Extracellular dissolved organic carbon released from phytoplankton as a source of carbon for heterotrophic bacteria in lakes of different humic content

The quantitative importance of photosynthetically produced dissolved organic carbon (PDOC) released from phytoplankton as a source of carbon for pelagic, heterotrophic bacteria was investigated in four temperate Swedish lakes, of which two had low (≈20 mg Pt 1⁻¹), and two moderately high (60–80 mg Pt 1⁻¹) humic content. The bacterial assimilation of PDOC was estimated with the ¹⁴C method, and the total production of the heterotrophic bacteria was estimated with the [3H]thymidine incorporation method. The release of PDOC from natural communities of phytoplankton was not restricted to periods of photosynthesis, but often continued during periods of darkness. Heterotrophic bacteria often assimilated the labile components of the PDOC at high rates (up to 73% of the released PDOC was assimilated during the incubation in our experiments). The contribution of PDOC to bacterial production exhibited large within-lake seasonal variations, but PDOC was at certain times, both in humic and non-humic lakes, a quantitatively very important carbon source for the heterotrophic bacteria. Under periods of comparatively low primary production, heterotrophic bacteria in humic lakes appear to utilize allochthonous, humic substances as a substrate.     

Export of DOC from forested catchments on the Precambrian Shield of Central Ontario: Clues from 13C and 14C

Export of dissolved organic carbon (DOC) from forested catchmentsis governed by competing processes of production, decomposition, sorptionand flushing. To examine the sources of DOC, carbon isotopes (¹⁴Cand ¹³C) were analyzed in DOC from surface waters, groundwatersand soils in a small forested catchment on the Canadian Shield in centralOntario. A significant fraction (greater than 50%) of DOCin major inflows to the lake is composed of carbon incorporated into organicmatter, solubilized and flushed into the stream within the last 40 years. Incontrast, ¹⁴C in groundwater DOC was old indicating extensiverecycling of forest floor derived organic carbon in the soil column beforeelution to groundwater in the lower B and C soil horizons. A small uplandbasin had a wide range in ¹⁴C from old groundwater values atbaseflow under dry basin conditions to relatively modern values during highflow or wetter antecedent conditions. Wetlands export mainly recently fixedcarbon with little seasonal range. DOC in streams entering the small lakemay be composed of two pools; an older recalcitrant pool delivered bygroundwater and a young labile pool derived from recent organic matter.The relative proportion of these two pools changes seasonally due thechanges in the water flowpaths and organic carbon dynamics. Althoughchanges in local climate (temperature and/or precipitation) may alterthe relative proportions of the old and young pools, the older pool islikely to be more refractory to sedimentation and decomposition in thelake setting. Delivery of older pool DOC from the catchment andsusceptibility of this older pool to photochemical decomposition mayconsequently be important in governing the minimum DOC concentrationlimit in lakes.     

Bacterial Contribution to Dissolved Organic Matter in Eutrophic Lake Kasumigaura,Japan

Incubation experiments using filtered waters from Lake Kasumigaura were conducted to examine bacterial contribution to a dissolved organic carbon (DOC) pool. Bacterial abundance, bacterial production, concentrations of DOC, total dissolved amino acids (TDAA), and total dissolved neutral sugars (TDNS) were monitored during the experiments. Bacterial production during the first few days was very high (20 to 35 μg C liter⁻¹ day⁻¹), accounting for 40 to 70% of primary production. The total bacterial production accounted for 34 to 55% of the DOC loss during the experiment, indicating high bacterial activities in Lake Kasumigaura. The DOC degradation was only 12 to 15%, whereas the degradation of TDAA and TDNS ranged from 30 to 50%, suggesting the preferential usage of TDAA and TDNS. The contribution of bacterially derived carbon to a DOC pool in Lake Kasumigaura was estimated using d-amino acids as bacterial biomarkers and accounted for 30 to 50% of the lake DOC. These values were much higher than those estimated for the open ocean (20 to 30%). The ratio of bacterially derived carbon to bulk carbon increased slightly with time, suggesting that the bacterially derived carbon is more resistant to microbial degradation than bulk carbon. This is the first study to estimate the bacterial contribution to a DOC pool in freshwater environments. These results indicate that bacteria play even more important roles in carbon cycles in freshwater environments than in open oceans and also suggests that recent increases in recalcitrant DOC in various lakes could be attributed to bacterially derived carbon. The potential differences in bacterial contributions to dissolved organic matter (DOM) between freshwater and marine environments are discussed.     

