Light affects competition for inorganic and organic nitrogen between maize and rhizosphere microorganisms

Effects of light on the short term competition for organic and inorganic nitrogen between maize and rhizosphere microorganisms were investigated using a mixture of amino acid, ammonium and nitrate under controlled conditions. The amount and forms of N added in the three treatments was identical, but only one of the three N forms was labeled with ¹⁵N. Glycine was additionally labeled with ¹⁴C to prove its uptake by maize and incorporation into microbial biomass in an intact form. Maize out-competed microorganisms for during the whole experiment under low and high light intensity. Microbial uptake of ¹⁵N and ¹⁴C was not directly influenced by the light intensity, but was indirectly related to the impact the light intensity had on the plant. More was recovered in microbial biomass than in plants in the initial 4 h under the two light intensities, although more ¹⁵N-glycine was incorporated into microbial biomass than in plants in the initial 4 h under low light intensity. Light had a significant effect on uptake by maize, but no significant effects on the uptake of or ¹⁵N-glycine. High light intensity significantly increased plant uptake of and glycine ¹⁴C. Based on ¹⁴C to ¹⁵N recovery ratios of plants, intact glycine contributed at least 13% to glycine-derived nitrogen 4 h after tracer additions, but it contributed only 0.5% to total nitrogen uptake. These findings suggest that light intensity alters the competitive relationship between maize roots and rhizosphere microorganisms and that C4 cereals such as maize are able to access small amounts of intact glycine. We conclude that roots were stronger competitor than microorganisms for inorganic N, but microorganisms out competed plants during a short period for organic N, which was mineralized into inorganic N within a few hours of application to the soil and was thereafter available for root uptake.     

Productivity, growth rates and cell size distributions of phytoplankton in the SW Tasman Sea: implications for carbon metabolism in the photic zone

Data are presented on primary productivity, cell size distributionsand the standing stocks of living and detrital paniculate organiccarbon (POC) in the waters of the SW Tasman Sea. Primary productivitywas measured by both standard 4- and 12-h incubations as wellas time-series incubations. Data are presented for ¹⁴C uptakeand loss in 12L/12D methods. The importance of time zero anddark controls is demonstrated. The uptake of ¹⁴C in the lightwas linear and the loss of label in the following dark periodranged from zero to 39%. The loss of label in the dark was correlatedwith the particle size distribution, being greatest in oligotrophicwaters dominated by small cells (25–30%) and least inspring bloom areas (0–20%) dominated by large diatoms.Kinetic data were strongly supportive of a major grazing impactby microphagous organisms. The data were an experimental confirmationof recent theoretical models of ¹⁴C uptake and grazing. Sizedistributions of phytoplankton and detritus were measured byHIAC and by microscopic counting. The phytoplankton consistedof a ubiquitous group of picoplankton, and variable contributionsfrom small flagellates and diatoms. The distribution of totalcell volume was dominated by large cells in spring bloom areas.Chlorophyll concentrations were strongly correlated with themean cell size of the phytoplankton. Comparison of the resultsof ¹⁴C uptake experiments with the results of experiments todetermine changes in POC, by counting particles, gave good correlation.In detail, the comparison of the methods revealed systematicerrors with the greatest discrepancy between the methods atlow apparent growth rates. The detritus showed constant sizedistributions in surface waters. The mean size of detritus particlesreduced rapidly with depth and declined in a way suggestingbiological reprocessing and removal by grazing. These resultsare discussed in the context of the patterns of carbon metabolismin the photic zone, the role of living and detrital POC andthe balance of ‘new’ versus ‘regenerated’production in surface waters.     

The influence of the spectral composition of irradiance on primary production in the Eastern Scheldt (The Netherlands)

The error inin vivo ¹⁴C incubator measurements of primary production in the Eastern Scheldt when neutral density filters were used and the error obtained when no account was taken of the spectral changes in submarine irradiance that occur with increasing depth, were evaluated theoretically. By multiplying the photosynthetic action spectra of two marine algae by calculated irradiance in the euphotic layer using Kd and Kd() respectively, the gross primary production P[Ed(400–700)] and P[Ed()] was computed. In the green-brown waters of the Eastern Scheldt estuary the use of neutral density filters was sufficient to simulate the underwater light conditions. In clear waters it can cause an overestimation of the gross production.     

