Sulfur and iron cycling in a coastal sediment: Radiotracer studies and seasonal dynamics

The seasonal variation in sulfate reduction ana the dynamics or sulfur ana iron geochemistry were studied throughout a year in sediment of Aarhus Bay, Denmark. A radiotracer method for measuring sulfate reduction rates was applied with incubation times down to 15 min and a depth resolution down to 2 mm in the oxidized surface layer of the sediment. The radiotracer data were analyzed by a mathematical model which showed that, due to partial, rapid reoxidation of radioactive sulfide during incubation, the actual reduction rates in this layer were probably underestimated 5-fold. In the deeper, sulfidic zone, measured rates appeared to be correct. Sulfate reduction followed the seasonal variation in temperature with maximum activity at 1–2 cm depth in late summer. In spite of its rapid production, free H₂S was detectable in the porewater only below the depth of free Fe²⁺ at 6–7 cm throughout the year. Following the massive sedimentation from a spring phytoplankton bloom, anaerobic degradation of phytoplankton detritus was strongly stimulated over several weeks. A transient reversed redox zonation developed with a thin, black zone on top of the brown, oxidized sediment layer due to intensive sulfate and iron reduction. Mineralization through sulfate reduction was equivalent to two thirds of the annual net sedimentation of organic matter.Author for correspondence     

Methane production and sulfate reduction in two Appalachian peatlands

Anaerobic carbon mineralization was evaluated over a 1-year period in two Sphagnum-dominated peatlands, Big Run Bog, West Virginia, and Buckle's Bog, Maryland. In the top 35 cm of peat, mean rates of methane production, anaerobic carbon dioxide production, and sulfate reduction at Big Run Bog were 63,406 and 146 mol L^-1 d^-1, respectively, and at Buckle's Bog were 18, 486 and 104 mol L^-1 d^-1. Annual anaerobic carbon mineralization to methane and carbon dioxide at Big Run Bog and Buckle's Bog was 52.8 and 57.2 mol m^-2, respectively. Rates of methane production were similar to rates reported for other freshwater peatlands, but methane production accounted for only 11.7 and 2.8%, respectively, of the total anaerobic carbon mineralization at these two sites. Carbon dioxide production, resulting substantially from sulfate reduction, dominated anaerobic carbon mineralization. Considerable sulfate reduction despite low instantaneous dissolved sulfate concentrations (typically < 300 mol L^-1 of substrate) was apparently fueled by oxidation and rapid turnover of the reduced inorganic sulfur pool.The coincidence of high sulfate inputs to the Big Run Bog and Buckle's Bog watersheds through acid precipitation with the unexpected importance of sulfate reduction leads us to suggest a new hypothesis: peatlands not receiving high sulfate loading should exhibit low rates of anaerobic decomposition, and a predominance of methane production over sulfate reduction; however, if such peatlands become subjected to high rates of sulfur deposition, sulfate reduction may be enhanced as an anaerobic mineralization pathway with attendant effects on carbon balance and peat accumulation.     

Origin and fate of acetate in an acidic fen

Acetate is a central intermediate in the anaerobic degradation of organic matter, and the resolution of its metabolism necessitates integrated strategies. This study aims to (1) estimate the contribution of acetogenesis to acetate formation in an acidic fen (pH ~ 4.9), (2) assess the genetic potential for acetogenesis targeting the fhs gene encoding formyltetrahydrofolate synthetase (FTHFS) and (3) unravel the in situ turnover of acetate using stable carbon isotope pore-water analysis. H(2)/CO(2)-supplemented peat microcosms yielded (13)C-depleted acetate (-37.2‰ vs. VPDB (Vienna Peedee belemnite standard) compared with -14.2‰ vs. VPDB in an unamended control), indicating the potential for H(2)-dependent acetogenesis. Molecular analysis revealed a high diversity and depth-dependent distribution of fhs phylotypes with the highest number of operational taxonomic units in 0-20 cm depth, but only few and distant relationships to known acetogens. In pore waters, acetate concentrations (0-170 μM) and δ(13)C-values varied widely (-17.4‰ to -3.4‰ vs. VPDB) and did not indicate acetogenesis, but pointed to a predominance of sinks, which preferentially consumed (12)C-acetate, like acetoclastic methanogenesis. However, depth profiles of methane and δ(13)C(CH4) revealed a temporarily and spatially restricted role of this acetate sink and suggest other processes like sulfate and iron reduction played an important role in acetate turnover.     

