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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.
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   

3.
Metal pollution, in combination with other environmental stressors such as acid deposition and climate change, may disturb metal biogeochemical cycles. To investigate the influence of dissolved organic carbon, acidity and seasonality on metal geochemistry, this study has described concentrations of 19 metals as they pass through an acidified forested catchment on the Precambrian Shield in south-central Ontario, Canada. Metal, dissolved organic carbon (DOC) and sulphate (SO4 2−) concentrations fluctuate throughout the catchment compartments as the water passes through and interacts with vegetation, soils and bedrock. Relationships among metals, DOC and SO4 2− are most pronounced in compartments where DOC and SO4 2− exhibit high variability, namely in the throughfall, organic horizon soil water, and wetland-draining stream. Metal, DOC and SO4 2− concentrations varied seasonally in the streams, and temporal coherence occurred among metal, DOC and SO4 2− concentrations in the organic horizon soil water and the wetland-draining stream (PC1). In the wetland-draining stream, the highest DOC, Cr, Cu, Fe, Pb, and V concentrations occur in the summer, whereas concentrations of SO4 2− and most other metals peak in the fall after a period of drought. Despite the rural location, provincial water quality objectives for surface water were exceeded for many metals when the peak fall values occurred.  相似文献   

4.
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   

5.
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.  相似文献   

6.
Nitrogen from atmospheric deposition serves as the dominant source of new nitrogen to forested ecosystems in the northeastern U.S. By combining isotopic data obtained using the denitrifier method, with chemical and hydrologic measurements we determined the relative importance of sources and control mechanisms on nitrate (NO3 ) export from five forested watersheds in the Connecticut River watershed. Microbially produced NO3 was the dominant source (82–100%) of NO3 to the sampled streams as indicated by the δ15N and δ18O of NO3 . Seasonal variations in the δ18O–NO3 in streamwater are controlled by shifting hydrologic and temperature affects on biotic processing, resulting in a relative increase in unprocessed NO3 export during winter months. Mass balance estimates find that the unprocessed atmospherically derived NO3 stream flux represents less than 3% of the atmospherically delivered wet NO3 flux to the region. This suggests that despite chronically elevated nitrogen deposition these forests are not nitrogen saturated and are retaining, removing, and reprocessing the vast majority of NO3 delivered to them throughout the year. These results confirm previous work within Northeastern U.S. forests and extend observations to watersheds not dominated by a snow-melt driven hydrology. In contrast to previous work, unprocessed atmospherically derived NO3 export is associated with the period of high recharge and low biotic activity as opposed to spring snowmelt and other large runoff events.  相似文献   

7.
Downstream from metropolitan Paris (France), a large amount of ammonium is discharged into the Seine River by the effluents of the wastewater treatment plant at Achères. To assess the extent of nitrification and denitrification in the water column, concentrations and isotopic compositions of ammonium (δ15N–NH4+) and nitrate (δ15N–NO3, δ18O–NO3) were measured during summer low-flow conditions along the lower Seine and its estuary. The results indicated that most of the ammonium released from the wastewater treatment plant is nitrified in the lower Seine River and its upper estuary, but there was no evidence for water-column denitrification. In the lower part of the estuary, however, concentration and isotopic data for nitrate were not consistent with simple mixing between riverine and marine nitrate. A significant departure of the nitrate isotopic composition from what would be expected from simple mixing of freshwater and marine nitrates suggested coupled nitrification and denitrification in the water, in spite of the apparent conservative behavior of nitrate. Denitrification rates of approximately 0.02 mg N/L/h were estimated for this part of the estuary.  相似文献   

8.
Dissolved organic carbon (DOC) and NO3 are important forms of C and N in stream water. Hypotheses concerning relationships between DOC and NO3 concentrations have been proposed, but there are no reports demonstrating a relationship between them in stream water. We observed 35 natural streams in the Lake Biwa watershed, central Japan, and found an inverse relationship between DOC and NO3 concentrations. This relationship was also found in observations of their seasonal variations in the Lake Biwa watershed. Moreover, this relationship was also found to apply to watersheds in other regions in Japan. These results suggest that forest biogeochemical processes which control DOC and NO3 concentrations in Japanese streams are closely related. Excess N availability together with a C (energy) deficit in a soil environment may explain this relationship. DOC and NO3 concentrations in streams will thus be a useful index indicating C and N availability in catchments.  相似文献   

