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1.
This study investigates the occurrence of greenhouse gases (GHGs) and the role of groundwater as an indirect pathway of GHG emissions into surface waters in a gaining stretch of the Triffoy River agricultural catchment (Belgium). To this end, nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) concentrations, the stable isotopes of nitrate, and major ions were monitored in river and groundwater over 8 months. Results indicated that groundwater was strongly oversaturated in N2O and CO2 with respect to atmospheric equilibrium (50.1 vs. 0.55 μg L?1 for N2O and 14,569 vs. 400 ppm for CO2), but only marginally for CH4 (0.45 vs. 0.056 μg L?1), suggesting that groundwater can be a source of these GHGs to the atmosphere. Nitrification seemed to be the main process for the accumulation of N2O in groundwater. Oxic conditions prevailing in the aquifer were not prone for the accumulation of CH4. In fact, the emissions of CH4 from the river were one to two orders of magnitude higher than the inputs from groundwater, meaning that CH4 emissions from the river were due to CH4 in-situ production in riverbed or riparian zone sediments. For CO2 and N2O, average emissions from groundwater were 1.5?×?105 kg CO2 ha?1 year?1 and 207 kg N2O ha?1 year?1, respectively. Groundwater is probably an important source of N2O and CO2 in gaining streams but when the measures are scaled at catchment scale, these fluxes are probably relatively modest. Nevertheless, their quantification would better constrain nitrogen and carbon budgets in natural systems.  相似文献   

2.
Much uncertainty exists in spatial and temporal variations of nitrous oxide (N2O) emissions from coastal marshes in temperate regions. To investigate the spatial and temporal variations of N2O fluxes and determine the environmental factors influencing N2O fluxes across the coastal marsh dominated by Suaeda salsa in the Yellow River estuary, China, in situ measurements were conducted in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in autumn and winter during 2011–2012. Results showed that mean N2O fluxes and cumulative N2O emission indicated intertidal zone of the examined marshes as N2O sources over all sampling seasons with range of 0.0051 to 0.0152 mg N2O m?2 h?1 and 7.58 to 22.02 mg N2O m?2, respectively. During all times of day and the seasons measured, N2O fluxes from the intertidal zone ranged from ?0.0004 to 0.0644 mg N2O m?2 h?1. The freeze/thaw cycles in sediments during early winter (frequent short-term cycle) and midwinter (long-term cycle) were one of main factors affecting the temporal variations of N2O emission. The spatial variations of N2O fluxes in autumn were mainly dependent on tidal fluctuation and plant composition. The ammonia-nitrogen (NH4 +–N) in sediments of MF significantly affected N2O emissions (p < 0.05), and the high concentrations of Fe in sediments might affect the spatial variation of N2O fluxes. This study highlighted the large spatial variation of N2O fluxes across the coastal marsh (coefficient of variation (CV) = 127.86 %) and the temporal variation of N2O fluxes during 2011–2012 (CV = 137.29 %). Presently, the exogenous C and N loadings of the Yellow River estuary are increasing due to human activities; thus, the potential effects of exogenous C and N loadings on N2O emissions during early winter should be paid more attention as the N2O inventory is assessed precisely.  相似文献   

3.
Freshwater marshes could be a source of greenhouse gases emission because they contain large amounts of soil carbon and nitrogen. These emissions are strongly influenced by exogenous nitrogen. We investigate the effects of exogenous nitrogen on ecosystem respiration (CO2), CH4 and N2O emissions from freshwater marshes in situ in the Sanjiang Plain Northeast of China during the growing seasons of 2004 and 2005, using a field fertilizer experiment and the static opaque chamber/GC techniques. The results show that there were no significant differences in patterns of seasonal variations of CO2 and CH4 among the fertilizer and non-fertilizer treatments, but the seasonal patterns of N2O emission were significantly influenced by the exogenous nitrogen. Seasonal averages of the CO2 flux from non-fertilizer and fertilizer were 987.74 and 1,344.35 mg m −2 h −1, respectively, in 2004, and 898.59 and 2,154.17 mg m −2 h −1, respectively, in 2005. And the CH4 from the control and fertilizer treatments were 6.05 and 13.56 mg m −2 h −1 and 0.72 and 1.88 mg m −2 h −1, respectively, in 2004 and 2005. The difference of N2O flux between the fertilizer and non-fertilizer treatments is also significant either in 2004 and 2005. On the time scale of 20-, 100-, and 500-year periods, the integrated global warming potential (GWP) of CO2 + CH4 + N2O released during the two growing seasons for the treatment of fertilizer was 97, 94 and 89%, respectively, higher than that for the control, which suggested that the nitrogen fertilizer can enhance the GWP of the CH4 and N2O either in long time or short time scale.  相似文献   