Cross-correlation analysis of the <Emphasis Type="Italic">Desulfovibrio</Emphasis> growth parameters of intestinal species isolated from people with colitis

Sulfate-reducing bacteria can be involved in inflammatory bowel disease. Cross-correlation parameters of their metabolic process in the gut have never been reported before. The aim of the research was to statistically (cross-correlation) evaluate the parameters of growth (biomass) of Desulfovibrio species from colitis people and healthy as well as to investigate the change in dissimilatory sulfate reduction of these bacterial strains. The microbiological, biochemical, chemical, and statistical methods were used in the research. The cross-correlation analysis indicates that the strains isolated from people with colitis shifted to the right side of Y axe by biomass accumulation, sulfate consumption, lactate oxidation as well as hydrogen sulfide and acetate production, compared with the strains isolated from healthy individuals. Different percentages were observed in shifting to the right side of Y axe: biomass accumulation 26%, sulfate consumption 1.5%, sulfide production 5%, lactate oxidation 3% and acetate production 12%. The biomass accumulation of intestinal sulfate-reducing bacteria and their hydrogen sulfide production are the main factors in inflammatory bowel disease development, including ulcerative colitis. Acetate production showed lesser influence while sulfate consumption and lactate oxidation are negligible factors in the inflammatory bowel disease.     

Partitioning effects during terminal carbon and electron flow in sediments of a low-salinity meltwater pond near Bratina Island, McMurdo Ice Shelf, Antarctica

A study of anaerobic sediments below cyanobacterial mats of a low-salinity meltwater pond called Orange Pond on the McMurdo Ice Shelf at temperatures simulating those in the summer season (<5 degrees C) revealed that both sulfate reduction and methane production were important terminal anaerobic processes. Addition of [2-(14)C]acetate to sediment samples resulted in the passage of label mainly to CO(2). Acetate addition (0 to 27 mM) had little effect on methanogenesis (a 1.1-fold increase), and while the rate of acetate dissimilation was greater than the rate of methane production (6.4 nmol cm(-3) h(-1) compared to 2.5 to 6 nmol cm(-3) h(-1)), the portion of methane production attributed to acetate cleavage was <2%. Substantial increases in the methane production rate were observed with H(2) (2.4-fold), and H(2) uptake was totally accounted for by methane production under physiological conditions. Formate also stimulated methane production (twofold), presumably through H(2) release mediated through hydrogen lyase. Addition of sulfate up to 50-fold the natural levels in the sediment (interstitial concentration, approximately 0.3 mM) did not substantially inhibit methanogenesis, but the process was inhibited by 50-fold chloride (36 mM). No net rate of methane oxidation was observed when sediments were incubated anaerobically, and denitrification rates were substantially lower than rates for sulfate reduction and methanogenesis. The results indicate that carbon flow from acetate is coupled mainly to sulfate reduction and that methane is largely generated from H(2) and CO(2) where chloride, but not sulfate, has a modulating role. Rates of methanogenesis at in situ temperatures were four- to fivefold less than maximal rates found at 20 degrees C.     

Sulfide and methane evolution in the hypolimnion of a subtropical lake: a three-year study

The differential impact of microbial sulfate reduction and methanogenesis on the mineralization of particulate organic carbon (POC) in warm monomictic Lake Kinneret (LK), Israel was studied during three consecutive lake cycles. The hypolimnetic accumulation of total sulfide and dissolved methane was examined in relation to the physical forcing of the water column and the settling flux of particulate matter. With the on-set of thermal stratification in spring, both solutes increased concomitantly with the depletion of oxygen, first in the benthic boundary layer, followed by the upper hypolimnion. Methane production was restricted to the sediments as emphasized by the persistently linear concentration gradient in the hypolimnion. Sulfate reduction occurred both in the sediments and the water column as revealed by the hypolimnetic distribution of sulfide and recurring metalimnetic sulfide peaks. Annual differences in the accumulation pattern of both solutes appeared to be primarily linked to the settling flux of POC and the length of the stratified season. Relatively lower hypolimnetic concentrations of dissolved methane during the stratified season of 2000 coincided with increased ebullition of gaseous methane, likely as the result of a nearly a 2 m drop in the lake level. Overall, sulfate reduction accounted for more than 60% of the POC settling flux, a finding that differs from similar studies made in temperate lakes where methanogenesis was shown to be the primary mode of terminal carbon mineralization. Intensive organic carbon turnover at the sediment water interface and comparatively high sulfate concentrations in LK are the most likely reason.     

