共查询到20条相似文献,搜索用时 31 毫秒
1.
We evaluated (1) the longitudinal pattern of stream chemistry and (2) the effects of the riparian zone on this longitudinal
pattern for nitrate (NO3
−), dissolved organic carbon (DOC), and total dissolved iron (Fe). We selected two small watersheds; the “southern watershed”
had an extending riparian wetland and the “northern watershed” had a narrow riparian area. Stream NO3
− concentrations decreased from the spring to outlet of both watersheds. In the southern watershed, stream DOC concentration
decreased from the spring to midstream and then increased to the outlet. Stream Fe concentration in the southern watershed
longitudinally increased. On the other hand, the northern watershed exhibited no longitudinal pattern for DOC and Fe concentrations.
In both watersheds, while NO3
− concentrations in the soil and ground water were lower than those in the stream waters, DOC and Fe concentrations exhibited
the opposite patterns. The longitudinal decreases of NO3
− concentrations in both streams and increase of stream Fe in the southern watershed mainly resulted from the inflow of the
soil and ground water to the stream. The decrease in stream DOC from the spring to midstream in the southern watershed was
due to the deep groundwater having low DOC, while the subsequent increase to the surrounding soil and ground water. Moreover,
considerations of stream solute flow with soil and ground water chemistry suggested other mechanisms adding NO3
− and removing/diluting DOC and Fe, especially for the northern watershed; coexistence of oxidizing and reducing conditions
in the riparian zone might control the longitudinal concentration change in the stream water chemistry. 相似文献
2.
Michael R. McHale Douglas A. Burns Gregory B. Lawrence Peter S. Murdoch 《Biogeochemistry》2007,84(3):311-331
The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter
of 1996–1997. The interactions among acidity, nitrate (NO3−), aluminum (Al), and calcium (Ca2+) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and
concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations,
and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO3− concentration was about 20 μmol l−1 for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l−1 about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Alim) export increased by 4−fold during the first year after the harvest. Alim mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca2+ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium
with imogolite appeared to control Al solubility in the B horizon. Alim and NO3− concentrations were strongly correlated in B-horizon soil water after the clearcut (r
2 = 0.96), especially at NO3− concentrations greater than 100 μmol l−1. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations
of NO3− which mixed with acidic, high Alim soil water and decreased the concentration of Alim in streamwater after the harvest. Five years after the harvest soil water NO3− concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater
NO3− concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO3− concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because
of the contribution of groundwater to the stream. Streamwater NO3− and Alim concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because
the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable
base cations and caused long-term acidification of the soil. 相似文献
3.
Since 1987 we have studied weekly change in winter (December–April) precipitation, snowpack, snowmelt, soil water, and stream
water solute flux in a small (176-ha) Northern Michigan watershed vegetated by 65–85 year-old northern hardwoods. Our primary
study objective was to quantify the effect of change in winter temperature and precipitation on watershed hydrology and solute
flux. During the study winter runoff was correlated with precipitation, and forest soils beneath the snowpack remained unfrozen.
Winter air temperature and soil temperature beneath the snowpack increased while precipitation and snowmelt declined. Atmospheric
inputs declined for H+, NO3−, NH4+, dissolved inorganic nitrogen (DIN), and SO42−. Replicated plot-level results, which could not be directly extrapolated to the watershed scale, showed 90% of atmospheric
DIN input was retained in surface shallow (<15 cm deep) soils while SO42− flux increased 70% and dissolved organic carbon (DOC) 30-fold. Most stream water base cation (CB), HCO3−, and Cl− concentrations declined with increased stream water discharge, K+, NO3−, and SO42− remained unchanged, and DOC and dissolved organic nitrogen (DON) increased. Winter stream water solute outputs declined or
were unchanged with time except for NO3− and DOC which increased. DOC and DIN outputs were correlated with the percentage of winter runoff and stream discharge that
occurred when subsurface flow at the plot-level was shallow (<25 cm beneath Oi). Study results suggest that the percentage
of annual runoff occurring as shallow lateral subsurface flow may be a major factor regulating solute outputs and concentrations
in snowmelt-dominated ecosystems. 相似文献
4.