Carbon dioxide consumption during soil development

Carbon is sequestered in soils by accumulation of recalcitrant organic matter and by bicarbonate weathering of silicate minerals. Carbon fixation by ecosystems helps drive weathering processes in soils and that in turn diverts carbon from annual photosynthesis-soil respiration cycling into the long-term geological carbon cycle. To quantify rates of carbon transfer during soil development in moist temperate grassland and desert scrubland ecosystems, we measured organic and inorganic residues derived from the interaction of soil biota and silicate mineral weathering for twenty-two soil profiles in arkosic sediments of differing ages. In moist temperate grasslands, net annual removal of carbon from the atmosphere by organic carbon accumulation and silicate weathering ranges from about 8.5 g m^–2 yr^–1 for young soils to 0.7 g M^–2 yr^–1 for old soils. In desert scrublands, net annual carbon removal is about 0.2 g m^–2 yr^–1 for young soils and 0.01 g m^–2 yr^–1 for old soils. In soils of both ecosystems, organic carbon accumulation exceeds CO₂ removal by weathering, however, as soils age, rates of CO₂ consumption by weathering accounts for greater amounts of carbon sequestration, increasing from 2% to 8% in the grassland soils and from 2% to 40% in the scrubland soils. In soils of desert scrublands, carbonate accumulation far outstrips organic carbon accumulation, but about 90% of this mass is derived from aerosolic sources that do not contribute to long-term sequestration of atmospheric carbon dioxide.     

Characterization and origin of polar dissolved organic matter from the Great Salt Lake

Polar dissolved organic matter (DOM) was isolated from a surface-water sample from the Great Salt Lake by separating it from colloidal organic matter by membrane dialysis, from less-polar DOM fractions by resin sorbents, and from inorganic salts by a combination of sodium cation exchange followed by precipitation of sodium salts by acetic acid during evaporative concentration. Polar DOM was the most abundant DOM fraction, accounting for 56% of the isolated DOM. Colloidal organic matter was ¹⁴C-age dated to be about 100% modern carbon and all of the DOM fractions were ¹⁴C-age dated to be between 94 and 95% modern carbon. Average structural models of each DOM fraction were derived that incorporated quantitative elemental and infrared, ¹³C-NMR, and electrospray/mass spectrometric data. The polar DOM model consisted of open-chain N-acetyl hydroxy carboxylic acids likely derived from N-acetyl heteropolysaccharides that constituted the colloidal organic matter. The less polar DOM fraction models consisted of aliphatic alicyclic ring structures substituted with carboxyl, hydroxyl, ether, ester, and methyl groups. These ring structures had characteristics similar to terpenoid precursors. All DOM fractions in the Great Salt Lake are derived from algae and bacteria that dominate DOM inputs in this lake.     

A preliminary carbon budget for a part of the Ems estuary: The Dollard

During 1975, measurements were made to quantify all sources of input of organic matter in the Dollard. This made a comparison possible between in situ primary production, import from natural sources and organic waste discharges in terms of organic carbon. In order to make a carbon budget, mineralization and the amount of organic matter buried in the sediment was also measured. Input of organic carbon was mainly based on primary production on the tidal flats (measured in situ as O₂ production, 9.3×10⁶ kg C · year^–1), accumulation of suspended matter originating from the North Sea and the River Ems (maximal 37.1×10⁶ kg C · year^–1) and discharge of heavily polluted water (33.0×10⁶ kg C · year^–1). Input from primary production in the water phase was negligibly low (0.7×10⁶ kg C · year^–1). Loss of organic carbon was due to mineralization in the sediment (measured in situ as oxygen consumption, 18.2×10⁶ kg C · year^–1), mineralization in the water phase (using the BOD technique, 7.2×10⁶ kg C · year^–1) and burying of organic matter in the sediment (9.9·10⁶ kg C · year^–1). The loss of dissolved organic matter to the adjacent Waddensea was not measured but must be considerable. Allochthonous detritus was the main source of energy for the food-webs in the Dollard. The role of bacteria as an important source of food for higher organisms in the Dollard is discussed.     

