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1.
Temperate grasslands are considered to be a significant sink for CH 3Br, although large uncertainties exist about the magnitude of this sink because of a paucity of field measurements. Here, we report the results of a combined field and laboratory study that investigated the effects of water, temperature, and plant community composition on CH 3Cl and CH 3Br fluxes in a semiarid temperate grassland. A novel stable isotope tracer technique was also employed to deconvolute simultaneous production and oxidation of CH 3Cl and CH 3Br. Net and gross fluxes were measured from different landforms (ridges, floodplains) and cover types (grass‐dominated, shrub‐dominated) to capture a representative range of hydrologic regimes, temperatures, and plant communities. In field experiments, net CH 3Cl and CH 3Br uptake was observed at all grass‐dominated sites (?400±77 nmol CH 3Cl m ?2 day ?1 and ?3.4±0.9 nmol CH 3Br m ?2 day ?1), while net CH 3Cl emission (439±58 nmol CH 3Cl m ?2 day ?1) was observed at sites dominated by the shrub Atriplex canescens, indicating that this plant is a strong CH 3Cl producer. Gross CH 3Cl and CH 3Br oxidation were comparable with estimates from other dryland ecosystems (507±115 nmol CH 3Cl m ?2 day ?1 and 9.1±2.2 nmol CH 3Br m ?2 day ?1), although CH 3Br oxidation rates were at least five times lower than those observed in more mesic temperate grasslands. We suggest that estimates of the temperate grassland CH 3Br sink should be reduced by ≥19% (≥1.8 Gg yr ?1) to account for the weaker sink strength of semiarid environments. Identification of A. canescens as a ‘new’ CH 3Cl source may have important ramifications for the global atmospheric budget of CH 3Cl, given the global distribution of this plant and its congeners and their widespread presence in many dryland ecosystems. Laboratory experiments revealed that soil water was the chief regulator of CH 3Cl and CH 3Br oxidation, while temperature had no observed effect between 14 and 26 °C. Oxidation rates rose most rapidly between 0.4% and 5% volumetric water content, suggesting that methyl halide‐oxidizing bacteria respond strongly to small inputs of water under the very driest conditions. Soil drying and rewetting experiments did not appear to affect the oxidation of CH 3Cl and CH 3Br by soil microorganisms, which are presumably adapted to frequent wet/dry cycles. 相似文献
2.
Methyl bromide (CH 3Br) and methyl chloride(CH 3Cl) emission rates from southernCalifornia coastal salt marshes show largespatial and temporal variabilities that arestrongly linked to biological and environmentalfactors. Here we discuss biogeochemical linesof evidence pointing to vegetation as theprimary source of CH 3Br and CH 3Clemissions from salt marshes. Sediments andmacroalgae do not appear to be major producersof these compounds, based on observations thatthe highest fluxes are not inhibited by soilinundation; their emissions are not correlatedwith those of certain gases produced in soils;and emissions from mudflat- andmacroalgae-dominated sites are relativelysmall. In contrast, the seasonal and spatialvariabilities of methyl halide fluxes in thesesalt marshes are consistent with the productionof these compounds by vascular plants, althoughthe possibility of production by microflora orfungi associated with the salt marsh vegetationis not ruled out. Flux chamber measurements ofemission rates are largely correlated to theoverall plant biomass enclosed in the chamber,but appear also to be highly dependent on thepredominant plant species. Emission ratesfollow a diurnal trend similar to the trends ofambient air temperature and photosyntheticallyactive radiation, but not surface soiltemperature. Diurnal variabilities in thecarbon isotope compositions of CH 3Cl andCH 3Br and their relative ratios ofemissions are consistent with simultaneouslycompeting mechanisms of uptake andproduction. 相似文献
3.