A. Bryce Cooper 《Plant and Soil》1986,93(3):383-394
Summary Nitrate-N losses to stream waters and soil inorganic N pools, nitrifying potentials and NO3-N production rates were measured in 2 adjacent watersheds, one used as pasture and the other planted in exotic conifer forest
(Pinus radiata D. Don). Estimated NO3-N loss to stream waters draining the pine and pasture watersheds were 0.6kg ha−1 y−1 and 7.6 kg ha−1 y−1 respectively. Ammonium-N pool sizes were not significantly different between soils in the two watersheds but NO3−N pools and nitrifying potentials were always lower in the pine watershed soil samples. Laboratory incubation experiments
indicated that suppression of NO3−N formation in pine watershed soils required the presence of live tree roots and was not due to the direct action of allelopathic
chemicals on nitrifiers. 相似文献
5.
Jonathan M. O’Brien Walter K. Dodds Kymberly C. Wilson Justin N. Murdock Jessica Eichmiller 《Biogeochemistry》2007,84(1):31-49
We conducted 15NO3− stable isotope tracer releases in nine streams with varied intensities and types of human impacts in the upstream watershed
to measure nitrate (NO3−) cycling dynamics. Mean ambient NO3− concentrations of the streams ranged from 0.9 to 21,000 μg l−1 NO3−–N. Major N-transforming processes, including uptake, nitrification, and denitrification, all increased approximately two
to three orders of magnitude along the same gradient. Despite increases in transformation rates, the efficiency with which
stream biota utilized available NO3−-decreased along the gradient of increasing NO3−. Observed functional relationships of biological N transformations (uptake and nitrification) with NO3− concentration did not support a 1st order model and did not show signs of Michaelis–Menten type saturation. The empirical
relationship was best described by a Efficiency Loss model, in which log-transformed rates (uptake and nitrification) increase
with log-transformed nitrate concentration with a slope less than one. Denitrification increased linearly across the gradient
of NO3− concentrations, but only accounted for ∼1% of total NO3− uptake. On average, 20% of stream water NO3− was lost to denitrification per km, but the percentage removed in most streams was <5% km−1. Although the rate of cycling was greater in streams with larger NO3− concentrations, the relative proportion of NO3− retained per unit length of stream decreased as NO3− concentration increased. Due to the rapid rate of NO3− turnover, these streams have a great potential for short-term retention of N from the landscape, but the ability to remove
N through denitrification is highly variable. 相似文献
6.
Riparian nitrogen dynamics in two geomorphologically distinct tropical rain forest watersheds: subsurface solute patterns 总被引:3,自引:3,他引:0
Nitrate, ammonium, dissolved organic N, and dissolved oxygen were measured in stream water and shallow groundwater in the
riparian zones of two tropical watersheds with different soils and geomorphology. At both sites, concentrations of dissolved
inorganic N (DIN; NH4
+- and NO3
−-N) were low in stream water (< 110 ug/L). Markedly different patterns in DIN were observed in groundwater collected at the
two sites. At the first site (Icacos watershed), DIN in upslope groundwater was dominated by NO3
−-N (550 ug/L) and oxygen concentrations were high (5.2 mg/L). As groundwater moved through the floodplain and to the stream,
DIN shifted to dominance by NH4
+-N (200–700 ug/L) and groundwater was often anoxic. At the second site (Bisley watershed), average concentrations of total
dissolved nitrogen were considerably lower (300 ug/L) than at Icacos (600 ug/L), and the dominant form of nitrogen was DON
rather than inorganic N. Concentrations of NH4
+ and NO3
− were similar throughout the riparian zone at Bisley, but concentrations of DON declined from upslope wells to stream water.