4.
Sediment oxygen uptake and net sediment-water fluxes of dissolved inorganic and organic nitrogen and phosphorus were measured at two sites in Fourleague Bay, Louisiana, from August 1981, through May 1982. This estuary is an extension of Atchafalaya Bay which receives high discharge and nutrient loading from the Atchafalaya River. Sediment O2 uptake averaged 49 mg m?2 h?1. On the average, ammonium (NH4 +) was released from the sediments (mean flux =+129 μmol m?2 h?1), and NO3 ? was taken up (mean flux =?19 μmol m?2h?1). However, very different NO3 ? fluxes were observed at the two sites, with sediment uptake at the upper, river-influenced, high NO3 ? site (mean flux =?112 μmol m?2 h?1) and release at the lower, marine-influenced low NO3 ? site (mean flux =+79 μmol m?2 h?1). PO4 3? fluxes were low and often negative (mean flux =?8 μmol m?2 h?1), while dissolved organic phosphorus fluxes were high and positive (mean flux =+124 μmol m?2 h?1). Dissolved organic nitrogen fluxes varied greatly, ranging from a mean of +305 μmol m?2 h?1 at the lower bay, to ?710 μmol m?2 h?1 at the upper bay. Total dissolved nitrogen and phosphorus fluxes indicated the sediments were a nitrogen (mean flux =+543 μmol m?2 h?1) and phosphorus source (mean flux =+30 μmol m?2 h?1) at the lower bay, and a nitrogen sink (mean flux =?553 μmol m?2 h?1) and phosphorus source (mean flux =+17 μmol m?2 h?1) in the upper bay. Mean annual O∶N ration of the positive inorganic sediment fluxes were 27∶1 at the upper bay and 18∶1 at the lower bay. Based on these data we hypothesize that nitrification and denitrification are important sediment processes in the upper bay. We further hypothesize that Atchafalaya River discharge affects sediment-water fluxes through seasonally high nutrient loading which leads to net nutrient uptake by sediments in the upper bay and release in the lower bay, where there is less river influnces.  相似文献   

5.
Fluxes of methane (CH4) and carbon dioxide (CO2) to the atmosphere at 52 sites within a salt marsh were measured by a dark static chamber technique from mid July to mid September. Mean CH4 fluxes ranged from 0.2 mg m?2 d?1 to 11.0 mg m?2 d?1, with an overall average of 1.6 mg m?2 d?1. Flux of CH4 was inversely correlated (r2=0.23, p = 0.001) with salinity of the upper porewater at the site, suggesting the dominant role of SO4 2? in inhibiting methanogenesis in salt-marsh sediments. The combination of salinity and water table position was able to explain only 29% of the variance in CH4 emission. Mean soil flux of CO2 ranged from 0.3 g m?2 d?1 to 3.7 g m?2 d?1, with an overall average of 2.5 g m?2 d?1; it was correlated with aboveground biomass (positive, r2=0.38, p = 0.001) and position of the water table (negative, r2 = 0.55, p = 0.001). The combination of biomass and water table position accounted for 63% of the variance in CO2 flux. There were high variations in gas flux within the six plant communities. The sequences were CH4: upland edge > panne > pool > middle marsh > low marsh > high marsh, and CO2: middle marsh > low marsh > upland edge > high marsh > panne > pool. Compared to other salt-marsh systems, this Bay of Fundy marsh emits small amounts of CH4 and CO2.  相似文献   