Soils provide the largest terrestrial carbon store, the largest atmospheric CO 2 source, the largest terrestrial N 2O source and the largest terrestrial CH 4 sink, as mediated through root and soil microbial processes. A change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease. We measured soil–atmosphere exchange of CO 2, N 2O and CH 4 in four adjacent land‐use systems (native eucalypt woodland, clover‐grass pasture, Pinus radiata and Eucalyptus globulus plantation) for short, but continuous, periods between October 2005 and June 2006 using an automated trace gas measurement system near Albany in southwest Western Australia. Mean N 2O emission in the pasture was 26.6 μg N m −2 h −1, significantly greater than in the natural and managed forests (< 2.0 μg N m −2 h −1). N 2O emission from pasture soil increased after rainfall events (up to 100 μg N m −2 h −1) and as soil water content increased into winter, whereas no soil water response was detected in the forest systems. Gross nitrification through 15N isotope dilution in all land‐use systems was small at water holding capacity < 30%, and under optimum soil water conditions gross nitrification ranged between < 0.1 and 1.0 mg N kg −1 h −1, being least in the native woodland/eucalypt plantation < pine plantation < pasture. Forest soils were a constant CH 4 sink, up to −20 μg C m −2 h −1 in the native woodland. Pasture soil was an occasional CH 4 source, but weak CH 4 sink overall (−3 μg C m −2 h −1). There were no strong correlations ( R < 0.4) between CH 4 flux and soil moisture or temperature. Soil CO 2 emissions (35–55 mg C m −2 h −1) correlated with soil water content ( R < 0.5) in all but the E. globulus plantation. Soil N 2O emissions from improved pastures can be considerable and comparable with intensively managed, irrigated and fertilised dairy pastures. In all land uses, soil N 2O emissions exceeded soil CH 4 uptake on a carbon dioxide equivalent basis. Overall, afforestation of improved pastures (i) decreases soil N 2O emissions and (ii) increases soil CH 4 uptake. 相似文献
4.
Based on theories of mire development and responses to a changing climate, the current role of mires as a net carbon sink has been questioned. A rigorous evaluation of the current net C-exchange in mires requires measurements of all relevant fluxes. Estimates of annual total carbon budgets in mires are still very limited. Here, we present a full carbon budget over 2 years for a boreal minerogenic oligotrophic mire in northern Sweden (64°11′N, 19°33′E). Data on the following fluxes were collected: land–atmosphere CO 2 exchange (continuous Eddy covariance measurements) and CH 4 exchange (static chambers during the snow free period); TOC (total organic carbon) in precipitation; loss of TOC, dissolved inorganic carbon (DIC) and CH 4 through stream water runoff (continuous discharge measurements and regular C-concentration measurements). The mire constituted a net sink of 27±3.4 (±SD) g C m −2 yr −1 during 2004 and 20±3.4 g C m −2 yr −1 during 2005. This could be partitioned into an annual surface–atmosphere CO 2 net uptake of 55±1.9 g C m −2 yr −1 during 2004 and 48±1.6 g C m −2 yr −1 during 2005. The annual NEE was further separated into a net uptake season, with an uptake of 92 g C m −2 yr −1 during 2004 and 86 g C m −2 yr −1 during 2005, and a net loss season with a loss of 37 g C m −2 yr −1 during 2004 and 38 g C m −2 yr −1 during 2005. Of the annual net CO 2-C uptake, 37% and 31% was lost through runoff (with runoff TOC>DIC≫CH 4) and 16% and 29% through methane emission during 2004 and 2005, respectively. This mire is still a significant C-sink, with carbon accumulation rates comparable to the long-term Holocene C-accumulation, and higher than the C-accumulation during the late Holocene in the region. 相似文献
5.