Differences in speciation and concentration of nitrogen in groundwater collected at the two sites appear to be controlled
by differences in redox conditions and accessibility of dissolved N to plant roots, which are themselves the result of geomorphological
differences between the two watersheds. At the Icacos site, a deep layer of coarse sand conducts subsurface water to the stream
below the rooting zone of riparian vegetation and through zones of strong horizontal redox zonation. At the Bisley site, infiltration
is impeded by dense clays and saturated flow passes through the variably oxidized rooting zone. At both sites, hydrologic
export of nitrogen is controlled by intense biotic activity in the riparian zone. However, geomorphology appears to strongly
modify the importance of specific biotic components. 相似文献
7.
M. J. Mitchell K. B. Piatek S. Christopher B. Mayer C. Kendall P. Mchale 《Biogeochemistry》2006,78(2):217-246
Understanding the effects of climate change including precipitation patterns has important implications for evaluating the
biogeochemical responses of watersheds. We focused on four storms in late summer and early fall that occurred after an exceptionally
dry period in 2002. We analyzed not only the influence of these storms on episodic chemistry and the role of different water
sources in affecting surface water chemistry, but also the relative contributions of these storms to annual biogeochemical
mass balances. The study site was a well studied 135-ha watershed in the Adirondack Park of New York State (USA). Our analyses
integrated measurements on hydrology, solute chemistry and the isotopic composition of NO3−(δ15N and δ18O) and SO42−(δ34S and δ18O) to evaluate how these storms affected surface water chemistry. Precipitation amounts varied among the storms (Storm 1:
Sept. 14–18, 18.5 mm; Storm 2: Sept. 21–24, 33 mm; Storm 3: Sept. 27–29, 42.9 mm; Storm 4: Oct. 16–21, 67.6 mm). Among the
four storms, there was an increase in water yields from 2 to 14%. These water yields were much less than in studies of storms
in previous years at this same watershed when antecedent moisture conditions were higher. In the current study, early storms
resulted in relatively small changes in water chemistry. With progressive storms the changes in water chemistry became more
marked with particularly major changes in Cb (sum of base cations), Si, NO3−, and SO42−, DOC and pH. Analyses of the relationships between Si, DOC, discharge and water table height clearly indicated that there
was a decrease in ground water contributions (i.e., lower Si concentrations and higher DOC concentrations) as the watershed
wetness increased with storm succession. The marked changes in chemistry were also reflected in changes in the isotopic composition
of SO42− and NO3−. There was a strong inverse relationship between SO42− concentrations and δ34S values suggesting the importance of S biogeochemical redox processes in contributing to SO42− export. The isotopic composition of NO3− in stream water indicated that this N had been microbially processed. Linkages between SO42− and DOC concentrations suggest that wetlands were major sources of these solutes to drainage waters while the chemical and
isotopic response of NO3− suggested that upland sources were more important. Although these late summer and fall storms did not play a major role in
the overall annual mass balances of solutes for this watershed, these events had distinctive chemistry including depressed
pH and therefore have important consequences to watershed processes such as episodic acidification, and the linkage of these
processes to climate change. 相似文献
8.
Spatial variability in nutrient concentration and biofilm nutrient limitation in an urban watershed 总被引:1,自引:0,他引:1
Nutrient enrichment threatens river ecosystem health in urban watersheds, but the influence of urbanization on spatial variation
in nutrient concentrations and nutrient limitation of biofilm activity are infrequently measured simultaneously. In summer
2009, we used synoptic sampling to measure spatial patterns of nitrate (NO3
−), ammonium (NH4
+), and soluble reactive phosphorus (SRP) concentration, flux, and instantaneous yield throughout the Bronx River watershed
within New York City and adjacent suburbs. We also quantified biofilm response to addition of NO3
−, phosphate (PO4
3−), and NO3
− + PO4
3− on organic and inorganic surfaces in the river mainstem and tributaries. Longitudinal variation in NO3
− was low and related to impervious surface cover across sub-watersheds, but spatial variation in NH4
+ and SRP was higher and unrelated to sub-watershed land-use. Biofilm respiration on organic surfaces was frequently limited
by PO4
3− or NO3
− + PO4
3−, while primary production on organic and inorganic surfaces was nutrient-limited at just one site. Infrequent NO3
− limitation and low spatial variability of NO3
− throughout the watershed suggested saturation of biological N demand. For P, both higher biological demand and point-sources
contributed to greater spatial variability. Finally, a comparison of our data to synoptic studies of forested, temperate watersheds
showed lower spatial variation of N and P in urban watersheds. Reduced spatial variation in nutrients as a result of biological
saturation may represent an overlooked effect of urbanization on watershed ecology, and may influence urban stream biota and
downstream environments. 相似文献
9.