土地利用方式转变后灰色森林土有机碳矿化的温度响应特征

通过室内培育试验,分析了土地利用方式转变后灰色森林土有机碳矿化过程及其对温度变化的响应特征.结果表明：原始林转变为农田后,0～10 cm、10～20 cm的土壤有机碳和全氮含量分别下降了68.5%、76.8%和40.5%、44.4%;而农田土壤有机碳的平均矿化速率和累积矿化量仅分别为原始林的24.4%～43.2%和9.20％～13.7％.低温条件下（<25 ℃）土壤有机碳矿化的温度敏感性显著高于高温条件下（>25 ℃）.低温条件下（<25 ℃）两种利用方式的土壤有机碳矿化对温度变化的敏感性没有显著差异;但高温条件下（>25 ℃）,农田0～10 cm土壤有机碳矿化的温度敏感性高于原始林,而农田10～20 cm土壤有机碳矿化的温度敏感性明显较低.     

Dynamics of Bacterial Sulfate Reduction in a Eutrophic Lake

Bacterial sulfate reduction in the surface sediment and the water column of Lake Mendota, Madison, Wis., was studied by using radioactive sulfate (³⁵SO₄²⁻). High rates of sulfate reduction were observed at the sediment surface, where the sulfate pool (0.2 mM SO₄²⁻) had a turnover time of 10 to 24 h. Daily sulfate reduction rates in Lake Mendota sediment varied from 50 to 600 nmol of SO₄²⁻ cm⁻³, depending on temperature and sampling date. Rates of sulfate reduction in the water column were 10³ times lower than that for the surface sediment and, on an areal basis, accounted for less than 18% of the total sulfate reduction in the hypolimnion during summer stratification. Rates of bacterial sulfate reduction in the sediment were not sulfate limited at sulfate concentrations greater than 0.1 mM in short-term experiments. Although sulfate reduction seemed to be sulfate limited below 0.1 mM, Michaelis-Menten kinetics were not observed. The optimum temperature (36 to 37°C) for sulfate reduction in the sediment was considerably higher than in situ temperatures (1 to 13°C). The response of sulfate reduction to the addition of various electron donors metabolized by sulfate-reducing bacteria in pure culture was investigated. The degree of stimulation was in this order: H₂ > n-butanol > n-propanol > ethanol > glucose. Acetate and lactate caused no stimulation.     

Turnover of glucose and acetate coupled to reduction of nitrate, ferric iron and sulfate and to methanogenesis in anoxic rice field soil

Turnover of glucose and acetate in the presence of active reduction of nitrate, ferric iron and sulfate was investigated in anoxic rice field soil by using [U-(14)C]glucose and [2-(14)C]acetate. The turnover of glucose was not much affected by addition of ferrihydrite or sulfate, but was partially inhibited (60%) by addition of nitrate. Nitrate addition also strongly reduced acetate production from glucose while ferrihydrite and sulfate addition did not. These results demonstrate that ferric iron and sulfate reducers did not outcompete fermenting bacteria for glucose at endogenous concentrations. Nitrate reducers may have done so, but glucose fermentation may also have been inhibited by accumulation of toxic denitrification intermediates (nitrite, NO, N(2)O). Addition of nitrate resulted in complete inhibition of CH(4) production from [U-(14)C]glucose and [2-(14)C]acetate. However, addition of ferrihydrite or sulfate decreased the production of (14)CH(4) from [U-(14)C]glucose by only 70 and 65%, respectively. None of the electron acceptors significantly increased the production of (14)CO(2) from [U-(14)C]glucose, but all increased the production of (14)CO(2) from [2-(14)C]acetate. Uptake of acetate was faster in the presence of either nitrate, ferrihydrite or sulfate than in the unamended control. Addition of ferrihydrite and sulfate reduced (14)CH(4) production from [2-(14)C]acetate by 83 and 92%, respectively. Chloroform completely inhibited the methanogenic consumption of acetate. It also inhibited the oxidation of acetate, completely in the presence of sulfate, but not in the presence of nitrate or ferrihydrite. Our results show that, besides the possible toxic effect of products of nitrate reduction (NO, NO(2)(-) and N(2)O) on methanogens, nitrate reducers, ferric iron reducers and sulfate reducers were active enough to outcompete methanogens for acetate and channeling the flow of electrons away from CH(4) towards CO(2) production.     