Concentrations and fluxes of organic carbon substrates in the aquatic environment

Data concerning concentrations and fluxes of dissolved organic compounds (DOC) from marine and lacustrine environments are reviewed and discussed. Dissolved free amino acids and carbohydrates comprised the main fraction in the labile organic carbon pool. Dissolved free amino acids in marine waters varied between 3–1400 nM and those of freshwaters between 2.6–4124 nM. Dissolved free carbohydrates varied between 0.4–5000 nM in marine systems and between 14–1111 nM in freshwaters. The turnover times of both substrate pools varied in marine waters between 1.4 hours and 948 days and in freshwaters between 2 hours and 51 days. Measurements of stable^12/13C-ratio and¹⁴C-isotope dating in ocean deep water samples revealed DOC turnover times between 2000–6000 years. Studies on carbon flows within the aquatic food webs revealed that about 50% of photosynthetically fixed carbon was channelled via DOC to the bacterioplankton. Excreted organic carbon varied between 1–70% of photosynthetically fixed carbon in marine waters and between 1–99% in freshwaters. The labile organic carbon pool represented only 10–30% of the DOC. The majority (70–90%) of the DOC was recalcitrant to microbial assimilation. Only 10–20% of the DOC could be easily chemically identified. Most of the large bulk material represented dissolved humic matter and neither the chemical structure nor the ecological function of the DOC is as yet clearly understood.Abbreviations ATP Adenosine Tri-Phosphate - AMS Accelerated Mass Spectrometry - BSA Bovine Serum Albumin - GlAse GlucosidAse activity - DAA Dissolved Amino Acids - DCAA Dissolved Combined Amino Acids - DFAA Dissolved Free Amino Acids - DTAA Dissolved Total Amino Acids - DCHO Dissolved Carbohydrates - DCCHO Dissolved Combined Carbohydrates - DFCHO Dissolved Free Carbohydrates - DTCHO Dissolved Total Carbohydrates - DLCFaAc Dissolved Long Chain Fatty Acids - DSCFaAc Dissolved Short Chain Fatty Acids - DOC Dissolved Organic Carbon - DOM Dissolved Organic Matter - DHM Dissolved Humic Matter - DTPhOH Dissolved Total Phenolic compounds - DCPhOH Dissolved Combined Phenolic compounds - DFPhOH Dissolved Free Phenolic conpounds - EOC Excreted Organic Carbon - HS Humic Substances - HPLC High Performance Liquid Chromatography - HTCO High-Temperature Catalytic Oxidation - (K_t+S_n) Transport Constant + Natural Substrate from Michaelis Menten Kinetics - LOCP Labile Organic Carbon Pool - OM Organic Matter - MEE Microbial Extracellular Enzymes - PER Percent of Extracellular Release - PhDOC Photosynthetically derived Dissolved Organic Carbon - POC Particulate Organic Carbon - ROCP Recalcitrant Organic Carbon Pool - T_t Turnover time - UDOC Utilizable Dissolved Organic Carbon - V_max Maximum Uptake Velocity - WCO Wet Chemical Oxidation Dedicated to Prof. Drs. J. Overbeck on the occasion of his 70th birthday     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Phytoplankton production, exudation and bacterial reassimilation in the River Meuse (Belgium)

Extracellular release of phytoplankton was investigated in theupper middle River Meuse (Belgium) in order to estimate thecontribution of this process as a source of dissolved organicmatter in the system. Particulate primary production, exudationand reassimilation of exudates by free-living and attached bacteriawere estimated in parallel using¹⁴C labelling and selectivefiltration techniques. The results showed that total phytoplantonexudation (EOCt) represented on average 7% of the particulateprimary production. Of this excreted organic carbon, 77% wastaken up by bacteria; attached bacteria contributed 30% of thisuptake. Calculations showed that in this river system, the excretionof organic carbon by algae accounted on average for 22% of thebacterial carbon demand.     