This paper presents results of 1 year (from March 25, 2003 to March 24, 2004, 366 days) of continuous measurements of net ecosystem CO 2 exchange (NEE) above a steppe in Mongolia using the eddy covariance technique. The steppe, typical of central Mongolia, is dominated by C 3 plants adapted to the continental climate. The following two questions are addressed: (1) how do NEE and its components: gross ecosystem production (GEP) and total ecosystem respiration ( Reco) vary seasonally? (2) how do NEE, GEP, and Reco respond to biotic and abiotic factors? The hourly minimal NEE and the hourly maximal Reco were −3.6 and 1.2 μmol m −2 s −1, respectively (negative values denoting net carbon uptake by the canopy from the atmosphere). Peak daily sums of NEE, GEP, and Reco were −2.3, 3.5, and 1.5 g C m −2 day −1, respectively. The annual sums of GEP, Reco, and NEE were 179, 138, and −41 g C m −2, respectively. The carbon removal by sheep was estimated to range between 10 and 82 g C m −2 yr −1 using four different approaches. Including these estimates in the overall carbon budget yielded net ecosystem productivity of −23 to +20 g C m −2 yr −1. Thus, within the remaining experimental uncertainty the carbon budget at this steppe site can be considered to be balanced. For the growing period (from April 23 to October 21, 2003), 26% and 53% of the variation in daily NEE and GEP, respectively, could be explained by the changes in leaf area index. Seasonality of GEP, Reco, and NEE was closely associated with precipitation, especially in the peak growing season when GEP and Reco were largest. Water stress was observed in late July to early August, which switched the steppe from a carbon sink to a carbon source. For the entire growing period, the light response curves of daytime NEE showed a rather low apparent quantum yield ( α=−0.0047 μmol CO 2 μmol −1 photons of photosynthetically active radiation). However, the α values varied with air temperature ( Ta), vapor pressure deficit, and soil water content. 相似文献
6.
Arctic ecosystems are characterized by a wide range of soil moisture conditions and thermal regimes and contribute differently to the net methane (CH 4) budget. Yet, it is unclear how climate change will affect the capacity of those systems to act as a net source or sink of CH 4. Here, we present results of in situ CH 4 flux measurements made during the growing season 2014 on Disko Island (west Greenland) and quantify the contribution of contrasting soil and landscape types to the net CH 4 budget and responses to summer warming. We compared gas flux measurements from a bare soil and a dry heath, at ambient conditions and increased air temperature, using open‐top chambers (OTCs). Throughout the growing season, bare soil consumed 0.22 ± 0.03 g CH 4‐C m ?2 (8.1 ± 1.2 g CO 2‐eq m ?2) at ambient conditions, while the dry heath consumed 0.10 ± 0.02 g CH 4‐C m ?2 (3.9 ± 0.6 g CO 2‐eq m ?2). These uptake rates were subsequently scaled to the entire study area of 0.15 km 2, a landscape also consisting of wetlands with a seasonally integrated methane release of 0.10 ± 0.01 g CH 4‐C m ?2 (3.7 ± 1.2 g CO 2‐eq m ?2). The result was a net landscape sink of 12.71 kg CH 4‐C (0.48 tonne CO 2‐eq) during the growing season. A nonsignificant trend was noticed in seasonal CH 4 uptake rates with experimental warming, corresponding to a 2% reduction at the bare soil, and 33% increase at the dry heath. This was due to the indirect effect of OTCs on soil moisture, which exerted the main control on CH 4 fluxes. Overall, the net landscape sink of CH 4 tended to increase by 20% with OTCs. Bare and dry tundra ecosystems should be considered in the net CH 4 budget of the Arctic due to their potential role in counterbalancing CH 4 emissions from wetlands – not the least when taking the future climatic scenarios of the Arctic into account. 相似文献
7.