Electron Donors Utilized by Sulfate-Reducing Bacteria in Eutrophic Lake Sediments

Mineralization rates of ¹⁴C-labeled substrates were determined in the presence and absence of Na₂MoO₄, an inhibitor of sulfate reduction, in the profundal sediments of a shallow eutrophic lake. Sulfate reduction was inhibited by Na₂MoO₄ at all concentrations tested (0.2 to 200 mM), whereas methane production was inhibited at Na₂MoO₄ concentrations greater than 20 mM. Initial mineralization rates of glucose were unaffected by Na₂MoO₄; however, Na₂MoO₄ decreased the mineralization rates of lactate (58%), propionate (52%), an amino acid mixture (85%), and acetate (14%). These decreases in the rates of mineralization were attributed to inhibition of sulfate reduction. Hydrogen stimulated the reduction of ³⁵SO₄²⁻ 2.5- to 2.8-fold, demonstrating potential hydrogen oxidation by sulfate-reducing bacteria. These results indicate that sulfate reducers utilize an array of substrates as electron donors and are of potential significance to the in situ mineralization of lactate, propionate, and free amino acids in these sediments.     

Competitive oxidation of volatile fatty acids by sulfate- and nitrate-reducing bacteria from an oil field in Argentina

Acetate, propionate, and butyrate, collectively referred to as volatile fatty acids (VFA), are considered among the most important electron donors for sulfate-reducing bacteria (SRB) and heterotrophic nitrate-reducing bacteria (hNRB) in oil fields. Samples obtained from a field in the Neuquén Basin, western Argentina, had significant activity of mesophilic SRB, hNRB, and nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB). In microcosms, containing VFA (3 mM each) and excess sulfate, SRB first used propionate and butyrate for the production of acetate, which reached concentrations of up to 12 mM prior to being used as an electron donor for sulfate reduction. In contrast, hNRB used all three organic acids with similar kinetics, while reducing nitrate to nitrite and nitrogen. Transient inhibition of VFA-utilizing SRB was observed with 0.5 mM nitrite and permanent inhibition with concentrations of 1 mM or more. The addition of nitrate to medium flowing into an upflow, packed-bed bioreactor with an established VFA-oxidizing SRB consortium led to a spike of nitrite up to 3 mM. The nitrite-mediated inhibition of SRB led, in turn, to the transient accumulation of up to 13 mM of acetate. The complete utilization of nitrate and the incomplete utilization of VFA, especially propionate, and sulfate indicated that SRB remained partially inhibited. Hence, in addition to lower sulfide concentrations, an increase in the concentration of acetate in the presence of sulfate in waters produced from an oil field subjected to nitrate injection may indicate whether the treatment is successful. The microbial community composition in the bioreactor, as determined by culturing and culture-independent techniques, indicated shifts with an increasing fraction of nitrate. With VFA and sulfate, the SRB genera Desulfobotulus, Desulfotignum, and Desulfobacter as well as the sulfur-reducing Desulfuromonas and the NR-SOB Arcobacter were detected. With VFA and nitrate, Pseudomonas spp. were present. hNRB/NR-SOB from the genus Sulfurospirillum were found under all conditions.     

Process development and optimisation of lactic acid purification using electrodialysis

Cell free sodium lactate solutions were subjected to purification based on mono- and bi-polar electrodialysis. Lactate concentration in the product stream increased to a maximum of 15% during mono-polar electrodialysis. Stack energy consumption averaged 0.6 kW h kg(-1) lactate transported at current efficiencies in the 90% range. Under optimum feed concentration (125 g l(-1)) and process conditions (auto-current mode with conductivity setpoints of minimum 5 and maximum 40 mS cm(-1)), lactate flux reached 300 g m(-2) h(-1) and water flux were low for mono-polar electrodialysis averaging 0.3 kg H(2)O per M lactate transported. Glucose in the concentrate stream solutions was reduced to < 2 g l(-1). Acetate impurities enriched from about 0.5 g l(-1) in the feed stream to 1.5 g l(-1) in the concentrate stream solutions. After mono-polar electrodialysis, the concentrated sodium lactate solutions were further purified using bi-polar electrodialysis. Water transport during bi-polar electrodialysis reached figures of 0.070 - 0.222 kg H(2)O per M lactate. Free lactic acid concentration reached 16% with lactate flux of up to 300 g m(-2) h(-1). Stack energy consumption ranged from 0.6 to 1 kW h per kg lactate. Under optimised process conditions current efficiency during bi-polar electrodialysis was consistently around 90%. Glucose was further reduced from 2 to <1 g l(-1) in the free lactic acid solution. Acetic acid impurity remained at around 1 g l(-1). Significant reduction in colour and minerals in the product streams was observed during electrodialysis purification.     