A simple and inexpensive multiple tube system for thein situ measurement of phytoplankton production by the14C method

A cheap and simple multiple tube incubation system is described for the in situ measurement of phytoplankton production by the ¹⁴C method. Incubation takes place in Pyrex glass tubes sealed with rubber suba-seal caps which are suitable for field, laboratory and incubator work. Comparison of production in the tubes with that in standard pyrex bottles showed no significant difference during both field and laboratory incubations. The in situ suspension system is transported in units and can be assembled into ladders in the field.     

Effect of vertical cycling on photosynthesis during the annual algal succession in the northern Baltic

During the phytoplankton succession in 1984 and 1985, the effect of fluctuating light on algal photosynthesis (incorporation of ¹⁴C, acidified water sample) was studied in the northern Baltic. Bottles were mounted on moving racks that mimicked vertical transport caused by Langmuir circulations in the trophogenic layer. Assuming that the photoinhibition observed near the surface in fixed-depth incubations (from 1 to 8% of integral photosynthesis) was avoided in cycled samples, vertical cycling conducted around noon resulted in on average 10% lower photosynthesis than fixed-depth incubations (n = 17). This difference lies within the 5% confidence limits of the measurement, and hence it was concluded that the lack of short-term fluctuations in light associated with the vertical circulation of natural phytoplankton communities does not seriously bias conventional in situ ¹⁴CO₂ fixation measurements performed at fixed depths in the study area.     

Application of <Superscript>13</Superscript>C mineral carbon for assessment of the primary production of organic matter in aquatic environments

The possibility of measuring the rates of light and dark CO₂ assimilation using ¹³C carbonate was demonstrated on Lake Kichier (Marii El). The application of methods utilizing the stable ¹³C and the radioactive ¹⁴C isotopes resulted in comparable values of the rates of light and dark CO₂ fixation. Due to its absolute environmental safety, the method with ¹³C mineral carbon can be recommended as an alternative to radioisotope methods for qualitative measurements of CO₂ fixation rates in aquatic ecosystems.     

Phloem transport in Picea abies (L.) Karst. in mid-winter

Summary Translocation of ¹⁴C assimilates was studied on four different transport systems of Picea abies branches after induced activation in January. ¹⁴CO₂ assimilation of terminal shoots for 48 h at 25° C resulted in phloem loading and basipetal transport of ¹⁴C photosynthate into the following, older shoot generations. ¹⁴C import was enhanced, when these older shoot generations were kept in the dark. Microautoradiographs of the labelled terminal shoots showed that ¹⁴C assimilates were exported from needles via sieve elements of the leaf traces and loaded into the latest increment of the axial secondary phloem. No ¹⁴C label appeared in the obliterated sieve cells or in the tracheids. In addition, ¹⁴C photosynthate accumulated densely in the chlorophyllous cells of the cortex and in cells of the resin ducts, indicating certain sink activity. In the darkened 2-year-old shoot, imported ¹⁴C photosynthate was concentrated in the functional secondary phloem, while some ¹⁴C label was unloaded into the latest xylem increment. When 6-year-old shoots were exposed to ¹⁴CO₂ for 48 h in the light, ¹⁴C assimilates accumulated in the phloem of the leaf trace and in the latest increment of the axial secondary phloem. However, a substantial amount of radioactivity was unloaded into ray cells and phloem parenchyma cells. Thus, the presence of functioning phloem in needles and twigs of P. abies during winter allows long-distance translocation and radial distribution of assimilates according to existing source-sink relations.     

Microautoradiographic studies of phloem loading and transport in the leaf of Zea mays L.