Factors Controlling Sediment Denitrification in Midwestern Streams of Varying Land Use 总被引:1,自引:0,他引:1
We investigated controls on stream sediment denitrification in nine headwater streams in the Kalamazoo River Watershed, Michigan,
USA. Factors influencing denitrification were determined by using experimental assays based on the chloramphenicol-amended
acetylene inhibition technique. Using a coring technique, we found that sediment denitrification was highest in the top 5 cm
of the benthos and was positively related to sediment organic content. To determine the effect of overlying water quality
on sediment denitrification, first-order stream sediments were assayed with water from second- and third-order downstream
reaches, and often showed higher denitrification rates relative to assays using site-specific water from the first-order stream
reach. Denitrification was positively related to nitrate (NO3
−) concentration, suggesting that sediments may have been nutrient-limited. Using stream-incubated inorganic substrata of varying
size classes, we found that finer-grained sand showed higher rates of denitrification compared to large pebbles, likely due
to increased surface area per volume of substratum. Denitrification was measurable on both inorganic substrata and fine particulate
organic matter loosely associated with inorganic particles, and denitrification rates were related to organic content. Using
nutrient-amended denitrification assays, we found that sediment denitrification was limited by NO3
− or dissolved organic carbon (DOC, as dextrose) variably throughout the year. The frequency and type of limitation differed
with land use in the watershed: forested streams were NO3
−-limited or co-limited by both NO3
− and DOC 92% of the time, urban streams were more often NO3
−-limited than DOC-limited, whereas agricultural stream sediments were DOC-limited or co-limited but not frequently limited
by NO3
− alone. 相似文献
10.
Isotopic signature of nitrate in two contrasting watersheds of Brush Brook,Vermont, USA 总被引:2,自引:0,他引:2
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. 相似文献
11.
Impact of N-Saturated Upland Forests on Downstream N Pollution in the Tatara River Basin, Japan 总被引:1,自引:0,他引:1
This study evaluated whether nitrogen (N) saturated upland forests can degrade downstream water quality in the Tatara River
Basin, northern Kyushu, western Japan. Our hypothesis is that elevated atmospheric N deposition degrades downstream water
quality in a watershed containing N-saturated forests because a considerable amount of atmospherically deposited N passes
into the streams without being retained. Synoptic stream water samplings were conducted at 23 sites across a wide range of
land-use categories in the basin over 1 year. A long-term temporal analysis of downstream water quality over the last 30 years
(1977–2007) was conducted and compared with long-term trends in related factors such as urban/agricultural activity, sewage
wastewater treatment, atmospheric N deposition, and forest condition. The results showed that atmospherically deposited N
to N-saturated forests can be a large enough non-point source of N leaving the watershed to impact downstream water quality.
This was highlighted by the reduction in pollutant exports derived from urban/agricultural activities, an increase in atmospheric
N deposition, and the maturation of coniferous plantation forests in the past 30 years. These have led to reductions in total
phosphorus and organic nitrogen concentrations in downstream water, whereas downstream nitrate (NO3
−) concentrations increased over the last 30 years. The consequent increase in the downstream N:P ratio indicated P limitation.
Reducing the NO3
− exports from N-saturated upland forests is suggested as a strategy to improve regional downstream NO3
− pollution, but involves intercontinental-scale action in reducing atmospheric N emissions. 相似文献
12.