广西典型喀斯特地区深层土壤有机碳矿化及其影响因素

以广西典型峰丛洼地草地和原生林深层土壤(70～100cm)为对象,利用微生物交叉接种培养试验,研究不同土地利用类型、土壤微生物群落和通气条件对深层土壤有机碳矿化的影响。在124d的培养期内,微生物接种改变了0～28d原生林和0～81d草地深层土壤有机碳矿化速率,而通气条件变化对这一过程没有明显影响。3因素方差分析结果显示,深层土壤有机碳累积矿化率受土地利用类型、微生物群落和通气条件的影响显著(P<0.01),且存在3因素间交互效应。研究结果对于深入认识喀斯特深层土壤有机碳稳定机制和评估碳储量及其周转具有重要意义。     

帽儿山温带落叶阔叶林细根生物量、生产力和周转率

细根在森林生态系统能量流动与物质循环中占有重要地位,但其生物量、生产和周转测定尚存在很大的不确定性,而且局域尺度空间变异机制尚不清楚。本研究分析了帽儿山温带天然次生林活细根生物量和死细根生物量在0～100 cm剖面的垂直分布与0～20 cm细根的季节动态、生产力和周转率,对比了采用连续根钻法(包括决策矩阵法和极差法)和内生长袋(直径3和5 cm)估测细根生产力和细根周转率,并探讨了可能影响细根的林分因子。结果表明: 76.8%的活细根生物量和62.9%的死细根生物量均集中在0～20 cm土层,随着深度增加,二者均呈指数形式减少。活细根生物量和死细根生物量的季节变化不显著,可能与冬季几乎无降雪而夏季降雨异常多有关。2种直径内生长袋估计的细根生产力无显著差异;对数转换后决策矩阵、极差法和内生长法估计的细根生产力和细根周转率差异显著。随着土壤养分增加,活细根生物量和死细根生物量比值显著增加,死细根生物量显著减少,但活细根生物量、细根生产力和细根周转率均无显著变化;细根周转率与前一年地上木质生物量增长量呈显著正相关,但与当年地上木质生物量增长量无显著相关关系。     

Comparison of Acetate Turnover in Methanogenic and Sulfate-Reducing Sediments by Radiolabeling and Stable Isotope Labeling and by Use of Specific Inhibitors: Evidence for Isotopic Exchange

Acetate turnover in the methanogenic freshwater anoxic sediments of Lake Vechten, The Netherlands, and in anoxic sediments from the Tamar Estuary, United Kingdom, and the Grosser Jasmunder Bodden, Germany, the latter two dominated by sulfate reduction, was determined. Stable isotopes and radioisotopes, inhibitors (chloroform and fluoroacetate), and methane flux were used to provide independent estimates of acetate turnover. Pore water acetate pool sizes were determined by gas chromatography with a flame ionization detector, and stable isotope-labeled acetate was determined by gas chromatography-mass spectrometry. The appearance of acetates with a different isotope labeling pattern from that initially added demonstrated that isotopic exchange occurred during methanogenic acetate metabolism. The predominant exchange processes were (i) D-H exchange in the methyl group and (ii) (sup13)C-(sup12)C exchange at the carboxyl carbon. These exchanges are most probably caused by the activity of the enzyme complex carbon monoxide dehydrogenase and subsequent methyl group dehydrogenation by tetrahydromethanopterine or a related enzyme. The methyl carbon was not subject to exchange during transformation to methane, and hence acetate with the methyl carbon labeled will provide the most reliable estimate of acetate turnover to methane. Acetate turnover rate estimates with these labels were consistent with independent estimates of acetate turnover (acetate accumulation after inhibition and methane flux). Turnover rates from either radioisotope- or stable isotope-labeled methyl carbon isotopes are, however, dependent on accurate determination of the acetate pool size. The additions of large amounts of stable isotope-labeled acetate elevate the acetate pool size, stimulating acetate consumption and causing deviation from steady-state kinetics. This can, however, be overcome by the application of a non-steady-state model. Isotopic exchange in sediments dominated by sulfate reduction was minimal.     