6.
In order to examine the fluxes of methane (CH4) from the Indian estuaries, measurements were carried out by collecting samples from 26 estuaries along the Indian coast during high discharge (wet) and low water discharge (dry) periods. The CH4 concentrations in the estuaries located along the west coast of India were significantly higher (113?±?40 nM) compared to the east coast of India (27?±?6 nM) during wet and dry periods (88?±?15 and 63?±?12 nM, respectively). Supersaturation of CH4 was observed in the Indian estuaries during both periods ((0.18 to 22.3?×?103 %). The concentrations of CH4 showed inverse relation with salinity indicating that freshwater is a significant source. Spatial variations in CH4 saturation were associated with the organic matter load suggesting that its decomposition may be another source in the Indian estuaries. Fluxes of CH4 ranged from 0.01 to 298 μmol m?2 day?1 (mean 13.4?±?5 μmol m?2 day?1) which is ~30 times lower compared to European estuaries (414 μmol m?2 day?1). The annual emission from Indian estuaries, including Pulicat and Adyar, amounted to 0.39?×?1010 g CH4?year?1 with the surface area of 0.027?×?106 km2 which is significantly lower than that in European estuaries (2.7?±?6.8?×?1010 g CH4?year?1 with the surface area of 0.03?×?106 km2). This study suggests that Indian estuaries are a weak source for atmospheric CH4 than European estuaries and such low fluxes were attributed to low residence time of water and low decomposition of organic matter within the estuary. The CH4 fluxes from the Indian estuaries are higher than those from Indian mangroves (0.01?×?1010 g CH4?year?1) but lower than those from Indian inland waters (210?×?1010 g CH4?year?1).  相似文献   

7.
Assessing nitrogen dynamics in the estuarine landscape is challenging given the unique effects of individual habitats on nitrogen dynamics. We measured net N2 fluxes, sediment oxygen demand, and fluxes of ammonium and nitrate seasonally from five major estuarine habitats: salt marshes, seagrass beds (SAV), oyster reefs, and intertidal and subtidal flats. Net N2 fluxes ranged from 332?±?116 μmol?N-N2?m?2?h?1 from oyster reef sediments in the summer to ?67?±?4 μmol?N-N2?m?2?h?1 from SAV in the winter. Oyster reef sediments had the highest rate of N2 production of all habitats. Dissimilatory nitrate reduction to ammonium (DNRA) was measured during the summer and winter. DNRA was low during the winter and ranged from 4.5?±?3.0 in subtidal flats to 104?±?34 μmol?15NH 4 + ?m?2?h?1 in oyster reefs during the summer. Annual denitrification, accounting for seasonal differences in inundation and light, ranged from 161.1?±?19.2 mmol?N-N2?m?2?year?1 for marsh sediments to 509.9?±?122.7 mmol?N-N2?m?2?year?1 for SAV sediments. Given the current habitat distribution in our study system, an estimated 28.3?×?106?mol of N are removed per year or 76 % of estimated watershed nitrogen load. These results indicate that changes in the area and distribution of habitats in the estuarine landscape will impact ecosystem function and services.  相似文献   

8.
Measuring fluxes of greenhouse gases (GHGs) is fundamental to estimating their impact on global warming. We examined diurnal variations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) vertical fluxes in a tidal marsh ecosystem. Measurements were recorded on neap and spring tide days in April and September 2010 in the Shanyutan wetland of the Min River estuary, southeast China. Here, we define a positive flux as directing into the atmosphere. CH4 fluxes on the diurnal scale were positive throughout, and CH4 emissions into the atmosphere on neap tide days were higher than on spring tide days. CH4 releases from the marsh ecosystem on neap tide days were higher in the daytime; however, on spring tide days, daily variations of CH4 emissions were more complex. The marsh ecosystem plays a twofold role in both releasing and assimilating CO2 and N2O gases on the diurnal scale. Average CO2 fluxes were positive on the daily scale both on neap and spring days and were greater on the neap tide days than on spring tide days. Diurnal variations of N2O fluxes fluctuated more. Over the diurnal period, soil temperature markedly controlled variations of CH4 emissions compared to other soil factors, such as salinity and redox potential. Tidal water height was a key factor influencing GHGs fluxes at the water–air interface. Compared with N2O, the diurnal course of CO2 and CH4 fluxes in the marsh ecosystem appeared to be directly controlled by marsh plants. These results have implications for sampling and scaling strategies for estimating GHGs fluxes in tidal marsh ecosystems.  相似文献   

