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991.
Niklaus Pascal A. Wohlfender Monika Siegwolf Rolf Körner Christian 《Plant and Soil》2001,233(2):189-202
Stimulated plant production and often even larger stimulation of photosynthesis at elevated CO2 raise the possibility of increased C storage in plants and soils. We analysed ecosystem C partitioning and soil C fluxes in calcareous grassland exposed to elevated CO2 for 6 years. At elevated CO2, C pools increased in plants (+23%) and surface litter (+24%), but were not altered in microbes and soil organic matter. Soils were fractionated into particle size and density separates. The amount of low-density macroorganic C, an indicator of particulate soil C inputs from root litter, was not affected by elevated CO2. Incorporation of C fixed during the experiment (Cnew) was tracked by C isotopic analysis of soil fractions which were labelled due to 13C depletion of the commercial CO2 used for atmospheric enrichment. This data constrains estimates of C sequestration (absolute upper bound) and indicates where in soils potentially sequestered C is stored. Cnew entered soils at an initial rate of 210±42 g C m–2 year–1, but only 554±39 g Cnew m–2 were recovered after 6 years due to the low mean residence time of 1.8 years. Previous process-oriented measurements did not indicate increased plant–soil C fluxes at elevated CO2 in the same system (13C kinetics in soil microbes and fine roots after pulse labelling, and minirhizotron observations). Overall experimental evidence suggests that C storage under elevated CO2 occurred only in rapidly turned-over fractions such as plants and detritus, and that potential extra soil C inputs were rapidly re-mineralised. We argue that this inference does not conflict with the observed increases in photosynthetic fixation at elevated CO2, because these are not good predictors of plant growth and soil C fluxes for allometric reasons. C sequestration in this natural system may also be lower than suggested by plant biomass responses to elevated CO2 because C storage may be limited by stabilisation of Cnew in slowly turned-over soil fractions (a prerequisite for long-term storage) rather than by the magnitude of C inputs per se. 相似文献
992.
Tree root systems, which play a major role in below-ground carbon (C) dynamics, are one of the key research areas for estimating long-term C cycling in forest ecosystems. In addition to regulating major C fluxes in the present conditions, tree root systems potentially hold numerous controls over forest responses to a changing environment. The predominant contribution of tree root systems to below-ground C dynamics has been given little emphasis in forest models. We developed the TRAP model, i.e. Tree Root Allocation of Photosynthates, to predict the partitioning of photosynthates between the fine and coarse root systems of trees among series of soil layers. TRAP simulates root system responses to soil stress factors affecting root growth. Validation data were obtained from two Belgian experimental forests, one mostly composed of beech (Fagus sylvatica L.) and the other of Scots pine (Pinus sylvestris L.). TRAP accurately predicted (R = 0.88) night-time CO2 fluxes from the beech forest for a 3-year period. Total fine root biomass of beech was predicted within 6% of measured values, and simulation of fine root distribution among soil layers was accurate. Our simulations suggest that increased soil resistance to root penetration due to reduced soil water content during summer droughts is the major mechanism affecting the distribution of root growth among soil layers of temperate Belgian forests. The simulated annual rate of C input to soil litter due to the fine root turnover of the Scots pine was 207 g C m–2 yr–1. The TRAP model predicts that fine root turnover is the single most important source of C to the temperate forest soils of Belgium. 相似文献
993.
994.
Diurnal variations in the carbon chemistry of two acidic peatland streams in north-east Scotland 总被引:1,自引:0,他引:1
1. Two acidic peatland upland streams in north‐east Scotland draining catchments of 1.3 and 41.4 km2 were sampled each season for 2 years to investigate diurnal variations in dissolved and gaseous forms of carbon. Stream metabolism, alkalinity, discharge, pH, air and water temperatures were measured to aid data interpretation. 2. Free CO2 showed marked diurnal variation with lowest concentrations during the period from late morning to early afternoon and highest during the hours of darkness. Although alkalinity and pH also showed some diurnal fluctuations, in comparison with other more productive alkaline systems, variation was small. Dissolved organic carbon (DOC) showed no significant diurnal pattern. However, significant changes in stream discharge influenced DOC concentrations, as well as over‐riding diurnal patterns of free CO2, alkalinity and pH. 3. The highest diurnal ratios (maximum concentration/minimum concentration) in CO2, gross primary productivity (GPP) and community respiration (CR) occurred in spring and summer and the lowest in autumn and winter. Variation in biotic in‐stream processes caused changes in CO2 concentrations and temperature affected both the solubility of CO2 and changes in up‐stream CO2 inputs. There was no significant difference in diurnal fluctuations between the two orders of stream studied. 4. The mean GPP (as CO2) was 0.81 g CO2 m?2 day?1 and mean CR 2.67 g CO2 m?2 day?1. The mean primary production/respiration (P/R) ratio was 0.26 ± 0.09 and 0.33 ± 0.15 in the first and second order streams, respectively. These values are low compared with published data because these heterotrophic headwater streams are dominated by benthic respiration and upstream allochthonous inputs with little autotrophic metabolism, particularly during the colder autumn and winter months. 5. The results have implications for the calculation of dissolved inorganic carbon (DIC) fluxes in streamwater. Samples taken during daylight hours tend to have lower concentrations of free CO2 and HCO3? than samples taken during darkness. During spring, concentrations of free CO2 were measured up to 2.4 (annual mean 1.8) times higher at night than during the day at a similar discharge. It is suggested that fluxes based on daytime measurements alone will under‐estimate the annual flux of these determinands in streamwater by as much as 40%. 相似文献
995.