Potentially Active Iron,Sulfur, and Sulfate Reducing Bacteria in Skagerrak and Bothnian Bay Sediments

In many marine surface sediments, the reduction of manganese (Mn) and iron (Fe) oxides is obscured by sulfate reduction, which is regarded as the predominant anaerobic microbial respiration process. However, many dissimilatory sulfate and sulfur reducing microorganisms are known to utilize alternative electron acceptors such as metal oxides. In this study, we tested whether sulfate and sulfur reducing bacteria are linked to metal oxide reduction based on biogeochemical modeling of porewater concentration profiles of Mn²⁺ and Fe²⁺ in Bothnian Bay (BB) and Skagerrak (SK) sediments. Steady-state modeling of Fe²⁺ and Mn²⁺ porewater profiles revealed zones of net Fe (0–9 cm BB; ～10 and 20 cm SK) and Mn (0–5 cm BB; 2–8 cm SK) species transformations. 16S rRNA pyrosequencing analysis of the in-situ community showed that Desulfobacteraceae, Desulfuromonadaceae and Desulfobulbaceae were present in the zone of Fe-reduction of both sediments. Rhodobacteraceae were also detected at high relative abundance in both sediments. BB sediments appeared to harbor a greater diversity of potential Fe-reducers compared to SK. Additionally, when the upper 10 cm of sediment from the SK was incubated with lepidocrocite and acetate, Desulfuromonas was the dominant bacteria. Real-time quantitative polymerase chain reaction (qPCR) results showed decreasing dsrA gene copy numbers with depth coincided with decreased Fe-reduction activity. Our results support the idea that sulfur and sulfate reducing bacteria contribute to Fe-reduction in the upper centimeters of both sediments.     

六盘山林区几种土地利用方式对土壤有机碳矿化影响的比较

 应用土壤培养法,比较分析了六盘山林区天然次生林（杂灌林、山杨（Populus davidanda）和辽东栎（Quercus liaotungensis）林）、农田、草地和人工林（13 a、18 a和25 a华北落叶松（Larix principis-rupprechtii））土壤在30℃和60%田间饱和含水量条件下培养180 d有机碳矿化速率的差异（以180 d累计释放的CO2-C计）。结果显示：农田和草地土壤碳矿化释放的CO2-C含量（180 d释放的gCO2-C·kg-1干土）分别比天然次生林低65%和23%,差异主要在0～40土层;人工林比农田和草地分别高155%和 17%,差异主要在0～70 cm土层。农田土壤碳矿化释放的CO2-C分配比例（即180 d释放CO2-C/土壤 C）比天然次生林平均低12%,草地比天然次生林平均高18%,差异主要在0～40 cm土层;人工林比农田平均高29%,草地比人工林平均高9%,差异主要在0～50 cm土层。不同土地利用方式土壤碳矿化释放的CO2-C含量的差异比其分配比例的差异大。土壤碳矿化释放的CO2-C含量和分配比例总体上都随土层加深而递减。分配比例随土层加深而递减的幅度方面,不同土地利用方式间的差异不大;含量随土层加深而递减的幅度方面,天然次生林和人工林比农田和草地中大;随土层递减的幅度方面,土壤碳矿化释放的CO2-C含量比其分配比例大。这主要由不同土地利用方式土壤碳输入和稳定性等差异所致。结果说明土壤碳矿化速率随天然次生林变成农田或草地而下降,随在农田或草地上造林而增加, 矿化速率变化幅度比分配比例的变化幅度大。另外,土地利用变化也使不同土层土壤碳矿化速率和分配比例差异改变,其中速率改变的幅度比分配比例改变的幅度大。     