Northern lakes are a source of greenhouse gases to the atmosphere and contribute substantially to the global carbon budget. However, the sources of methane (CH4) to northern lakes are poorly constrained limiting our ability to the assess impacts of future Arctic change. Here we present measurements of the natural groundwater tracer, radon, and CH4 in a shallow lake on the Yukon-Kuskokwim Delta, AK and quantify groundwater discharge rates and fluxes of groundwater-derived CH4. We found that groundwater was significantly enriched (2000%) in radon and CH4 relative to lake water. Using a mass balance approach, we calculated average groundwater fluxes of 1.2 ± 0.6 and 4.3 ± 2.0 cm day−1, respectively as conservative and upper limit estimates. Groundwater CH4 fluxes were 7—24 mmol m−2 day−1 and significantly exceeded diffusive air–water CH4 fluxes (1.3–2.3 mmol m−2 day−1) from the lake to the atmosphere, suggesting that groundwater is an important source of CH4 to Arctic lakes and may drive observed CH4 emissions. Isotopic signatures of CH4 were depleted in groundwaters, consistent with microbial production. Higher methane concentrations in groundwater compared to other high latitude lakes were likely the source of the comparatively higher CH4 diffusive fluxes, as compared to those reported previously in high latitude lakes. These findings indicate that deltaic lakes across warmer permafrost regions may act as important hotspots for CH4 release across Arctic landscapes. 相似文献
8.
Background Biogenic emissions of methyl halides (CH 3Cl, CH 3Br and CH 3I) are the major source of these compounds in the atmosphere; however, there are few reports about the halide profiles and
strengths of these emissions. Halide ion methyltransferase (HMT) and halide/thiol methyltransferase (HTMT) enzymes concerning
these emissions have been purified and characterized from several organisms including marine algae, fungi, and higher plants;
however, the correlation between emission profiles of methyl halides and the enzymatic properties of HMT/HTMT, and their role
in vivo remains unclear. 相似文献
9.
Throughout the Holocene, northern peatlands have both accumulated carbon and emitted methane. Their impact on climate radiative forcing has been the net of cooling (persistent CO 2 uptake) and warming (persistent CH 4 emission). We evaluated this by developing very simple Holocene peatland carbon flux trajectories, and using these as inputs to a simple atmospheric perturbation model. Flux trajectories are based on estimates of contemporary CH 4 flux (15–50 Tg CH 4 yr −1), total accumulated peat C (250–450 Pg C), and peatland initiation dates. The contemporary perturbations to the atmosphere due to northern peatlands are an increase of ∼100 ppbv CH 4 and a decrease of ∼35 ppmv CO 2. The net radiative forcing impact northern peatlands is currently about −0.2 to −0.5 W m −2 (a cooling). It is likely that peatlands initially caused a net warming of up to +0.1 W m −2, but have been causing an increasing net cooling for the past 8000–11 000 years. A series of sensitivity simulations indicate that the current radiative forcing impact is determined primarily by the magnitude of the contemporary methane flux and the magnitude of the total C accumulated as peat, and that radiative forcing dynamics during the Holocene depended on flux trajectory, but the overall pattern was similar in all cases. 相似文献
10.
The environmental importance of methyl bromide (CH 3Br) arises from its contribution to stratospheric ozone loss processes and, as a consequence, its emissions from anthropogenic sources are subject to the Montreal Protocol. A better understanding of the natural budget of CH 3Br is required for assessing the benefit of anthropogenic emission reductions and for understanding any potential effects of environmental change on global CH 3Br concentrations. Measurements of CH 3Br flux in temperate woodland ecosystems, in particular, are very sparse, yet these cover a large fraction of terrestrial land surface. Results presented here from 18 months of field measurements of CH 3Br fluxes in four static flux chambers in a woodland in Scotland and from enclosures of rotting wood and deciduous and coniferous leaf litter suggest net emissions from temperate woodlands. Net CH 3Br fluxes in the woodland varied between the chambers, fluctuating between net uptake and net emissions (?73 to 279 ng m ?2 h ?1 across 161 individual measurements), and with no strong seasonality, but with time‐averaged net emission overall [27±57 (1 SD)] ng m ?2 h ?1]. This work demonstrates that scale‐up needs to be based on sufficient individual measurements to provide a reasonably constrained estimate of the long‐term mean. Mean (±1 SD) net CH 3Br emissions from deciduous and coniferous leaf litter were 43 (±33) ng kg ?1 (dry weight) h ?1 and 80 (±37) ng kg ?1 (dry weight) h ?1, respectively, and ~1–2 ng kg ?1 (fresh weight) h ?1 from rotting woody litter. Despite the intrinsic variability, data obtained here consistently point to the conclusion that the temperate forest soil/litter ecosystem is a net source of CH 3Br to the atmosphere. 相似文献
11.