9.
We estimated CO2 and CH4 emissions from mangrove-associated waters of the Andaman Islands by sampling hourly over 24 h in two tidal mangrove creeks (Wright Myo; Kalighat) and during transects in contiguous shallow inshore waters, immediately following the northeast monsoons (dry season) and during the peak of the southwest monsoons (wet season) of 2005 and 2006. Tidal height correlated positively with dissolved O2 and negatively with pCO2, CH4, total alkalinity (TAlk) and dissolved inorganic carbon (DIC), and pCO2 and CH4 were always highly supersaturated (330–1,627 % CO2; 339–26,930 % CH4). These data are consistent with a tidal pumping response to hydrostatic pressure change. There were no seasonal trends in dissolved CH4 but pCO2 was around twice as high during the 2005 wet season than at other times, in both the tidal surveys and the inshore transects. Fourfold higher turbidity during the wet season is consistent with elevated net benthic and/or water column heterotrophy via enhanced organic matter inputs from adjacent mangrove forest and/or the flushing of CO2-enriched soil waters, which may explain these CO2 data. TAlk/DIC relationships in the tidally pumped waters were most consistent with a diagenetic origin of CO2 primarily via sulphate reduction, with additional inputs via aerobic respiration. A decrease with salinity for pCO2, CH4, TAlk and DIC during the inshore transects reflected offshore transport of tidally pumped waters. Estimated mean tidal creek emissions were ～23–173 mmol m?2 day?1 CO2 and ～0.11–0.47 mmol m?2 day?1 CH4. The CO2 emissions are typical of mangrove-associated waters globally, while the CH4 emissions fall at the low end of the published range. Scaling to the creek open water area (2,700 km2) gave total annual creek water emissions ～3.6–9.2?×?1010 mol CO2 and 3.7–34?×?107 mol CH4. We estimated emissions from contiguous inshore waters at ～1.5?×?1011 mol CO2?year?1 and 2.6?×?108 mol CH4?year?1, giving total emissions of ～1.9?×?1011 mol CO2?year?1 and ～3.0?×?108 mol CH4?year?1 from a total area of mangrove-influenced water of ～3?×?104 km2. Evaluating such emissions in a range of mangrove environments is important to resolving the greenhouse gas balance of mangrove ecosystems globally. Future such studies should be integral to wider quantitative process studies of the mangrove carbon balance.  相似文献   

10.
Nitrogen oxides (NO x ) are involved in acid rain and ozone formation, as well as destruction. NO x are climate-relevant trace gases in the atmosphere. Atmospheric NO x originate from anthropogenic emissions (mainly combustion processes). NO from natural processes derives from thunderstorms and soil microbial processes. They may play a crucial role in soil?Catmosphere feedback processes. This study aims to investigate NO x -emissions from soils under different land use, geographical and meteorological conditions. NO x -emissions were quantified in both field and laboratory experiments with a closed static chamber. Disturbed soil samples have been used for laboratory experiments. A climate chamber was used to regulate soil temperature of the samples. Field experiments showed that NO-soil emissions strongly depend on soil temperature. NO-emissions from a soil under meadow showed significant daily variations, unlike soil below spruce forest. Peak emission values were 18???g NO?CN?m?2?h?1 above meadow and 1.3???g NO?CN?m?2?h?1 under forest canopy. In addition, NO-emissions of meadow and forest soil were studied in a climate chamber, enhanced by an additional experiment with agricultural soil. These experiments revealed strong exponential correlations of NO-emissions and soil temperature. Maximum values reached above 400???g NO?CN?m?2?h?1 from agricultural soils at soil temperatures above 50°C. This study shows that soil NO-emissions strongly depend on temperature, vegetation type and geographical position. Consequently, NO-emissions may have a positive feedback effect on climate change.  相似文献   

11.
Sediment-water oxygen and nutrient (NH4 +, NO3 ?+NO2 ?, DON, PO4 3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l?1, apparently limiting SOC rates. Fluxes of NH4 + were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l?1), very large releases (>500 μmol N m?2 h?1) were observed. Nitrate + nitrite (NO3 ?+NO2 ?) exchanges were directed into sediments in areas where bottom water NO3 ?+NO2 ? concentrations were high (>18 μM N); sediment efflux of NO3 ?+NO2 ? occurred only in areas where bottom water NO3 ?+NO2 ? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m?2 h?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH4 ?, PO4 3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.  相似文献   