9.
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   

10.
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   

11.
Porewater equilibration samplers were used to obtain porewater inventories of inorganic nutrients (NH4+, NOx, PO43−), dissolved organic carbon (DOC) and nitrogen (DON), sulfate (SO42−), dissolved inorganic carbon (DIC), hydrogen sulfide (H2S), chloride (Cl), methane (CH4) and reduced iron (Fe2+) in intertidal creek-bank sediments at eight sites in three estuarine systems over a range of salinities and seasons. Sulfate reduction (SR) rates and sediment particulate organic carbon (POC) and nitrogen (PON) were also determined at several of the sites. Four sites in the Okatee River estuary in South Carolina, two sites on Sapelo Island, Georgia and one site in White Oak Creek, Georgia appeared to be relatively pristine. The eighth site in Umbrella Creek, Georgia was directly adjacent to a small residential development employing septic systems to handle household waste. The large data set (>700 porewater profiles) offers an opportunity to assess system-scale patterns of porewater biogeochemical dynamics with an emphasis on DOC and DON distributions. SO42− depletion (SO42−)Dep was used as a proxy for SR, and (SO42−)Dep patterns agreed with measured (35S) patterns of SR. There were significant system-scale correlations between the inorganic products of terminal metabolism (DIC, NH4+ and PO43−) and (SO42−)Dep, and SR appeared to be the dominant terminal carbon oxidation pathway in these sediments. Porewater inventories of DIC and (SO42−)Dep indicate a 2:1 stoichiometry across sites, and the C:N ratio of the organic matter undergoing mineralization was between 7.5 and 10. The data suggest that septic-derived dissolved organic matter with a C:N ratio below 6 fueled microbial metabolism and SR at a site with development in the upland. Seasonality was observed in the porewater inventories, but temperature alone did not adequately describe the patterns of (SO42−)Dep, terminal metabolic products (DIC, NH4+, PO43−), DOC and DON, and SR observed in this study. It appears that production and consumption of labile DOC are tightly coupled in these sediments, and that bulk DOC is likely a recalcitrant pool. Preferential hydrolysis of PON relative to POC when overall organic matter mineralization rates were high appears to drive the observed patterns in POC:PON, DOC:DON and DIC:DIN ratios. These data, along with the weak seasonal patterns of SR and organic and inorganic porewater inventories, suggest that the rate of hydrolysis limits organic matter mineralization in these intertidal creek-bank sediments.  相似文献   

12.
The Ferrous Wheel Hypothesis (Davidson et al. 2003) postulates the abiotic formation of dissolved organic N (DON) in forest floors, by the fast reaction of NO2 with dissolved organic C (DOC). We investigated the abiotic reaction of NO2 with dissolved organic matter extracted from six different forest floors under oxic conditions. Solutions differed in DOC concentrations (15–60 mg L−1), NO2 concentrations (0, 2, 20 mg NO2 -N L−1) and DOC/DON ratio (13.4–25.4). Concentrations of added NO2 never decreased within 60 min, therefore, no DON formation from added NO2 took place in any of the samples. Our results suggest that the reaction of NO2 with natural DOC in forest floors is rather unlikely.  相似文献   