Temperature Sensitivity of Soil Organic Carbon Mineralization along an Elevation Gradient in the Wuyi Mountains,China

Soil organic carbon (SOC) actively participates in the global carbon (C) cycle. Despite much research, however, our understanding of the temperature sensitivity of soil organic carbon (SOC) mineralization is still very limited. To investigate the responses of SOC mineralization to temperature, we sampled surface soils (0–10 cm) from evergreen broad-leaf forest (EBF), coniferous forest (CF), sub-alpine dwarf forest (SDF), and alpine meadow (AM) along an elevational gradient in the Wuyi Mountains, China. The soil samples were incubated at 5, 15, 25, and 35°C with constant soil moisture for 360 days. The temperature sensitivity of SOC mineralization (Q₁₀) was calculated by comparing the time needed to mineralize the same amount of C at any two adjacent incubation temperatures. Results showed that the rates of SOC mineralization and the cumulative SOC mineralized during the entire incubation significantly increased with increasing incubation temperatures across the four sites. With the increasing extent of SOC being mineralized (increasing incubation time), the Q₁₀ values increased. Moreover, we found that both the elevational gradient and incubation temperature intervals significantly impacted Q₁₀ values. Q₁₀ values of the labile and recalcitrant organic C linearly increased with elevation. For the 5–15, 15–25, and 25–35°C intervals, surprisingly, the overall Q₁₀ values for the labile C did not decrease as the recalcitrant C did. Generally, our results suggest that subtropical forest soils may release more carbon than expected in a warmer climate.     

Biochemical Composition of Dissolved Organic Carbon Derived from Phytoplankton and Used by Heterotrophic Bacteria

The molecular size distribution and biochemical composition of the dissolved organic carbon released from natural communities of lake phytoplankton (photosynthetically produced dissolved organic carbon [PDOC]) and subsequently used by heterotrophic bacteria were determined in three lakes differing in trophic status and concentration of humic substances. After incubation of epilimnetic lake water samples with H¹⁴CO₃^- over one diel cycle, the phytoplankton were removed by size-selective filtration. The filtrates, still containing most of the heterotrophic bacteria, were reincubated in darkness (heterotrophic incubation). Differences in the amount and composition of PDO¹⁴C between samples collected before the heterotrophic incubation and samples collected afterwards were considered to be a result of bacterial utilization. The PDO¹⁴C collected at the start of the heterotrophic incubations always contained both high (>10,000)- and low (<1,000)-molecular-weight (MW) components and sometimes contained intermediate-MW components as well. In general, bacterial turnover rates of the low-MW components were fairly rapid, whereas the high-MW components were utilized slowly or not at all. In the humic lake, the intermediate-MW components accounted for a large proportion of the net PDO¹⁴C and were subject to rapid bacterial utilization. This fraction probably consisted almost entirely of polysaccharides of ca. 6,000 MW. Amino acids and peptides, other organic acids, and carbohydrates could all be quantitatively important parts of the low-MW PDO¹⁴C that was utilized by the heterotrophic bacteria, but the relative contributions of these fractions differed widely. It was concluded that, generally, low-MW components of PDOC are quantitatively much more important to the bacteria than are high-MW components, that PDOC released from phytoplankton does not contain substances of quantitative importance as bacterial substrates in all situations, and that high-MW components of PDOC probably contribute to the buildup of refractory, high-MW dissolved organic carbon in pelagic environments.     