Central European grasslands are characterized by a wide range of different management practices in close geographical proximity. Site‐specific management strategies strongly affect the biosphere–atmosphere exchange of the three greenhouse gases (GHG) carbon dioxide (CO 2), nitrous oxide (N 2O), and methane (CH 4). The evaluation of environmental impacts at site level is challenging, because most in situ measurements focus on the quantification of CO 2 exchange, while long‐term N 2O and CH 4 flux measurements at ecosystem scale remain scarce. Here, we synthesized ecosystem CO 2, N 2O, and CH 4 fluxes from 14 managed grassland sites, quantified by eddy covariance or chamber techniques. We found that grasslands were on average a CO 2 sink (−1,783 to −91 g CO 2 m −2 year −1), but a N 2O source (18–638 g CO 2‐eq. m −2 year −1), and either a CH 4 sink or source (−9 to 488 g CO 2‐eq. m −2 year −1). The net GHG balance (NGB) of nine sites where measurements of all three GHGs were available was found between −2,761 and −58 g CO 2‐eq. m −2 year −1, with N 2O and CH 4 emissions offsetting concurrent CO 2 uptake by on average 21 ± 6% across sites. The only positive NGB was found for one site during a restoration year with ploughing. The predictive power of soil parameters for N 2O and CH 4 fluxes was generally low and varied considerably within years. However, after site‐specific data normalization, we identified environmental conditions that indicated enhanced GHG source/sink activity (“sweet spots”) and gave a good prediction of normalized overall fluxes across sites. The application of animal slurry to grasslands increased N 2O and CH 4 emissions. The N 2O‐N emission factor across sites was 1.8 ± 0.5%, but varied considerably at site level among the years (0.1%–8.6%). Although grassland management led to increased N 2O and CH 4 emissions, the CO 2 sink strength was generally the most dominant component of the annual GHG budget. 相似文献
12.
Arctic-boreal landscapes are experiencing profound warming, along with changes in ecosystem moisture status and disturbance from fire. This region is of global importance in terms of carbon feedbacks to climate, yet the sign (sink or source) and magnitude of the Arctic-boreal carbon budget within recent years remains highly uncertain. Here, we provide new estimates of recent (2003–2015) vegetation gross primary productivity (GPP), ecosystem respiration ( Reco), net ecosystem CO 2 exchange (NEE; Reco − GPP), and terrestrial methane (CH 4) emissions for the Arctic-boreal zone using a satellite data-driven process-model for northern ecosystems (TCFM-Arctic), calibrated and evaluated using measurements from >60 tower eddy covariance (EC) sites. We used TCFM-Arctic to obtain daily 1-km 2 flux estimates and annual carbon budgets for the pan-Arctic-boreal region. Across the domain, the model indicated an overall average NEE sink of −850 Tg CO 2-C year −1. Eurasian boreal zones, especially those in Siberia, contributed to a majority of the net sink. In contrast, the tundra biome was relatively carbon neutral (ranging from small sink to source). Regional CH 4 emissions from tundra and boreal wetlands (not accounting for aquatic CH 4) were estimated at 35 Tg CH 4-C year −1. Accounting for additional emissions from open water aquatic bodies and from fire, using available estimates from the literature, reduced the total regional NEE sink by 21% and shifted many far northern tundra landscapes, and some boreal forests, to a net carbon source. This assessment, based on in situ observations and models, improves our understanding of the high-latitude carbon status and also indicates a continued need for integrated site-to-regional assessments to monitor the vulnerability of these ecosystems to climate change. 相似文献
13.