13.
Organic and inorganic carbon (C) fluxes transported by water were evaluated for dominant hydrologic flowpaths on two adjacent headwater catchments in the Brazilian Amazon with distinct soils and hydrologic responses from September 2003 through April 2005. The Ultisol-dominated catchment produced 30% greater volume of storm-related quickflow (overland flow and shallow subsurface flow) compared to the Oxisol-dominated catchment. Quickflow fluxes were equivalent to 3.2 ± 0.2% of event precipitation for the Ultisol catchment, compared to 2.5 ± 0.3% for the Oxisol-dominated watershed (mean response ±1 SE, n = 27 storms for each watershed). Hydrologic responses were also faster on the Ultisol watershed, with time to peak flow occurring 10 min earlier on average as compared to the runoff response on the Oxisol watershed. These different hydrologic responses are attributed primarily to large differences in saturated hydraulic conductivity (K s). Overland flow was found to be an important feature on both watersheds. This was evidenced by the response rates of overland flow detectors (OFDs) during the rainy season, with overland flow intercepted by 54 ± 0.5% and 65 ± 0.5% of OFDs for the Oxisol and Ultisol watersheds respectively during biweekly periods. Small volumes of quickflow correspond to large fluxes of dissolved organic C (DOC); DOC concentrations of the hydrologic flowpaths that comprise quickflow are an order of magnitude higher than groundwater flowpaths fueling base flow (19.6 ± 1.7 mg l−1 DOC for overland flow and 8.8 ± 0.7 mg l−1 DOC for shallow subsurface flow versus 0.50 ± 0.04,mg l−1 DOC in emergent groundwater). Concentrations of dissolved inorganic C (DIC, as dissolved CO2–C plus HCO3–C) in groundwater were found to be an order of magnitude greater than quickflow DIC concentrations (21.5 mg l−1 DIC in emergent groundwater versus 1.1 mg l−1 DIC in overland flow). The importance of deeper flowpaths in the transport of inorganic C to streams is indicated by the 40:1 ratio of DIC:DOC for emergent groundwater. Dissolved CO2–C represented 92% of DIC in emergent groundwater. Results from this study illustrate a highly dynamic and tightly coupled linkage between the C cycle and the hydrologic cycle for both Ultisol and Oxisol landscapes: organic C fluxes strongly tied to flowpaths associated with quickflow, and inorganic C (particularly dissolved CO2) transported via deeper flowpaths.  相似文献   

14.
Considerable research has recently been devoted to understanding biogeochemical processes under winter snow cover, leading to enhanced appreciation of the importance of many winter ecological processes. In this study, a comprehensive investigation of the stable carbon isotope composition (δ13C) of CO2 within a high-elevation subalpine forest snowpack was conducted. Our goals were to study the δ13C of biological soil respiration under snow in winter, and to assess the relative importance of diffusion and advection (ventilation by wind) for gas transport within snow. In agreement with other studies, we found evidence of an active microbial community under a roughly 1-m deep snowpack during winter and into spring as it melted. Under-snow CO2 mole fractions were observed up to 3,500 μmol mol−1, and δ13C of CO2 varied from ~−22 to ~−8‰. The δ13C of soil respiration calculated from mixing relationships was −26 to −24‰, and although it varied in time, it was generally close to that of the bulk organic horizon (−26.0‰). Subnivean CO2 and δ13C were quite dynamic in response to changes in soil temperature, liquid water availability, and wind events. No clear biologically-induced isotopic changes were observed during periods when microbial activity and root/rhizosphere activity were expected to vary, although such changes cannot be eliminated. There was clear evidence of isotopic enrichment associated with diffusive transport as predicted by theory, but simple diffusive enrichment (4.4‰) was not observed. Instead, ventilation of the snowpack by sustained wind events in the forest canopy led to changes in the diffusively-enriched gas profile. The isotopic influence of diffusion on gases in the snowpack and litter was greatest at greater depths, due to the decreased relative contribution of advection at depth. There were highly significant correlations between the apparent isotopic content of respiration from the soil with wind speed and pressure. In summary, physical factors influencing gas transport substantially modified and potentially obscured biological factors in their effects on δ13C of CO2 within this subalpine forest snowpack.  相似文献   