The feeding biology of Hydrobia ventrosa (Montagu). II. Allocation of the components of the carbon-budget and the significance of the secretion of dissolved organic material

The deposit-feeding prosobranch Hydrobia ventrosa (Montagu) was fed ¹⁴C-labelled food for a short period. As food, sterile detritus (homogenously ¹⁴C-labelled, dried barley hay), detritus with attached bacteria, and pure bacteria were used. The distribution of the ingested ¹⁴C was followed for a 24-h period. It was found that the assimilation efficiencies of sterile hay, hay with bacteria, and pure bacteria were 34, 56, and 70 %, respectively. This indicates a significance of bacteria for deposit-feeders. There is a considerable loss of dissolved organic material, in part due to leakage from faecal pellets (13 % of the ingested C in the case of a pure bacterial meal and 7 % of the ingested C in the case of sterile hay). The animals also excrete about 30 % of the assimilated carbon. Excretion of mucus constitutes about 9 % of the assimilated carbon. The fraction of assimilated carbon respired depends on the nature of the food. For sterile hay, hay with bacteria, and pure bacteria the percentage respired was 53, 30, and 38 %, respectively. Growth efficiency is, therefore, higher when protein-rich bacteria are included in the diet.     

Phytoplankton, bacterial production and protozoan bacterivory in stratified, brackish-water Lake Shira (Khakasia, Siberia)

Rates of oxygenic and anoxygenic photosynthesis, chemoautotrophic and heterotrophic bacterial production and protozoan bacterivory were measured in the pelagic zone of the stratified brackish-water lake with the purpose to determine the vertical distribution of these processes and to estimate their significance in the functioning of planktonic community of the lake. In midsummer, total daily primary productivity was about 1.3 g C m^–2, of which 72% was produced by the phytoplankton, 24% by the chemoautotrophic bacteria, and only 4% by the phototrophic sulphur bacteria. Thus anoxygenic photosynthesis is a negligible source of organic matter in the lake. The production of heterotrophic bacteria averaged 1.5 g C m^–2 d^–1 and exceeded the total photosynthesis of phytoplankton and photosynthetic bacteria by a factor of 1.5. The estimated total primary production was too low to sustain the bacterial production. Probably the carbon cycle in the lake is dependent on the input of allochthonous organic matter. As a rule, the maximal rates of primary production and heterotrophic bacterial production were found in the chemocline or at the upper boundary of the chemocline. Heterotrophic flagellates dominated among the protozoan populations and were the major consumers of the bacterioplankton production in the lake. They showed maximal ingestion rates from 2.3 to 2.9 mg C m^–3 h^–1 at the upper boundary of the chemocline, where they consumed from 50 to 54% of the production of heterotrophic bacteria. Data obtained indicate that in Lake Shira the oxic-anoxic interface is the site of the most intensive production and mineralization of organic matter.     

Carbon balance of a tropical savanna of northern Australia

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO₂ efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204±53 ton C ha^–1, with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0–1 m) was 151±33 ton C ha^–1, accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53±20 ton C ha^–1, 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha^–1, of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha^–1 year^–1, of which 8.0 ton C ha^–1 (73%) was contributed by below-ground net primary production and 3.0 ton C ha^–1 (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha^–1 year^–1. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5–6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha^–1 year^–1.     

Protozoan Grazing, Bacterial Activity, and Mineralization in Two-Stage Continuous Cultures

In two-stage continuous cultures, at bacterial concentrations, biovolumes, and growth rates similar to values found in Lake Vechten, ingestion rates of heterotrophic nanoflagellates (HNAN) increased from 2.3 bacteria HNAN⁻¹ · h⁻¹ at a growth rate of 0.15 day⁻¹ to 9.2 bacteria · HNAN⁻¹ · h⁻¹ at a growth rate of 0.65 day⁻¹. On a yeast extract medium with a C/N/P ratio of 100:15:1.2 (Redfield ratio), a mixed bacterial population showed a yield of 18% (C/C) and a specific carbon content of 211 fg of C · μm⁻³. The HNAN carbon content and yield were estimated at 127 fg of C · μm⁻³ and 47% (C/C). Although P was not growth limiting, HNAN accelerated the mineralization of PO₄-P from dissolved organic matter by 600%. The major mechanism of P remineralization appeared to be direct consumption of bacteria by HNAN. N mineralization was performed mainly (70%) by bacteria but was increased 30% by HNAN. HNAN did not enhance the decomposition of the relatively mineral-rich dissolved organic matter. An accelerated decomposition of organic carbon by protozoa may be restricted to mineral-poor substrates and may be explained mainly by protozoan nutrient regeneration. Growth and grazing in the cultures were compared with methods for in situ estimates. Thymidine incorporation by actively growing bacteria yielded an empirical conversion factor of 1.1 × 10¹⁸ bacteria per mol of thymidine incorporated into DNA. However, nongrowing bacteria also showed considerable incorporation. Protozoan grazing was found to be accurately measured by uptake of fluorescently labeled bacteria, whereas artificial fluorescent microspheres were not ingested, and selective prokaryotic inhibitors blocked not only bacterial growth but also protozoan grazing.     