Microautoradiographs showed that [¹⁴C]sucrose taken up in the xylem of small and intermediate (longitudinal) vascular bundles of Zea mays leaf strips was quickly accumulated by vascular parenchyma cells abutting the vessels. The first sieve tubes to exhibit ¹⁴C-labeling during the [¹⁴C]sucrose experiments were thick-walled sieve tubes contiguous to the more heavily labeled vascular parenchyma cells. (These two cell types typically have numerous plasmodesmatal connections.) With increasing [¹⁴C]sucrose feeding periods, greater proportions of thick- and thin-walled sieve tubes became labeled, but few of the labeled thin-walled sieve tubes were associated with labeled companion cells. (Only the thin-walled sieve tubes are associated with companion cells.) When portions of leaf strips were exposed to ¹⁴CO₂ for 5 min, the vascular parenchyma cells-regardless of their location in relation to the vessels or sieve tubes-were the most consistently labeled cells of small and intermediate bundles, and label (¹⁴C-photosynthate) appeared in a greater proportion of thin-walled sieve tubes than thick-walled sieve tubes. After a 5-min chase with ¹²CO₂, the thin-walled sieve tubes were more heavily labeled than any other cell type of the leaf. After a 10-min chase with ¹²CO₂, the thin-walled sieve tubes were even more heavily labeled. The companion cells generally were less heavily labeled than their associated thin-walled sieve tubes. Although all of the thick-walled sieve tubes were labeled in portions of leaf strips fed ¹⁴CO₂ for 5 min and given a 10-min ¹²CO₂ chase, only five of 72 vascular bundles below the ¹⁴CO₂-exposed portions contained labeled thick-walled sieve tubes. Moreover, the few labeled thick-walledsieve tubes of the transport region always abutted ¹⁴C-labeled vascular parenchyma cells. The results of this study indicate that (1) the vascular parenchyma cells are able to retrieve at least sucrose from the vessels and transfer it to the thick-walled sieve tubes, (2) the thick-walled sieve tubes are not involved in long-distance transport, and (3) the thin-walled sieve tubes are capable themselves of accumulating sucrose and photosynthates from the apoplast, without the companion cells serving as intermediary cells.     

Comparison of Coulter Counter and 14C measurements of Acartia tonsa (Copepoda,Calanoida) grazing on Chlamydomonas sp. (Volvocales)

Both the Coulter Counter and ¹⁴C method were used to measure the grazing (clearance rates) of the marine calanoid copepod Acartia tonsa on different concentrations of a Chlamydomonas sp. culture. In most cases, clearance rates measured by the Coulter Counter method were higher than those measured by the ¹⁴C method by factors of 2 to 3. We explore several possibilities for the differences obtained between the two methods. We suggest that loss of radioactivity through grazer egestion might be the main reason for the discrepancy between methods. Food concentration did not affect the comparability of both methods' measurements.     

Below-ground carbon distribution in barley (Hordeum vulgare L.) with and without nitrogen fertilization

The distribution of net assimilated C in barley (Hordeum vulgare L.) grown at two N-levels was determined in a growth chamber. The N-fertilization involved 0 and 3.61 mol N g^-1 dry soil. After growth for seven weeks in an atmosphere with continuously ¹⁴C-labelled CO₂, ¹⁴C was determined in shoots, roots, rhizosphere respiration and soil. At the low N-level, 32% of the net assimilated ¹⁴C was translocated below ground, whereas at the high N-level 27% was translocated below ground. The release of C from roots (root respiration, microbial respiration originating from decomposition of ¹⁴C-labelled root material and ¹⁴C remaining in soil) was greater with no N-supply (19% of net assimilated ¹⁴C) than in the treatment with N-supply (15%). Thus, the effect of N-supply on both translocation of assimilated ¹⁴C below ground and the release of ¹⁴C from growing roots was relatively small.     

Soil organic carbon dynamics: variability with depth in forested and deforested soils under pasture in Costa Rica