15.
Abstract Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests of the Great Lakes Region commonly receive elevated levels of atmospheric nitrate (NO3) deposition, which can alter belowground carbon (C) cycling. Past research has demonstrated that chronic experimental NO3 deposition (3 g N m−2 y−1 above ambient) elicits a threefold increase in the leaching loss of dissolved organic carbon (DOC). Here, we used DOC collected from tension-cup lysimeters to test whether increased DOC export under experimental NO3 deposition originated from forest floor or mineral soil organic matter (SOM). We used DOC radiocarbon dating to quantify C sources and colorimetric assays to measure DOC aromaticity and soluble polyphenolic content. Our results demonstrated that DOC exports are primarily derived from new C (<50-years-old) in the forest floor under both ambient and experimental NO3 deposition. Experimental NO3 deposition increased soluble polyphenolic content from 25.03 ± 4.26 to 49.19 ± 4.23 μg phenolic C mg DOC−1, and increased total aromatic content as measured by specific UV absorbance. However, increased aromatic compounds represented a small fraction (<10%) of the total observed increased DOC leaching. In combination, these findings suggest that experimental NO3 deposition has altered the production or retention as well as phenolic content of DOC formed in forest floor, however exact mechanisms are uncertain. Further elucidation of the mechanism(s) controlling enhanced DOC leaching is important for understanding long-term responses of Great Lakes forests to anthropogenic N deposition and the consequences of those responses for aquatic ecosystems.  相似文献   

16.
Yoshioka  T.  Ueda  S.  Miyajima  T.  Wada  E.  Yoshida  N.  Sugimoto  A.  Vijarnsorn  P.  Boonprakub  S. 《Limnology》2002,3(1):51-59
The distributions of organic matter in the tropical swamps in southern Thailand are reported. The concentrations of particulate and dissolved organic carbon (POC and DOC) in the Bang Nara River, which drains swamp forests and nearby paddy fields, were 2.9 ± 2.0 and 6.2 ± 1.3 mg C l−1, respectively. Although the variation was large, DOC concentration in the Bang Nara River seemed to be higher than POC in November 1992 (DOC/POC ratio, 2.8 ± 2.2). River waters from the upland areas were characterized by low POC and DOC concentrations as compared with Bang Nara River water. The δ13C values of POC and river sediments were useful to distinguish between organic matter originating in upland and swamp areas. It is suggested that the distributions of organic matter and its isotopic composition reflect the difference in drainage characteristics between lowland swamp and upland areas. Isotopic analyses of plant leaves and soils revealed that the swamp forest ecosystems were characterized by low δ13C and low δ15N values, which suggested low efficiency of water use by plants and large contributions of atmospheric deposition of nitrogen, respectively. Although CO2 recycling in the forest might be an important factor determining the δ13C values of understory plants, the main process in carbon metabolism of tropical swamp forests would be CO2 exchange between the atmosphere and forest canopy. Received: May 1, 2001 / Accepted: September 28, 2001  相似文献   

17.
Arid ecosystems experience prolonged dry periods, as well as storms that vary in size, intensity and frequency. As a result, nitrogen (N) retention and export patterns may be a function of individual storm characteristics. Our objective was to determine how seasonal patterns in rainfall as well as individual storm characteristics influence N transport and retention on terrestrial hill slopes in a Sonoran Desert watershed. Regression models indicated that variation in runoff ammonium (NH4+) was best explained by antecedent conditions (cumulative seasonal rainfall, days since last storm) while variation in runoff nitrate (NO3) was best explained by single storm characteristics, primarily rain NO3. Increases in runoff NO3 along overland surface flowpaths were balanced by decreases in NH4+ during summer, with no change in dissolved inorganic nitrogen (DIN) concentration; a pattern consistent with nitrification. Nitrate increases along flowpaths were not as strong during winter storms. Results indicate that NH4+ is transported from hillslopes to other parts of the catchment, including streams, and that nitrification occurs along surface flowpaths, particularly during summer storms. These findings suggest that the extent to which a receiving patch is supplied with NH4+ or NO3 depends on the distance runoff has traveled (flowpath length) and the length of the antecedent dry period. The extent and configuration of fluvial reconnection amongst patches in the landscape following long drought periods likely determines the fate of available N, whether N is processed and retained in the terrestrial or in the aquatic component of the watershed, and the mechanisms involved. The nature of this fluvial reconnection is driven by the size, intensity and sequence of storms in space and time.  相似文献   