Effect of rice plants on methane production and rhizospheric metabolism in paddy soil

In order to elucidate the effects of rice plants on CH₄ production, we conducted experiments with soil slurries and planted rice microcosms. Methane production in anoxic paddy soil slurries was stimulated by the addition of rice straw, of unsterile or autoclaved rice roots, and of the culture fluid in which rice plants had axenically been cultivated. The addition of these compounds also increased the concentrations of acetate and H₂, precursors of CH₄ production, in the soil. Planted compared to unplanted paddy soil microcosms exhibited lower porewater CH₄ concentrations but higher CH₄ emission rates. They also exhibited higher sulfate concentrations but similar nitrate concentrations. Concentrations of acetate, lactate and H₂ were not much different between planted and unplanted microcosms. Pulse labeling of rice plants with¹⁴CO₂ resulted during the next 5 days in transient accumulation of radioactive lactate, propionate and acetate, and after the second day of incubation in the emission of¹⁴CH₄. Most of the radioactivity (40–70%) was incorporated into the above-ground biomass of rice plants. However, during a total incubation of 16 days about 3–6% of the applied radioactivity was emitted as¹⁴CH₄, demonstrating that plant-derived carbon was metabolized and significantly contributed to CH₄ production. The sequence of the appearance of radioactive products and their specific radioactivities indicate that CH₄ was produced from root exudates by a microbial community consisting of fermenting and methanogenic bacteria.     

Phloem bleeding from legume fruits—A technique for study of fruit nutrition

Summary Bleeding from phloem of cut distal tips of attached fruits was demonstrated in the genera Spartium, Genista, Lupinus and Jacksonia. Bleeding occurred over a 2–25 min period enabling 0.5–10 l of sap to be collected from a fruit. A detailed study of Lupinus albus L. showed that exudation rate declined exponentially after cutting, but without any change with time in solute levels in exudate. Bleeding resumed at its initial rate and solute concentration on recutting the fruit tip.Phloem exudates had a high pH (7.8-8.0), a sucrose content of 100–210 mg ml^-1 but only traces of monosaccharides. Surrounding pod tissues contained only 15–35 mg ml^-1 of sugars (tissue water basis) more than two thirds of this monosaccharide. Amino compounds were present in phloem exudates at 8–28 mg ml^-1, asparagine and glutamine predominating but a wide spectrum of other amino acids being also present. No significant differences in levels of organic solutes were observed in phloem exudates collected from tips of attached versus detached fruits, from phloem exudates collected from fruit tips versus pedicels, or from basal versus distal ends of a detached fruit.Potassium was the major cation (1.5–2.2 mg ml^-1) of the phloem exudate, Ca²⁺ was at a much lower level than either Mg²⁺ or Na⁺. Trace element levels in phloem exudates appeared to be influenced by availability to the plant from the rooting medium. Nitrate was absent though detectable in non-vascular tissues of the shoot. ¹⁴C- labelled assimilates were detected in exudates of L. albus one hour after feeding a source leaf ¹⁴CO₂; sucrose, organic acids and certain amino compounds achieved high specific labelling. ¹⁴CO₂ feeding studies coupled with the phloem bleeding technique revealed highly specific source-sink relationships between foliar organs and fruits of the primary inflorescence.     

