated δ¹⁵N in salmon streams, but also with proximity to both stream types suggesting that both salmon and landscape factors may affect δ¹⁵N. The amount of nitrogen incorporated from Pacific salmon was low for all organisms (<20%) and did not correlate with measures of quality or condition, probably due to elevated δ¹⁵N at salmon streams reflecting historical salmon runs instead of current contributions. Salmon runs in these Idaho streams have been declining, and associated riparian ecosystems have probably seen about a 90% reduction in salmon‐derived nitrogen since the 1950s. In addition, our results support those of other studies that have cautioned that inferences from natural abundance isotope data, particularly in conjunction with mixing models for salmon‐derived nutrient percentage estimates, may be confounded by biogeochemical transformations of nitrogen, physiological processes, and even historical legacies of nitrogen sources. Critically, studies should move beyond simply describing isotopic patterns to focusing on the consequences of salmon‐derived nutrients by quantifying the condition and fitness of organisms putatively using those resources.     

Trophic ecology of Pacific salmon (<Emphasis Type="Italic">Oncorhynchus</Emphasis> spp.) in the ocean: a synthesis of stable isotope research

Increasing interest in the marine trophic dynamics of Pacific salmon has been motivated by the recognition of their sensitivity to changing climate and to the competitive effects of hatchery fish on wild stocks. It has become more common to use stable isotopes to supplement traditional diet studies of salmon in the ocean; however, there have been no integrated syntheses of these data to determine whether stable isotope analyses support the existing conventional wisdom of feeding strategies of the Pacific salmon. We performed a meta-analysis of stable isotope data to examine the extent of trophic partitioning among five species of Pacific salmon during their marine lives. Pink, sockeye, and chum salmon showed very high overlap in resource use and there was no consistent evidence for chum relying on alternative food webs dominated by gelatinous zooplankton. δ¹⁵N showed that Chinook and coho salmon fed at trophic levels higher than the other three species. In addition, these two species were distinctly enriched in ¹³C, suggesting more extensive use of coastal food webs compared to the more depleted (pelagic) signatures of pink, sockeye, and chum salmon. This paper presents the first synthesis of stable isotope work on Pacific salmon and provides δ¹⁵N and δ¹³C values applicable to research on the fate of the marine derived nutrients these organisms transport to freshwater and riparian ecosystems.     

Marine-derived nitrogen and carbon in freshwater-riparian food webs of the Copper River Delta,southcentral Alaska

After rearing to adulthood at sea, coho salmon (Oncorhynchus kisutch) return to freshwater to spawn once and then die on or near their spawning grounds. We tested the hypothesis that spawning coho salmon return marine N and C to beaver (Castor canadensis) ponds of the Copper River Delta (CRD), Cordova, southcentral Alaska, thereby enhancing productivity of the aquatic food webs that support juvenile coho salmon. We sampled three types of pond treatment: (1) natural enrichment by spawning salmon, (2) artificial enrichment via addition of salmon carcasses and eggs, and (3) ponds with no salmon enrichment. All ponds supported juvenile coho salmon. Seasonal samples of stable isotopes revealed that juvenile coho salmon, threespine sticklebacks (Gasterosteus aculeatus), caddisfly larvae, leeches, and chironomid midge larvae were enriched with marine N and C. The aquatic vascular plants bur reed (Sparganium hyperboreum), pondweed (Potamogeton gramineus), and mare’s tail (Hippuris vulgaris) were enriched with marine N only. Riparian vegetation (Sitka alder Alnus viridis ssp. sinuata and willow Salix spp.) did not show enrichment. Artificial additions of adult carcasses and eggs of coho salmon increased the δ¹⁵N and δ¹³C values of juvenile coho salmon. In this dynamic and hydrologically complex coastal environment, spawning coho salmon contributed marine N and C comprising 10–50% of the dietary needs of juvenile coho salmon through direct consumption of eggs and carcass material. Invertebrates that have assimilated marine N and C yield a further indirect contribution. This perennial subsidy maintains the productivity of the ecosystem of the coho salmon on the CRD.     

Uptake of salmon-derived nitrogen by mosses and liverworts in coastal British Columbia

Throughout forested watersheds bordering the North Pacific, anadromous salmon ( Oncorhynchus spp . ) are transferred into coastal forests by numerous predators and scavengers with remnants providing an important nutrient subsidy to riparian zones. The contribution of these nutrients to mosses and liverworts, which are the dominant ground cover of coastal forests, has not been investigated. We examine here δ¹⁵N isotope signatures and foliar percent N of eight moss and liverwort species, as well as moss and liverwort community structure, in multiple habitat blocks that vary in access to salmon nutrients from two watersheds on the central coast of British Columbia. For the most common moss Rhytidiadelphus loreus , we also examine δ¹⁵N values and foliar percent N among salmon carcass micro-sites and along wildlife trails. Overall, δ¹⁵N signatures ranged from 2‰ to 7‰ higher below the falls near the salmon stream than above the small falls that are impassable to salmon, or at the adjacent control watershed that had no salmon. Among micro-sites, δ¹⁵N and %N values were highest near bony carcass remnants from transfer during previous years and high adjacent to wildlife trails indicating spatial heterogeneity in the salmon nutrient pools of these forests. Species richness and prevalence of nitrogen-rich soil indicators were also highest in forests adjacent to the salmon stream. These data suggest an important contribution of salmon-derived nutrients to the non-vascular plants of the riparian zone of old-growth forests throughout the Pacific Rim.     