Reactions of lithium halide (LiX, X = F, Cl, Br and I) and methyl halide (CH 3X, X = F, Cl, Br and I) have been investigated at the B3LYP/6-31G(d) level of theory using the microhydration model. Beginning
with hydrated lithium ion, four or two water molecules have been conveniently introduced to these aqueous-phase halogen-exchange
S N2 reactions. These water molecules coordinated with the center metal lithium ion, and also interacted with entering and leaving
halogen anion via hydrogen bond in complexes and transition state, which to some extent compensated hydration of halogen anion. At 298 K the
reaction profiles all involve central barriers Δ E
cent
which are found to decrease in the order F > Cl > Br > I. The same trend is also found for the overall barriers (Δ E
ovr
) of the title reaction. In the S N2 reaction of sodium iodide and methyl iodide, the activation energy agrees well with the aqueous conductometric investigation. 相似文献
14.
Carbon dioxide, energy flux measurements and methane chamber measurements were carried out in an arctic wet tussock grassland located on a flood plane of the Kolyma river in NE Siberia over a summer period of 155 days in 2002 and early 2003. Respiration was also measured in April 2004. The study region is characterized by late thaw of the top soil (mid of June) and periodic spring floods. A stagnant water table below the grass canopy is fed by thawing of the active layer of permafrost and by flood water. The climate is continental with average daily temperature in the warmest months of 13°C (maximum temperature at midday: 28°C by the end of July), dry air (maximum vapour pressure deficit at midday: 28 hPa) and low rainfall of 50 mm during summer (July–September). Summer evaporation (July–September: 103 mm) exceeded rainfall by a factor of 2. The daily average Bowen ratio ( H/LE) was 0.62 during the growing season. Net ecosystem CO 2 uptake reached 10 μmol m −2 s −1 and was related to photon flux density (PFD) and vapour pressure deficit (VPD). The cumulative annual net carbon flux from the atmosphere to the terrestrial surface was estimated to be about −38 g C m −2 yr −1 (negative flux depicts net carbon sink). Winter respiration was extrapolated using the Lloyd and Taylor function. The net carbon balance is composed of a high rate of assimilation in a short summer and a fairly large but uncertain respiration mainly during autumn and spring. Methane flux (about 12 g C m −2 measured over 60 days) was 25% of C uptake during the same period of time (end of July to end of September). Assuming that CH 4 was emitted only in summer, and taking the greenhouse gas warming potential of CH 4 vs. CO 2 into account (factor 23), the study site was a greenhouse gas source (at least 200 g C equivalent m −2 yr −1). Comparing different studies in wetlands and tundra ecosystems as related to latitude, we expect that global warming would rather increase than decrease the CO 2-C sink. 相似文献
15.
Greenhouse gas fluxes from vegetated drained lake basins have been largely unstudied, although these land features constitute up to 47% of the land cover in the Arctic Coastal Plain in northern Alaska. To describe current and to better predict future sink/source activity of the Arctic tundra, it is important to assess these vegetated drained lake basins with respect to the patterns of and controls on gross primary production (GPP), net ecosystem exchange, and ecosystem respiration (ER). We measured CO 2 fluxes and key environmental variables during the 2007 growing season (June through August) in 12 vegetated drained lake basins representing three age classes (young, drained about 50 years ago; medium, drained between 50 and 300 years ago; and old, drained between 300 and 2000 years ago, as determined by Hinkel et al., 2003) in the Arctic Coastal Plain. Young vegetated drained lake basins had both the highest average GPP over the summer (11.4 gCO 2 m ?2 day ?1) and the highest average summer ER (7.3 gCO 2 m ?2 day ?1), while medium and old vegetated drained lake basins showed lower and similar GPP (7.9 and 7.2 gCO 2 m ?2 day ?1, respectively), and ER (5.2 and 4 gCO 2 m ?2 day ?1, respectively). Productivity decreases with age as nutrients are locked up in living plant material and dead organic matter. However, we showed that old vegetated drained lakes basins maintained relatively high productivity because of the increased development of ice‐wedge polygons, the formation of ponds, and the re‐establishment of very productive species. Comparison of the seasonal CO 2 fluxes and concomitant environmental factors over this chronosequence provides the basis for better understanding the patterns and controls on CO 2 flux across the coastal plain of the North Slope of Alaska and for more accurately estimating current and future contribution of the Arctic to the global carbon budget. 相似文献
16.