抚育间伐对长白落叶松人工林土壤碳、氮及其组分的影响

抚育间伐作为重要的森林经营措施之一,能够改变林分结构和稳定性,进而影响森林生态系统的生物地球化学循环.然而,抚育间伐对森林土壤碳、氮循环的影响程度如何尚不明确,尤其缺少长期试验结果报道.本研究以黑龙江省孟家岗林场经过4种不同强度和频度的抚育间伐处理后的60年生长白落叶松人工林为研究对象(4次低强度的间伐,LT4;3次中等强度的间伐,MT3;2次高强度间伐,HT2;不进行间伐的对照,CK),从酸水解法划分土壤碳、氮库(活性碳、氮库Ⅰ,活性碳、氮库Ⅱ和惰性碳、氮库)的角度研究了抚育间伐对长白落叶松人工林土壤总有机碳、全氮的影响机制.结果表明: 抚育间伐显著增加了土壤有机碳和全氮含量,增幅分别高达48.7%～50.3%和28.9%～42.7%.抚育间伐均增加了3种碳、氮组分的含量,而增加的程度因碳、氮组分和抚育间伐措施的不同而异.与活性碳库Ⅰ和活性碳库Ⅱ的增加程度相比,惰性碳库的增加程度最大,LT4、MT3和HT2处理下惰性碳库分别增加71%、69%和75%.此外,抚育间伐也显著增加了惰性碳占土壤总有机碳的比例.LT4显著提高了土壤微生物生物量碳、氮含量和微生物熵,而MT3和HT2对微生物生物量碳、氮和微生物熵却无显著影响.抚育间伐可能通过产生较多的粗木质残体于土体中,增加土壤木栓质和木质素等顽固组分的输入,进而导致土壤惰性碳含量增加,降低有机质的分解,最终导致土壤有机碳增加.     

Photochemical mineralization of synthetic lignin in lake water indicates enhanced turnover of aromatic organic matter under solar radiation

The degradation of ¹⁴C-[ring]-labelled syntheticlignin (¹⁴C-DHP) and dissolved organic carbon(DOC) from lake water were studied simultaneously.¹⁴C-DHP was incubated in humic lake water (colour173 mg Pt l^-1) for 7 d in the dark or under solarradiation. In the dark <0.4% of the introduced¹⁴C-DHP label and 4% of the indigenous DOC weremineralized, indicating that the ¹⁴C-labelledaromatic rings of DHP and the humic DOC weremicrobiologically recalcitrant. Under solar radiation(116 MJ m^-2), 17–21% of the ¹⁴C-labelledcarbons in DHP and 18–23% of the indigenous DOC weremineralized in 7 d. Simultaneously the watersolubility of ¹⁴C-DHP increased. Solar radiationconverted the aromatic cores of synthetic lignin toCO₂ and soluble organic photoproducts. Theresults suggest that solar radiation plays a key rolein the decomposition of natural polyaromatic matter.     

Role of plankton in the turnover of organic matter on the Great Barrier Reef,Australia

Number, biomass and production of phytoplankton, bacteria, micro- and mesozooplankton and turnover of labile and stable organic matter were measured in waters over some Capricornia round reefs, and over the reefs of Lizard Island. Primary production was 10 to 40 mg C m^–3 d^–1 but was lower over the living reefs. Microbial wet biomass in reef waters varied from 100 to 500 mg m^–3, and production from 4 to 68 mg C m^–3 d^–1, which was commensurable with primary production. The biomass of microzooplankton (ciliates, zooflagellates and larvae) in waters of Lizard Island reefs reached 100–300 mg m^–3. Mesozooplankton biomass at night in reef waters of Heron Island varied from 200 to 800 mg m^–3. Its composition depended upon the tide phase. PB coefficients in bacterioplankton were 0.3 to 1.2 per day. The food demand of bacterioplankton in waters over the reefs was 5 to 20 times higher than the primary phytoplankton production. Labile organic matter (LOM) doubled in waters after it stayed over living reef for several hours. The turnover time of LOM in reef waters was as short as 1–2 weeks.