Whole-system Estimates of Nitrification and Nitrate Uptake in Streams of the Hubbard Brook Experimental Forest 总被引:3,自引:2,他引:1
Although they drain remarkably similar forest types, streams of the Hubbard Brook Experimental Forest (HBEF) vary widely in
their NO3
− concentrations during the growing season. This variation may be caused by differences in the terrestrial systems they drain
(for example, varying forest age or composition, hydrology, soil organic matter content, and so on) and/or by differences
between the streams themselves (for example, contrasting geomorphology, biotic nitrogen [N] demand, rates of instream nitrogen
transformations). We examined interstream variation in N processing by measuring NH4
+ and NO3
− uptake and estimating nitrification rates for 13 stream reaches in the HBEF during the summers of 1998 and 1999. We modeled
nitrification rates using a best-fit model of the downstream change in NO3
− concentrations following short-term NH4
+ enrichments. Among the surveyed streams, the fraction of NH4
+ uptake that was subsequently nitrified varied, and this variation was positively correlated with ambient streamwater NO3
− concentrations. We examined whether this variation in instream nitrification rates contributed significantly to the observed
variation in NO3
− concentrations across streams. In some cases, instream nitrification provided a substantial portion of instream NO3
− demand. However, because there was also substantial instream NO3
− uptake, the net effect of instream processing was to reduce rather than supplement the total amount of NO3
− exported from a watershed. Thus, instream rates of nitrification in conjunction with instream NO3
− uptake were too low to account for the wide range of streamwater NO3
−. The relationship between streamwater NO3
− concentration and rates of instream nitrification may instead be due to a shift in the competitive balance between heterotrophic
N uptake and nitrification when external inputs of NO3
− are relatively high.
Received 11 October 2000; accepted 14 December 2001. 相似文献
13.
An Unexpected Nitrate Decline in New Hampshire Streams 总被引:7,自引:2,他引:5
Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition
and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream
NO3
− concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376–381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and
chronic N deposition. Between 1973–74 and 1996–97, mean NO3
− concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 μmol/L), Ca2+ decreased by 24% (8 μmol/L), and Mg2+ decreased by 22% (5 μmol/L). Nitrate concentrations decreased in every stream in every season, but spatial patterns among
streams persisted: Streams draining old-growth stands maintained higher NO3
− concentrations than those draining successional stands. The cause of the NO3
− decline is not evident. Nitrogen deposition has changed little, and although mechanisms such as insect defoliation and soil
frost may contribute to the temporal patterns of nitrate loss, they do not appear to fully explain the NO3
− decline across the region. Although the role of climate remains uncertain, interannual climate variation and its effects
on biotic N retention may be responsible for the synchronous decrease in NO3
− across all streams, overriding expected increases due to chronic N deposition and forest aging.
Received 4 December 2001; accepted 30 May 2002. 相似文献
14.
Koop JH Grieshaber MK 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2000,170(1):75-83
Fluctuating salinities at different sites on the German salt-polluted rivers Werra and Weser were compared with extracellular
ion levels of specimens of Gammarus tigrinus (Sexton; Amphipoda, Crustacea), collected at the same sites. G. tigrinus regulated haemolymph concentrations of inorganic anions (Cl−, SO2−
4, PO3−
4) and cations (Na+, K+, Mg2+, Ca2+) during fluctuations of salt pollution in the upper Weser. This capacity to regulate varying levels of salt pollution in
the upper Weser, correlated well with the distribution of the brackish amphipods in this river ecosystem. G. tigrinus tolerated periods of Na+ and Cl− stress (>380 mmol l−1) without compensating these maxima by regulating extracellular Na+ and Cl−. However, during such bursts of Na+ and Cl− stress in Werra and Weser, the ability to regulate extracellular [K+] at river water K+ stress of ≥6.0 mmol l−1 may explain why this brackish species has been more successful in these rivers than its competitors like Gammarus pulex. The present investigation demonstrates that the water salinity affects the [NO−
3] in the haemolymph of G. tigrinus. With increasing hypo-osmotic stress the animals accumulate increasing amounts of NO−
3. A simultaneous increase in stream water [NO−
3] causes an additional accumulation of NO−
3 in the haemolymph. The high extent of accumulation indicates that active ion transport systems may be involved. The accumulation
of NO−
3 in the haemolymph has low physiological consequences to G. tigrinus, but when hypo-osmotically stressed under anoxic conditions, nitrite formed by the reduction of nitrate may have an adverse
affect on the metabolism of G. tigrinus.