The research performed in August 2004 within the framework of the Russian-American Long-term Census of the Arctic (RUSALCA) resulted in the first data concerning the rates of the key microbial processes in the water column and bottom sediments of the Bering strait and the Chukchi Sea. The total bacterial counts in the water column varied from 30 × 10 3 cells ml ?1 in the northern and eastern parts to 245 × 10 3 cells ml ?1 in the southern part. The methane content in the water column of the Chukchi sea varied from 8 nmol CH 4l ?1 in the eastern part of the sea to 31 nmol CH 4l ?1 in the northern part of the Herald Canyon. Microbial activity occurred in the upper 0–3 cm of the bottom sediments; the methane formation rate varied from 0.25 to 16 nmol CH 4dm ?3 day ?1. The rates of methane oxidation varied from 1.61 to 14.7 nmol CH 4dm ?3 day ?1. The rates of sulfate reduction varied from 1.35 to 16.2 μmol SO 4 2? dm ?1 day ?1. The rate of methane formation in the sediments increased with depth, while sulfate reduction rates decreased (less than 1 μmol SO 4 2? dm ?3 day ?1). These high concentrations of biogenic elements and high rates of microbial processes in the upper sediment layers suggest a specific type of trophic chain in the Chukchi Sea. The approximate calculated balance of methane emission from the water column into the atmosphere is from 5.4 to 57.3 μmol CH 4m ?2 day ?1. 相似文献
17.
qazTin-119 and phosphorus-31 NMR spectra have been recorded for a series of adducts of RSnX 3 (R Me, Ph; X Cl, Br) with halide, tributylphosphine (P) and tributylphosphine oxide (L). The adducts were either 1:1 five coordinate or 1:2 six coordinate complexes. The tin-ll9 NMR spectra of mixtures of corresponding chloro and bromo complexes reveal, in most cases, all possible mixed halide species but much additional structural information is obtained from these spectra which could not be extracted from the spectra of individual compounds themselves. Thus in some cases, in the five coordinate species the Berry pseudorotation between isomers within a particular stoichiometry could be slowed on the NMR timescale which allowed a determination of the molecular structure. An equimolar mixture of [PhSnCl 5] 2− and [PhSnBr 5] 2− shows eleven of the twelve geometries possible for [PhSnCl xBr 5−x] 2−. In the six coordinate series [RSnX 4P] − the tin-119 NMR spectra of the mixtures of [RSnCl 4P] − and [RSnBr 4P] − allow the geometry to be determined as trans. Application of the pairwise additivity model for calculation of the tin-119 chemical shift positions for the mixed halide systems are discussed. 相似文献
18.
Northern peatlands contain up to 25% of the world's soil carbon (C) and have an estimated annual exchange of CO 2‐C with the atmosphere of 0.1–0.5 Pg yr −1 and of CH 4‐C of 10–25 Tg yr −1. Despite this overall importance to the global C cycle, there have been few, if any, complete multiyear annual C balances for these ecosystems. We report a 6‐year balance computed from continuous net ecosystem CO 2 exchange (NEE), regular instantaneous measurements of methane (CH 4) emissions, and export of dissolved organic C (DOC) from a northern ombrotrophic bog. From these observations, we have constructed complete seasonal and annual C balances, examined their seasonal and interannual variability, and compared the mean 6‐year contemporary C exchange with the apparent C accumulation for the last 3000 years obtained from C density and age‐depth profiles from two peat cores. The 6‐year mean NEE‐C and CH 4‐C exchange, and net DOC loss are −40.2±40.5 (±1 SD), 3.7±0.5, and 14.9±3.1 g m −2 yr −1, giving a 6‐year mean balance of −21.5±39.0 g m −2 yr −1 (where positive exchange is a loss of C from the ecosystem). NEE had the largest magnitude and variability of the components of the C balance, but DOC and CH 4 had similar proportional variabilities and their inclusion is essential to resolve the C balance. There are large interseasonal and interannual ranges to the exchanges due to variations in climatic conditions. We estimate from the largest and smallest seasonal exchanges, quasi‐maximum limits of the annual C balance between 50 and −105 g m −2 yr −1. The net C accumulation rate obtained from the two peatland cores for the interval 400–3000 bp (samples from the anoxic layer only) were 21.9±2.8 and 14.0±37.6 g m −2 yr −1, which are not significantly different from the 6‐year mean contemporary exchange. 相似文献
19.