12.
Community Oxygen and Nutrient Fluxes in Seagrass Beds of Florida Bay, USA   总被引：1，自引：0，他引：1  
We used clear, acrylic chambers to measure in situ community oxygen and nutrient fluxes under day and night conditions in seagrass beds at five sites across Florida Bay five times between September 1997 and March 1999. Underlying sediments are biogenic carbonate with porosities of 0.7–0.9 and with low organic content (<1.6%). The seagrass communities always removed oxygen from the water column during the night and produced oxygen during daylight, and sampling date and site significantly affected both night and daytime oxygen fluxes. Net daily average fluxes of oxygen (?4.9 to 49 mmol m?2 day?1) ranged from net autotrophy to heterotrophy across the bay and during the 18-month sampling period. However, the Rabbit Key Basin site, located in the west-central bay and covered with a dense Thalassia testudinum bed, was always autotrophic with net average oxygen production ranging from 4.8 to 49 mmol m?2 day?1. In November 1998, three of the five sites were strongly heterotrophic and oxygen production was least at Rabbit, suggesting the possibility of hypoxic conditions in fall. Average ammonium (NH4) concentrations in the water column varied widely across the bay, ranging from a mean of 6.9 μmol l?1 at Calusa in the eastern bay to a mean of 0.6 μmol l?1 at Rabbit Key for the period of study. However, average NH4 fluxes by site and date (?240 to 110 μmol m?2 h?1) were not correlated with water column concentrations and did not vary in a consistent diel, seasonal, or spatial pattern. Concentrations of dissolved organic nitrogen (DON) in the water column, averaged by site (15–25 μmol l?1), were greater than mean NH4 concentrations, and the range of day and night DON fluxes (?920 to 1,300 μmol m?2 h?1), averaged by site and date, was greater than the range of mean NH4 fluxes. Average DON fluxes did not vary consistently from day to night, seasonally or spatially. Mean silicate fluxes ranged from ?590 to 860 μmol m?2 h?1 across all sites and dates, but mean net daily fluxes were less variable and most of the time contributed small amounts of silicate to the water column. Mean concentrations of filterable reactive phosphorus (FRP) in the water column across the bay were very low (0.021–0.075 μmol l?1); but site average concentrations of dissolved organic phosphorus (DOP) were higher (0.04–0.15 μmol l?1) and showed a gradient of increasing concentration from east to west in the bay. A pronounced gradient in average surficial sediment total phosphorus (1.1–12 μmol g DW?1) along an east-to-west gradient was not reflected in fluxes of phosphorus. FRP fluxes, averaged by site and date, were low (?5.2 to 52 μmol m?2 h?1), highly variable, and did not vary consistently from day to night or across season or location. Mean DOP fluxes varied over a smaller range (?8.7 to 7.4 μmol m?2 h?1), but also showed no consistent spatial or temporal patterns. These small DOP fluxes were in sharp contrast to the predominately organic phosphorus pool in surficial sediments (site means?=?0.66–7.4 μmol g DW?1). Significant correlations of nutrient fluxes with parameters related to seagrass abundance suggest that the seagrass community may play a major role in nutrient recycling. Integrated means of net daily fluxes over the area of Florida Bay, though highly variable, suggest that seagrass communities might be a source of DOP and NH4 to Florida Bay and might remove small amounts of FRP and potentially large amounts of DON from the waters of the bay.  相似文献   

13.
Groundwater may be highly enriched in dissolved carbon species, but its role as a source of carbon to coastal waters is still poorly constrained. Exports of deep and shallow groundwater-derived dissolved carbon species from a small subtropical estuary (Korogoro Creek, Australia, latitude ?31.0478°, longitude 153.0649°) were quantified using a radium isotope mass balance model (233Ra and 224Ra, natural groundwater tracers) under two hydrological conditions. In addition, air-water exchange of carbon dioxide and methane in the estuary was estimated. The highest carbon inputs to the estuary were from deep fresh groundwater in the wet season. Most of the dissolved carbon delivered by groundwater and exported from the estuary to the coastal ocean was in the form of dissolved inorganic carbon (DIC; 687 mmol m?2 estuary day?1; 20 mmol m?2 catchment day?1, respectively), with a large export of alkalinity (23 mmol m?2 catchment day?1). Average water to air flux of CO2 (869 mmol m?2 day?1) and CH4 (26 mmol m?2 day?1) were 5- and 43-fold higher, respectively, than the average global evasion in estuaries due to the large input of CO2- and CH4-enriched groundwater. The groundwater discharge contribution to carbon exports from the estuary for DIC, dissolved organic carbon (DOC), alkalinity, CO2, and CH4 was 22, 41, 3, 75, and 100 %, respectively. The results show that CO2 and CH4 evasion rates from small subtropical estuaries surrounded by wetlands can be extremely high and that groundwater discharge had a major role in carbon export and evasion from the estuary and therefore should be accounted for in coastal carbon budgets.  相似文献   