Lipozyme IM20 from Novo Nordisk (Denmark) was examined after various treatments. Conditions were chosen to reflect those that would be considered in the design of an industrial process. A two-level factorial design was employed to assess the effects of pressurization/depressurization cycles, rate of depressurization and exposure length. A significant three-factor interaction was observed. Lowest residual activity was observed for runs in which the depressurization rate was 86–89 bar min–1. Incubation for 12 h also yielded low residual activity but only when exposing the immobilized enzyme to one cycle. The highest residual activity was obtained for immobilized enzymes repeatedly exposed for periods of 12 h (5 times) with a depressurization rate of 4.3 to 4.45 bar min–1. This effect may be due to the extraction of an inhibiting compound. Tuning process parameters can lead to a seven-fold change in residual activity. 相似文献
996.
Organic matter chemistry and dynamics in clear-cut and unmanaged hardwood forest ecosystems 总被引:7,自引:1,他引:6
Forest harvesting alters the organic matter cycle by changing litter inputs and the decomposition regime. We hypothesized that these changes would result in differences in organic matter chemistry between clear-cut and uncut watershed ecosystems. We studied the chemistry of soil organic matter (SOM), and dissolved organic carbon (DOC) in soil solutions and stream samples in clear-cut and uncut sites at the Hubbard Brook Experimental Forest in New Hampshire using DOC fractionation techniques and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy.Alkyl-C (aliphatic compounds) and O-alkyl-C (carbohydrates) were the largest C fractions in soil and dissolved organic matter at Hubbard Brook. Alkyl-C ranged from 29–48% of soil C, 25–42% of soil solution C, and 22–42% of streamwater DOC. Carbohydrates comprised 32–49%, 36–43%, and 29–60% of C in soils, solutions, and streamwater, respectively. In both soils and soil solutions, the carbohydrate fraction decreased with increasing soil depth, while the aromaticity of organic matter increased with depth. There were no significant differences in the structural chemistry of SOM between clear-cut and uncut watersheds.The aromatic-C fractions in soil solutions at the clear-cut site ranged from 12–16%, approximately 40% greater than at the uncut site (8.5–11%). Thus, clear-cutting has resulted in the leaching of more highly decomposed organic matter, and depletion of more aliphatic compounds in the soluble organic pool. Because DOC fluxes are small compared to the SOM pool, large differences in soil solution chemistry do not substantially alter the overall composition of SOM. While the organic chemistry of stream DOC varied greatly among 3 sampling dates, there were no obvious clear-cutting effects. Thus, temporal variations in flowpaths and/or in-stream processes appear to be more important than disturbance in regulating the organic carbon chemistry of these streams. 相似文献
997.
Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem 总被引:21,自引:4,他引:17
Ross D. Fitzhugh Charles T. Driscoll Peter M. Groffman Geraldine L. Tierney Timothy J. Fahey Janet P. Hardy 《Biogeochemistry》2001,56(2):215-238
Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO3
–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH4
+) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha–1yr–1, significant in comparison to wet Ndeposition (530 mol ha–1 yr–1) andstream NO3
– export (25 mol ha–1yr–1) in this northern forest ecosystem. Soil solution fluxes of Pi from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha–1 yr–1;this elevated mobilization of Pi coincidedwith heightened NO3
– leaching. Elevated leaching of Pi from the forestfloor was coupled with enhanced retention ofPi in the mineral soil Bs horizon. Thequantities of Pi mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha–1) as well asnet P mineralization rates in the forest floor(180 mol ha–1 yr–1). Increased fineroot mortality was likely an important sourceof mobile N and Pi from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication. 相似文献
998.
The Reactions of Methanimine and Cyanogen with Carbon Monoxide in Prebiotic Molecular Evolution on Earth 总被引:2,自引:0,他引:2
A primeval atmosphere is proposedcontaining simple molecules such as formaldehyde, ammonia, carbon monoxide, cyanogen andhydrogen cyanide, which have been detected in space. Chemical reactions aredescribed for the formation ofaziridine-2-one and di-azirine-3-one derivatives aspotential precursors for the original synthesesis of amino-acids, proteins, pyrimidines,purines, nicotinamide and flavin. The reactions have been shown to be kinetically feasiblefrom the overall enthalpy changes in the ZKE approximation at the MP2/6-31G* level. 相似文献
999.
1000.
Activity of the Enzymes of Carbon Metabolism in Sulfobacillus sibiricus under Various Conditions of Cultivation 总被引:1,自引:0,他引:1
Zakharchuk L. M. Egorova M. A. Tsaplina I. A. Bogdanova T. I. Krasil'nikova E. N. Melamud V. S. Karavaiko G. I. 《Microbiology》2003,72(5):553-557
The thermoacidophilic iron-oxidizing chemolithotroph Sulfobacillus sibiricus N1T is characterized by steady growth and amplified cell yield when grown in vigorously aerated medium containing Fe2+, glucose, and yeast extract as energy sources. In this case, carbon dioxide, glucose, and yeast extract are used as carbon sources. Glucose is assimilated through the fructose-bisphosphate pathway and the pentose-phosphate pathway. The glyoxylate bypass does not function in S. sibiricus, and the tricarboxylic acid cycle is disrupted at the level of 2-oxoglutarate dehydrogenase. The presence of ribulose-bisphosphate carboxylase indicates that carbon dioxide fixation proceeds through the Calvin cycle. The activity of ribulose-bisphosphate carboxylase is highest in autotrophically grown cells. The cells also contain pyruvate carboxylase, phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase, and phosphoenolpyruvate carboxytransphosphorylase. 相似文献