Measurements of regional net ecosystem exchange (NEE) were made over a period of 21 days in summer 2002 in the South‐Central part of the Netherlands and extrapolated to an area of 13 000 km 2 using a combination of flux measurements made by a Sky Arrow ERA research aircraft, half‐hourly eddy covariance data from four towers, half‐hourly weather data recorded by three weather stations and detailed information on regional land use. The combination of this type of information allowed to estimate the net contribution of the terrestrial ecosystems to the overall regional carbon flux and to map dynamically the temporal and spatial variability of the fluxes. A regional carbon budget was calculated for the study period and the contributions of the different land uses to the overall regional flux, were assessed. Ecosystems were, overall, a small source of carbon to the atmosphere equivalent to to 0.23±0.025 g C m −2 day −1. When considered separately, arable and grasslands were a source of, respectively, 0.68±0.022 and 1.28±0.026 g C m −2 day −1. Evergreen and deciduous forests were instead a sink of −1.42±0.015 g C m −2 day −1. During the study period, forests offset approximately 3.5% of anthropogenic carbon emission estimates obtained from inventory data. Lacking of a robust validation, NEE values obtained with this method were compared with independent state of art estimates of the regional carbon balance that were obtained by applying a semi‐empirical model of NEE driven by MODIS satellite fAPAR data. The comparison showed an acceptable matching for the carbon balance of forest that was a sink in both cases, while a much larger difference for arable and grassland was found. Those ecosystems were a sink for satellite‐based estimates while they were a source for the combined aircraft and tower estimates. Possible causes of such differences are discussed and partly addressed. The importance of new methods for determining carbon balance at the regional scale, is outlined. 相似文献
20.
Maximum rates of CH 4 production in the littoral sediment were observed in 2–5 cm depth. The CH 4 production rates increased during the year from about 5 mmol m −2d −1 in December to a maximum of about 95 mmol m −2d −1 in September. CH 4 production rates showed a temperature optimum at 30°C and an apparent activation energy of 76 kJ mol −1. A large part of the seasonality of CH 4 production could be ascribed to the change of the sediment temperature. Most of the produced CH 4 was lost by ebullition. Gas bubbles contained about 60–70% CH 4 with an average δ 13C of −56.2% and δD of −354%, and 2% CO 2 with an average δ 13C of −14.1% indicating that CH 4 was produced from methyl carbon, i.e. mainly using acetate as methanogenic substrate. This result was confirmed by inhibition of methanogenesis with chloroform which resulted in an accumulation rate of acetate equivalent to 81% of the rate of CH 4 production. Most probable numbers of methanogenic bacteria were in the order of 10 4 bacteria g −1d.w. sediment for acetate-, methanol- or formate-utilizing, and of 10 5 for H 2-utilizing methanogens. The turnover times of acetate were in the order of 2.3–4.8 h which, with in situ acetate concentrations of about 25–50 μM, resulted in rates of acetate turnover which were comparable to the rates of CH 4 production. The respiratory index (RI) showed that [2− 14C]acetate was mainly used by methanogenesis rather than by respiratory processes, although the zone of CH 4 production in the sediment overlapped with the zone of sulfate reduction. 相似文献
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