Effects of spawning Pacific salmon on terrestrial invertebrates: Insects near spawning habitat are isotopically enriched with nitrogen‐15 but display no differences in body size

When Pacific salmon (Oncorhynchus spp.) spawn and die, they deliver marine‐derived nutrient subsidies to freshwater and riparian ecosystems. These subsidies can alter the behavior, productivity, and abundance of recipient species and their habitats. Isotopes, such as nitrogen‐15 (¹⁵N), are often used to trace the destination of marine‐derived nutrients in riparian habitats. However, few studies have tested for correlations between stable isotopes and physiological responses of riparian organisms. We examined whether increases in δ ¹⁵N in terrestrial insect bodies adjacent to salmon spawning habitat translate to changes in percent nitrogen content and body size. This involved comparisons between distance from a salmon‐bearing river, marine‐derived nutrients in soils and insects, soil moisture content, and body size and nitrogen content in two common beetle families (Coleoptera: Curculionidae, Carabidae). As predicted, δ¹⁵N in riparian soils attenuated with distance from the river but was unaffected by soil moisture. This gradient was mirrored by δ¹⁵N in the herbivorous curculionid beetles, whereas carabid beetles, which feed at a higher trophic level and are more mobile, did not show discernable patterns in their δ¹⁵N content. Additionally, neither distance from the river nor body δ¹⁵N content was related to beetle body size. We also found that nitrogen‐15 was not correlated with total percent nitrogen in insect bodies, meaning that the presence of spawning salmon did not increase the percent nitrogen content of these insects. We conclude that while salmon‐derived nutrients had entered terrestrial food webs, the presence of δ¹⁵N alone did not indicate meaningful physiological changes in these insects in terms of percent nitrogen nor body size. While stable isotopes may be useful tracers of marine‐derived nutrients, they cannot necessarily be used as a proxy for physiologically important response variables.     

Stormflow Dynamics of Dissolved Organic Carbon and Total Dissolved Nitrogen in a Small Urban Watershed

We examined patterns of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) loading to a small urban stream during baseflow and stormflow. We hypothesized that lower DOC and TDN contributions from impervious surfaces would dilute natural hydrologic flowpath (i.e., riparian) contributions during storm events in an urban watershed, resulting in lower concentrations of DOC and TDN during storms. We tested these hypotheses in a small urban watershed in Portland, Oregon, over a 3-month period during the spring of 2003. We compared baseflow and stormflow chemistry using Mann–Whitney tests (significant at p<0.05). We also applied a mass balance to the stream to compare the relative significance of impervious surface contributions versus riparian contributions of DOC and TDN. Results showed a significant increase in stream DOC concentrations during stormflows (median baseflow DOC = 2.00 mg l⁻¹ vs. median stormflow DOC = 3.46 mg l⁻¹). TDN streamwater concentrations, however, significantly decreased with stormflow (median baseflow TDN = 0.75 mg l⁻¹ vs. median stormflow TDN = 0.56 mg l⁻¹). During storms, remnant riparian areas contributed 70–74% of DOC export and 38–35% of TDN export to the stream. The observed pattern of increased DOC concentrations during stormflows in this urban watershed was similar to patterns found in previous studies of forested watersheds. Results for TDN indicated that there were relatively high baseflow nitrogen concentrations in the lower watershed that may have partially masked the remnant riparian signal during stormflows. Remnant riparian areas were a major source of DOC and TDN to the stream during storms. These results suggest the importance of preserving near-stream riparian areas in cities to maintain ambient carbon and nitrogen source contributions to urban streams.     

Inter-annual variation in responses of water chemistry and epilithon to Pacific salmon spawners in an Alaskan stream

1. Pacific salmon (Oncorhynchus spp.) deliver salmon‐derived nutrients (SDN) to the streams in which they spawn. However, many stream parameters, such as discharge and spawner abundance, can vary from year to year, which could alter the quantity and flux of SDN. 2. Over six consecutive years, we studied responses in streamwater chemistry and epilithon (i.e. the microbial community on submerged rocks) to salmon spawners in Fish Creek, southeastern Alaska, U.S.A. The lower reach of Fish Creek receives spawners of several salmon species, while the upper reach does not receive spawners because of an intervening waterfall. 3. We estimated salmon spawner biomass, analysed water chemistry [ammonium, nitrate, soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC)], and measured epilithon abundance [as chlorophyll a (chl a) and ash‐free dry mass (AFDM)] in Fish Creek. Measurements were made in both the upper and lower reaches, before, during and after the major salmon runs. 4. Absolute values and relative differences indicated that the presence of salmon spawners consistently increased dissolved ammonium (by 58 μg L⁻¹ on average, 41× over background), SRP (by 6 μg L⁻¹, 14×), epilithon chl a (by 35 mg m⁻², 16×), and epilithon AFDM (by 3 g m⁻², 8×). Salmon spawners did not increase nitrate or DOC in either absolute or relative amounts. The persistence and magnitude of spawner effects varied among years and appeared to reflect weather‐driven hydrology as well as spawner biomass. 5. Salmon‐derived nutrients can stimulate the growth of primary producers by increasing streamwater nutrient concentrations, but this positive influence may be modulated by other factors, such as water temperature and discharge. To better assess the ecological influence of SDN on stream biota, future studies should explicitly consider the role of key environmental factors and their temporal and spatial dynamics in stream ecosystems.     