14.
Rates of CO2 emission from bare salt-marsh sediments in areas of short and tall formSpartina alterniflora were measured monthly for 1 yr. Maximum emission rates, as high as 325 ml CO2m?2h?1, were observed during summer months, while minimum rates, 10.2 ml CO2 m?2h?1, were observed during the winter. An exponential function of inverse soil temperature explained most of the seasonal variability, but other factors are involved in regulating CO2 emissions as demonstrated by rates that were higher in spring than in late summer at equivalent temperatures. Annual CO2 emissions from bare sediments were 27.3 and 18.6 mol C m?2 yr?1 in communities of short and tallS. alterniflora, respectively. It was estimated that losses of dissolved inorganic carbon from the turnover of pore water, up to 14.6 mol C m?2 yr?1 at the creek bank (tall,S. alterniflora) site, and diffusion of CO2 from the root system ofS. alterniflora through the culms, 12.3 to 16.2 mol C m?2 yr?1, could also be important pathways of carbon loss from marsh sediments. If the internal flux of CO2 from the root system through the culm is refixed within the leaves, then the observed rate of 9.8 μI CO2 min?1 cm?2 of culm cross sectional area appears to make a small but significant contribution to total photosynthesis.  相似文献   

15.
In situ carbon flux measurements and calculated burial rates are utilized to construct an organic carbon budget for the upper meter of sediment at a single station in Cape Lookout Bight, a small marine basin located on the Outer Banks of North Carolina, U.S.A. (34°37′N, 76°33′W). Of 149 ± 20 mole · m?2 · yr?1 of total organic carbon deposited, 35.6 ± 5.2 mole · m?2 · yr?1 is recycled to overlying waters, 84 ± 18% as ∑CO2 and 16 ± 8% as CH4. Approximately 68 ± 20% of the upward carbon flux is supported by sulfate reduction while 32 ± 16% takes place as the result of underlying methanogenesis. Measured ∑CO2 and CH4 sediment-water fluxes range seasonally from 1900–6300 and 50–2500 μmole · m?2 · hr?1 respectively.The mean residence time of metabolizable organic carbon in the upper 80 cm of sediment is approximately four months with greater than 98% of the calculated total remineralization taking place within three years. In spite of large upward fluxes of methane, larger molecules derived from metabolizable sedimentary organic carbon appear to be the dominant reductants for dissolved sulfate.  相似文献   

16.
The present study evaluated the effect of plant species on methane (CH4) emission and microbial populations in three types of soil–plant systems. Results showed large variation of CH4 flux rate ranging from 1.35 to 212.61 mg CH4 m?2 h?1. Emission peak of CH4 occurred in July. No significant difference was found in the non-vegetation system spanning 2 years. Compared with non-vegetation, vegetation systems had much higher flux of CH4, and obvious seasonal variation was observed. The polyculture system planted with Zizania latifolia (Z. latifolia) and Phragmites australis (P. australis) released higher CH4 fluxes than the mono system (P. australis), reflecting that Z. latifolia growth could simulate CH4 emission. The fluorescence in situ hybridization (FISH) results support the characteristics of CH4 fluxes. Much higher methanotrophs amount and lower methanogens amount from the mono system than those from the polyculture system was observed indicating that Z. latifolia growth may limit the oxygen transportation resulting in higher CH4 emission. The polyculture system has the highest potential of CH4 emission.  相似文献   

17.
The application of chemical and isotopic geothermometry to geothermal systems is reviewed, pointing out the uses and limitations of specific reactions in estimating deep temperatures from well, hot-spring and fumarole discharges.At present the most reliable indicators are: the silica-water equilibria; the Na/K ratio; the isotopic distributions Δ2H(H2“H2O), Δ2H(H2“CH4), Δ18O(H2O“HSO?4); and the gas reactions CO2 + 4H2 ? CH4 + 2H2O, and 2NH3 ? N2 + 3H2. Many other qualitative chemical indicators exist.  相似文献   