丹江口库区覆膜土壤不同土层氮素矿化速率及其影响因素

以丹江口库区五龙池小流域玉米黄棕壤为例,利用原位土壤氮矿化试验,研究覆膜条件下0～10、10～20、20～30 cm土层土壤氮素矿化速率及其影响因素.结果表明: 玉米生长期内,土壤氨化速率随土层加深呈逐渐降低的趋势;土壤硝化速率在苗期、拔节期、抽穗期表现为10～20 cm＞0～10 cm＞20～30 cm,在成熟期随土层加深呈逐渐升高的趋势;土壤氮矿化速率在苗期、拔节期、抽穗期随土层加深呈逐渐降低的趋势,在成熟期随土层加深呈逐渐升高的趋势.与无覆膜相比,覆膜会加快0～10 cm土壤氨化过程,也会提高拔节期、抽穗期和成熟期0～10、10～20 cm土层土壤硝化速率和氮矿化速率,但苗期覆膜土壤硝化速率、氮矿化速率均低于无覆膜土壤.逐步回归分析显示,土壤含水量和全氮含量是0～10 cm土壤氮矿化速率的主要影响因子,土壤温度、含水量、全氮含量是10～20 cm土壤氮矿化速率的主要影响因子,土壤温度是20～30 cm土壤氮矿化速率的主要影响因子.     

Effect of Fall Turnover on Terminal Carbon Metabolism in Lake Mendota Sediments

The carbon and electron flow pathways and the bacterial populations responsible for the transformation of H₂-CO₂, formate, methanol, methylamine, acetate, ethanol, and lactate were examined in eutrophic sediments collected during summer stratification and fall turnover. The rate of methane formation averaged 1,130 μmol of CH₄ per liter of sediment per day during late-summer stratification versus 433 μmol of CH₄ per liter of sediment per day during the early portion of fall turnover, whereas the rate of sulfate reduction was 280 μmol of sulfate per liter of sediment per day versus 1,840 μmol of sulfate per liter of sediment per day during the same time periods, respectively. The sulfate-reducing population remained constant while the methanogenic population decreased by one to two orders of magnitude during turnover. The acetate concentration increased from 32 to 81 μmol per liter of sediment while the acetate transformation rate constant decreased from 3.22 to 0.70 per h, respectively, during stratification versus turnover. Acetate accounted for nearly 100% of total sedimentary methanogenesis during turnover versus 70% during stratification. The fraction of ¹⁴CO₂ produced from all ¹⁴C-labeled substrates examined was 10 to 40% higher during fall turnover than during stratification. The addition of sulfate, thiosulfate, or sulfur to stratified sediments mimicked fall turnover in that more CO₂ and CH₄ were produced. The addition of Desulfovibrio vulgaris to sulfate-amended sediments greatly enhanced the amount of CO₂ produced from either [¹⁴C]methanol or [2-¹⁴C]acetate, suggesting that H₂ consumption by sulfate reducers can alter methanol or acetate transformation by sedimentary methanogens. These data imply that turnover dynamically altered carbon transformation in eutrophic sediments such that sulfate reduction dominated over methanogenesis principally as a consequence of altering hydrogen metabolism.     

Cycling of inorganic and organic sulfur in peat from Big Run Bog,West Virginia

Total S concentration in the top 35 cm of Big Run Bog peat averaged 9.7 mol·g — wet mass^–1 (123 mol·g dry mass^–1). Of that total, an average of 80.8% was carbon bonded S, 10.4% was ester sulfate S, 4.5% was FeS₂­S, 2.7% was FeS­S, 1.2% was elemental S, and 0.4% was SO₄ ^2–­S. In peat collected in March 1986, injected with³⁵S­SO₄ ^2– and incubated at 4 °C, mean rates of dissimilatory sulfate reduction (formation of H₂S + S⁰ + FeS + FeS₂), carbon bonded S formation, and ester sulfate S formation averaged 3.22, 0.53, and 0.36 nmol·g wet mass^–1·h^–1, respectively. Measured rates of sulfide oxidation were comparable to rates of sulfate reduction. Although dissolved SO₄ ^2– concentrations in Big Run Bog interstitial water (< 200 µM) are low enough to theoretically limit sulfate reducing bacteria, rates of sulfate reduction integrated throughout the top 30–35 cm of peat of 9 and 34 mmol·m^–2·d^–1 (at 4 °C are greater than or comparable to rates in coastal marine sediments. We suggest that sulfate reduction was supported by a rapid turnover of the dissolved SO₄ ^2– pool (average turnover time of 1.1 days). Although over 90% of the total S in Big Run Bog peat was organic S, cycling of S was dominated by fluxes through the inorganic S pools.