Accepted: 4 October 1999 相似文献
15.
Stream export of nitrogen (N) as nitrate (NO3−; the most mobile form of N) from forest ecosystems is thought to be controlled largely by plant uptake of inorganic N, such
that reduced demand for plant N during the non-growing season and following disturbances results in increased stream NO3− export. The roles of microbes and soils in ecosystem N retention are less clear, but are the dominant controls on N export
when plant uptake is low. We used a mass balance approach to investigate soil N retention during winter (December through
March) at the Hubbard Brook Experimental Forest by comparing NO3− inputs (atmospheric deposition), internal production (soil microbial nitrification), and stream output. We focused on months
when plant N uptake is nearly zero and the potential for N export is high. Although winter months accounted for only 10–15%
of annual net nitrification, soil NO3− production (0.8–1.0 g N m−2 winter−1) was much greater than stream export (0.03–0.19 N m−2 winter−1). Soil NO3− retention in two consecutive winters was high (96% of combined NO3− deposition and soil production; year 1) even following severe plant disturbance caused by an ice-storm (84%; year 2) We show
that soil NO3− retention is surprisingly high even when N demand by plants is low. Our study highlights the need to better understand mechanisms
of N retention during the non-growing season to predict how ecosystems will respond to high inputs of atmospheric N, disturbance,
and climate change. 相似文献
16.
Wetlands are often highly effective nitrogen (N) sinks. In the Lake Waco Wetland (LWW), near Waco, Texas, USA, nitrate (NO3−) concentrations are reduced by more than 90% in the first 500 m downstream of the inflow, creating a distinct gradient in
NO3− concentration along the flow path of water. The relative importance of sediment denitrification (DNF), dissimilatory NO3− reduction to ammonium (DNRA), and N2 fixation were examined along the NO3− concentration gradient in the LWW. “Potential DNF” (hereafter potDNF) was observed in all months and ranged from 54 to 278 μmol N m−2 h−1. “Potential DNRA” (hereafter potDNRA) was observed only in summer months and ranged from 1.3 to 33 μmol N m−2 h−1. Net N2 flux ranged from 184 (net denitrification) to −270 (net N2 fixation) μmol N m−2 h−1. Nitrogen fixation was variable, ranging from 0 to 426 μmol N m−2 h−1, but high rates ranked among the highest reported for aquatic sediments. On average, summer potDNRA comprised only 5% (±2%
SE) of total NO3− loss through dissimilatory pathways, but was as high as 36% at one site where potDNF was consistently low. Potential DNRA
was higher in sediments with higher sediment oxygen demand (r
2 = 0.84), and was related to NO3− concentration in overlying water in one summer (r
2 = 0.81). Sediments were a NO3− sink and accounted for 50% of wetland NO3− removal (r
2 = 0.90). Sediments were an NH4+ source, but the wetland was often a net NH4+ sink. Although DNRA rates in freshwater wetlands may rival those observed in estuarine systems, the importance of DNRA in
freshwater sediments appears to be minor relative to DNF. Furthermore, sediment N2 fixation can be extremely high when NO3− in overlying water is consistently low. The data suggest that newly fixed N can support sustained N transformation processes
such as DNF and DNRA when surface water inorganic N supply rates are low. 相似文献
17.