18.
Methane and CO2 emissions from the two most active mud volcanoes in central Japan, Murono and Kamou (Tokamachi City, Niigata Basin), were measured in from both craters or vents (macro-seepage) and invisible exhalation from the soil (mini- and microseepage). Molecular and isotopic compositions of the released gases were also determined. Gas is thermogenic (δ13CCH4 from −32.9‰ to −36.2‰), likely associated with oil, and enrichments of 13C in CO2 (δ13CCO2 up to +28.3‰) and propane (δ13CC3H8 up to −8.6‰) suggest subsurface petroleum biodegradation. Gas source and post-genetic alteration processes did not change from 2004 to 2010. Methane flux ranged within the orders of magnitude of 101-104 g m−2 d−1 in macro-seeps, and up to 446 g m−2 d−1 from diffuse seepage. Positive CH4 fluxes from dry soil were widespread throughout the investigated areas. Total CH4 emission from Murono and Kamou were estimated to be at least 20 and 3.7 ton a−1, respectively, of which more than half was from invisible seepage surrounding the mud volcano vents. At the macro-seeps, CO2 fluxes were directly proportional to CH4 fluxes, and the volumetric ratios between CH4 flux and CO2 flux were similar to the compositional CH4/CO2 volume ratio. Macro-seep flux data, in addition to those of other 13 mud volcanoes, supported the hypothesis that molecular fractionation (increase of the “Bernard ratio” C1/(C2 + C3)) is inversely proportional to gas migration fluxes. The CH4 “emission factor” (total measured output divided by investigated seepage area) was similar to that derived in other mud volcanoes of the same size and activity. The updated global “emission-factor” data-set, now including 27 mud volcanoes from different countries, suggests that previous estimates of global CH4 emission from mud volcanoes may be significantly underestimated.  相似文献   

19.
The metabolic rate of individual habitats can differ significantly in their contribution to the total system productivity of estuaries. Changing environmental conditions such as those created by tidal exchange can frequently alter these rates. In an effort to quantify these rate responses, metabolic rates were measured for macroalgal and sediment habitats at different salinities. Microcosms representing the two habitats were incubated at three salinity ranges (high: 25 to 31‰; moderate: 12 to 18‰; and low: 0 to 4‰) and production and respiration rates were estimated. The production rates for both habitats were proportional to the salinity of the water in the incubation, with the lowest metabolic rates associated with the lowest salinity. Average macroalgal habitat net production rates were 879 mg O2 m?2 h?1, 609 mg O2 m?2 h?1, and 451 mg O2 m?2 h?1 at high, moderate, and low salinity treatments, respectively, and the dark respiration rates were ?401 mg O2 m?2 h?1, ?341 mg O2 m?2 h?1, and ?333 mg O2 m?2 h?1. Average sediment habitat net production rates were 60 mg O2 m?2 h?1, 13 mg O2 m?2 h?1 and 10 mg O2 m?2 h?1 and the respiration rates were ?114 mg O2 m?2 h?1, ?55 mg O2 m?2 h?1, and ?31 mg O2 m?2 h?1 at high, moderate, and low salinity treatments. The larger contribution of macroalgal habitats to system metabolism may account for observed diurnal changes in water column oxygen levels in some estuaries. Macroalgal production rates explained 83% of the increase in water column oxygen levels during daylight hours and macroalgal respiration rates explained 65% of the decline in oxygen levels during the night. The contribution of macroalgal metabolism to the system can be influenced by even short-term changes in water column salinity. Environmental processes that alter salinity levels on hourly time scales may moderate the effect of macroalgal metabolism on oxygen levels.  相似文献   

20.
Cycling of methane (CH4) in Tomales Bay, a 28-km2 temperature estuary in northern California with relatively low inputs of organic carbon, was studied over a 1-yr period. Water column CH4 concentrations showed spatial and temporal variability (range=8–100 nM), and were supersaturated with respect to the atmosphere by a factor of 2–37. Rates of net water column CH4 production-oxidation were determined by in situ experiments, and were not found to be significantly different from zero. Fluxes across the sediment-water interface, determined by direct measurement using benthic chambers, varied from ?0.1 μmol m?2 d?1 to +16 μmol m?2 d?1 (positive fluxes into water). Methane concentrations in the two perennial creeks feeding the bay varied annually (140–950 nM); these creeks were a significant CH4 source to the bay during winter. In addition, mass-balance calculations indicate a significant additional inter CH4 source, which is hypothesized to result from storm-related runoff from dairy farms adjacent to the bay. Systemwide CH4 budgets of the 16-km2 inner bay indicate benthic production (110 mol d?1) and atmospheric evasion (110 mol d?1) dominated during summer, while atmospheric evasion (160 mol d?1) and runoff from dairy farms (90 mol d?1) dominated during winter.  相似文献   

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