18.
Based on data from three German forest ecosystems severely disturbed by windthrow events, correlation patterns between NO3 and SO4 2− dynamics in the soil solution are described. Each of the correlation types was related to site-specific SO4 2− retention processes. The relative importance of SO4 2− adsorption/desorption and precipitation/dissolution was found to be different for the studied soils depending on their contents of SO4 2− -adsorbing sesquioxides and of A1 hydroxy sulfate minerals. Enhanced NO3 concentrations in the soil solution resulting from excess nitrification promoted either SO4 2− adsorption or the dissolution of Al hydroxy sulfates. Both processes result in different ecological consequences: Whereas the former reaction reduces anion concentration peaks in the soil solution, the latter increases them. Thus, a prediction of cation export from soil ecosystems subjected to excess nitrification has to regard site-specific interrelationships between NO3 and SO4 2−. As a third type of conelation the independence of NO3 and SO4 2− concentrations in the soil solution is presented. This type is suggested to be typical for soils with low SO4 2−. adsorption capacity and absence of A1 hydroxy sulfates.  相似文献   

19.
The abundance and distribution of dissolved CH4 were determined from 1987–1990 in Lake Fryxell, Antarctica, an amictic, permanently ice-covered lake in which solute movement is controlled by diffusion. CH4 concentrations were < 1 υM in the upper oxic waters, but increased below the oxycline to 936 μM at 18 m. Sediment CH4 was 1100 μmol (1 sed)−1 in the 0–5 cm zone. Upward flux from the sediment was the source of the CH4, NH4 +, and DOC in the water column; CH4 was 27% of the DOC+CH4 carbon at 18 m. Incubations with surficial sediments indicated that H14CO3 reduction was 0.4 μmol (1 sed)−1 day−1 or 4× the rate of acetate fermentation to CH4. There was no measurable CH4 production in the water column. However, depth profiles of CH4, NH4, and DIC normalized to bottom water concentrations demonstrated that a significant CH4 sink was evident in the anoxic, sulfate-containing zone of the water column (10–18 m). The δ13CH4 in this zone decreased from −72 % at 18 m to −76% at 12 m, indicating that the consumption mechanism did not result in an isotopic enrichment of 13CH4. In contrast, δ13CH4 increased to −55 % at 9 m due to aerobic oxidation, though this was a minor aspect of the CH4 cycle. The water column CH4 profile was modeled by coupling diffusive flux with a first order consumption term; the best-fit rate constant for anaerobic CH4 consumption was 0.012 yr−1. On a total carbon basis, CH4 consumption in the anoxic water column exerted a major effect on the flux of carbonaceous material from the underlying sediments and serves to exemplify the importance of CH4 to carbon cycling in Lake Fryxell.  相似文献   

20.
Diurnal variations of needle water isotopic ratios in two pine species   总被引:1,自引:0,他引:1  
Diurnal fluctuations of leaf water isotope ratios (δ18O and δD) were measured for Jeffrey (Pinus jeffreyi Balf.) and lodgepole (Pinus contorta Douglas ex Louden) pine. Two trees per species were sampled every few hours on 15–16 October 2005 and 19–20 June 2006. Diurnal gas exchange was measured during the summer sampling. In fall 2005, leaf water δ18O ranged from 0.7 to 9.0‰, and leaf water δD ranged from −70 to −50‰. In summer 2006, leaf water δ18O ranged from 7.7 to 20.7‰, and leaf water δD ranged from −61 to −24‰. Diurnal variation of leaf water isotope values typically reached a maximum in early afternoon, began decreasing around midnight, and reached a minimum in mid-morning. Both periods showed a high degree of enrichment relative to source water, with leaf water–source water enrichments ranging up to 37.8‰ for δ18O, and up to 95‰ for δD. Leaf water enrichment varied by season with summer enrichment being greater than fall enrichment. A steady-state model (i.e., modified Craig–Gordon modeling) for leaf water isotope compositions did not provide a good fit to measured values of leaf water. In summer, a non-steady state model provided a better fit to the measured data than the steady-state model. Our findings demonstrate substantial leaf water enrichment above source water and diurnal variations in the isotopic composition of leaf water, which has application to understanding short-term variability of atmospheric gases (water vapor, CO2, O2), climate studies based on the isotopic composition of tree rings, and ecosystem water fluxes.  相似文献   

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