Urban streams often contain elevated concentrations of nitrogen (N) which can be amplified in systems receiving effluent from
wastewater treatment plants (WWTP). In this study, we evaluated the importance of denitrification in a stream draining urban
Greensboro, NC, USA, using two approaches: (1) natural abundance of 15N–NO3− in conjunction with background NO3−–N concentrations along a 7 km transect downstream of a WWTP; and (2) C2H2 block experiments at three sites and at three habitat types within each site. Overall lack of a longitudinal pattern of δ15N–NO3− and NO3−–N, combined with high concentrations of NO3−–N suggested that other factors were controlling NO3−–N flux in the study transect. However, denitrification did appear to be significant along one portion of the transect. C2H2 block experiments showed that denitrification rates were much higher downstream of the WWTP compared to upstream, and showed
that denitrification rates were highest in erosional and depositional areas downstream of the WWTP and in erosional areas
upstream of the plant. Thus, the combination of the two methods for evaluating denitrification provided more insight into
the spatial dynamics of denitrification activity than either approach alone. Denitrification appeared to be a significant
sink for NO3−–N upstream of the WWTP, but not downstream. Approximately 46% of the total NO3−–N load was removed via denitrification in the upstream, urban section of the stream, while only 2.3% of NO3−–N was lost downstream of the plant. This result suggests that controlling NO3−–N loading from the plant could result in considerable improvement of downstream water quality. 相似文献
18.
J. Alan Yeakley David C. Coleman Bruce L. Haines Brian D. Kloeppel Judy L. Meyer Wayne T. Swank Barry W. Argo James M. Deal Sharon F. Taylor 《Ecosystems》2003,6(2):0154-0167
We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented
on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic
carbon (DOC), K+, Na+, Ca2+, Mg2+, NO3
−-N, NH4
+-N, PO4
3−-P, and SO4
2− were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees
on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations
of NO3
−-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO4
2−, Ca2+, and Mg2+. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO3
−-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO3
−-N increased four fold, and streamwater NO3
−-N doubled. Significant changes also occurred in soilwater pH, DOC, SO4
2−, Ca2+, and Mg2+. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope.
Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient
uptake by canopy trees is a key control on NO3
−-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes
significant nutrient loss to streams.
Received 30 January 2001; accepted 25 July 2002. 相似文献
19.
Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia 总被引:1,自引:0,他引:1
Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years
in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing
vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate
the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong
seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge
increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients
accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast,
stream water DOC concentrations did not exhibit seasonal variation.
Higher DOC and NO3
− concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of
leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge
from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there
was an additional NO3
− source or reduction of a NO3
− sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence
of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated
area. 相似文献
20.
R. Thomas James Wayne S. Gardner Mark J. McCarthy Stephen A. Carini 《Hydrobiologia》2011,669(1):199-212
Total nitrogen (TN) in Lake Okeechobee, a large, shallow, turbid lake in south Florida, has averaged between 90 and 150 μM
on an annual basis since 1983. No TN trends are evident, despite major storm events, droughts, and nutrient management changes
in the watershed. To understand the relative stability of TN, this study evaluates nitrogen (N) dynamics at three temporal/spatial
levels: (1) annual whole lake N budgets, (2) monthly in-lake water quality measurements in offshore and nearshore areas, and
(3) isotope addition experiments lasting 3 days and using 15N-ammonium (15NH4
+) and 15N-nitrate (15NO3
−) at two offshore locations. Budgets indicate that the lake is a net sink for N. TN concentrations were less variable than
net N loads, suggesting that in-lake processes moderate these net loads. Monthly NO3
− concentrations were higher in the offshore area and higher in winter for both offshore and nearshore areas. Negative relationships
between the percentage of samples classified as algal blooms (defined as chlorophyll a > 40 μg l−1) and inorganic N concentrations suggest N-limitation. Continuous-flow experiments over intact sediment cores measured net
fluxes (μmol N m−2 h−1) between 0 and 25 released from sediments for NH4
+, 0–60 removed by sediments for NO3
−, and 63–68 transformed by denitrification. Uptake rates in the water column (μmol N m−2 h−1) determined by isotope dilution experiments and normalized for water depth were 1,090–1,970 for NH4
+ and 59–119 for NO3
−. These fluxes are similar to previously reported results. Our work suggests that external N inputs are balanced in Lake Okeechobee
by denitrification. 相似文献