Fate and Transport of Organic Nitrogen in Minimally Disturbed Montane Streams of Colorado,USA

In two montane watersheds that receive minimal deposition of atmospheric nitrogen, 15–71% of dissolved organic nitrogen (DON) was bioavailable in stream water over a 2-year period. Discharge-weighted concentrations of bulk DON were between 102 and 135 μg/l, and the C:N ratio differed substantially between humic and non-humic fractions of DON. Approximately 70% of DON export occurred during snowmelt, and 40% of that DON was biologically available to microbes in stream sediments. Concentrations of bioavailable DON in stream water were 2–16 times greater than dissolved inorganic nitrogen (DIN) during the growing season, and bioavailable DON was depleted within 2–14 days during experimental incubations. Uptake of DON was influenced by the concentration of inorganic N in stream water, the concentration of non-humic DON in stream water, and the C:N ratio of the non-humic fraction of dissolved organic matter (DOM). Uptake of DON declined logarithmically as the concentration of inorganic N in stream water increased. Experimental additions of inorganic N also caused a decline in uptake of DON and net production of DON when the C:N ratio of non-humic DOM was high. This study indicates that the relative and absolute amount of bioavailable DON can vary greatly within and across years due to interactions between the availability of inorganic nutrients and composition of DOM. DOM has the potential to be used biotically at a high rate in nitrogen-poor streams, and it may be generated by heterotrophic microbes when DIN and labile DOM with low relative nitrogen content become abundant.     

Drainage Size, Stream Intermittency, and Ecosystem Function in a Sonoran Desert Landscape

Understanding the interactions between terrestrial and aquatic ecosystems remains an important research focus in ecology. In arid landscapes, catchments are drained by a channel continuum that represents a potentially important driver of ecological pattern and process in the surrounding terrestrial environment. To better understand the role of drainage networks in arid landscapes, we determined how stream size influences the structure and productivity of riparian vegetation, and the accumulation of organic matter (OM) in soils beneath plants in an upper Sonoran Desert basin. Canopy volume of velvet mesquite (Prosopis velutina), as well as overall plant cover, increased along lateral upland–riparian gradients, and among riparian zones adjacent to increasingly larger streams. Foliar δ¹³C signatures for P. velutina suggested that landscape patterns in vegetation structure reflect increases in water availability along this arid stream continuum. Leaf litter and annual grass biomass production both increased with canopy volume, and total aboveground litter production ranged from 137 g m⁻² y⁻¹ in upland habitat to 446 g m⁻² y⁻¹ in the riparian zone of the perennial stream. OM accumulation in soils beneath P. velutina increased with canopy volume across a broad range of drainage sizes; however, in the riparian zone of larger streams, flooding further modified patterns of OM storage. Drainage networks represent important determinants of vegetation structure and function in upper Sonoran Desert basins, and the extent to which streams act as sources of plant-available water and/or agents of fluvial disturbance has implications for material storage in arid soils.     

Effects of sediment removal on nitrogen uptake by riparian plants in the higher floodplain of the Chikuma River, Japan

Riparian plants can use nitrogen (N) from soil and river water, but the use of river water N might be limited in higher floodplain environments of the Chikuma River. The purpose of this study is to reveal the relationship between N uptake by riparian plants and the floodplain topography (relative height and distance from a river channel). We examined the hypothesis that surface sediment removal from the higher floodplain increases river water N uptake by riparian plants by using a stable isotope analysis. The δ¹⁵N value of river water samples (ca. 8‰) were significantly higher than those of the soil extracts (ca. 3‰) in the study area. The δ¹⁵N value of riparian plants increased from +3.0‰ (standard deviation, SD ±2.1‰) before sediment removal to +9.6‰ (±2.1‰) after sediment removal, although there was no significant change in the δ¹⁵N value in N sources of soil and river water. The sediment removal enhanced frequency of flood disturbance, relative ground water level, and river water N uptake by riparian plants on the floodplain.     

Hydrogen and oxygen isotope ratios of tree ring cellulose for field-grown riparian trees

The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of –125/–15‰ (δD/δ¹⁸O), –48/–6‰, and –67/–7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water (both δD and δ¹⁸O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ¹⁸O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ¹⁸O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating that the model parameterization from controlled experiments was robust even under uncontrolled and variable field conditions. Received: 7 April 1999 / Accepted: 8 December 1999