Dynamics of soil organic carbon (SOC) inchronosequences of soils below forests that had beenreplaced by grazed pastures 3–25 years ago, wereinvestigated for two contrasting soil types (AndicHumitropept and Eutric Hapludand) in the Atlantic Zoneof Costa Rica. By forest clearing and subsequentestablishment of pastures, photosynthesis changes froma C-3 to a C-4 pathway. The accompanying changes inC-input and its ¹³C and ¹⁴Csignals, were used to quantify SOC dynamics. C-input from rootturnover at a pasture site was measured by sequentialharvesting and ¹⁴C-pulse labelling. With aspatial resolution of 5 cm, data on total SOC,¹³C and ¹⁴C of soil profileswere interpreted with a model that distinguishes threepools of SOC: active C, slow C and passive C,each with a 1^-st order decomposition rate(k_a, k_s and k_p). The modelincludes carbon isotope fractionation and depth-dependentdecomposition rates. Transport of C between soillayers was described as a diffusion process, whichaccounts for physical and biotic mixing processes.Calibrated diffusion coefficients were 0.42 cm²yr-¹ for the Humitropept and 3.97 cm²yr-¹ for the Hapludand chronosequence.Diffusional transport alone was insufficient foroptimal simulation; it had to be augmented bydepth-dependent decomposition rates to explain thedynamics of SOC, ¹³C and¹⁴C. Decomposition rates decreasedstrongly with depth. Upon increased diffusion,differences between calibrated decomposition rates ofSOC fractions between surface soils and subsoilsdiminished, but the concept of depth-dependentdecomposition had to be retained, to obtain smallresiduals between observed and simulated data. At areference depth of 15–20 cm k_s was 90 yr-¹in the Humitropept and 146 yr-¹ in the Hapludand.Slow C contributed most to total organic C in surfacesoils, whereas passive C contributed most below 40 cmdepth. After 18–25 years of pasture, net loss of C was2180 g C m-² for the Hapludand and 150 g m-²for the Humitropept soil.     

Spatial distribution of calcification and photosynthesis in the scleractinian coral Galaxea fascicularis

The spatial heterogeneity of photosynthesis and calcification of single polyps of the coral Galaxea fascicularis was investigated. Photosynthesis was investigated with oxygen microsensors. The highest rates of gross photosynthesis (Pg) were found on the tissue covering the septa, the tentacles, and the tissues surrounding the mouth opening of the polyp. Lower rates were found on the tissues of the wall and the coenosarc. Calcification was investigated by radioactive tracers. The incorporation pattern of ⁴⁵Ca and ¹⁴C in the corallites was imaged with use of a Micro-Imager. The -images obtained showed that the incorporation of the radioactive tracers coincided with the Pg distribution pattern with the highest incorporation rates found in the corallite septa. Thus, the high growth rate of the septa is supported by the high rates of Pg by the symbiont in the adjacent tissues. The total incorporation rates were higher in light than in dark, however, the distribution pattern of the radioisotope incorporation was not affected by illumination. This further emphasizes the close relation between calcification and photosynthesis.     

Residue fractionation and decomposition: The significance of the active fraction

This paper describes an incubation experiment with homogeneously ¹⁴C labeled maize-straw and its insoluble fraction. The role of the soluble fraction in the decomposition process was assessed, using three independently measured characteristics: (1) fractionation of the maize-straw, resulting in kinetically different fractions; (2) microbial biomass C and its ¹⁴C activity determined by a fumigation extraction method, and (3) the ¹⁴C activity of the released CO₂-C. The fumigation extraction method was proved to be useful from 9 days after the application of the maize-straw onwards. The fractionation method yielded a soluble (48%), a (hemi) cellulosic (47%), and a lignin fraction (1%). Nine days after addition of either the complete residue or its insoluble fraction, the microbial biomass C increased from 53 to 337 and 217 mg C kg^-1 dry soil, respectively. Similar values were maintained up to day 40. The large increase in microbial activity was accompanied by a N-immobilization of 65 and 29 mg N Kg^-1 dry soil for the maize-straw treatment and its insoluble fraction, respectively, resulting in biomass C/N values of 5.5 and 5.6 A genuine priming effect (10 and 7% of the total CO₂-C production) on the mineralization of native soil organic C was caused by an increase in decomposition of the native C rather than by an increase in turnover of the microbial biomass in the soil amended with maize straw. The soluble fraction caused a priming effect on the decomposition of the less decomposable cell-wall fraction. Calculations by nonlinear regression confirmed this observation.     

The symbiotic relationship between the hydrocoral Millepora dichotoma and the barnacle Savignium milleporum

Translocation of radioactive ¹⁴C and ³²P between the pyrgomatine barnacle Savignium milleporum and the hydrocoral Millepora dichotoma in the Red Sea was investigated in order to discover any mutual nutritional benefits. Translocation of photosynthetic products from endosymbiotic zooxanthellae to the hydrocoral was demonstrated. There was no evidence that carbon was further translocated to the barnacle. However, hydrocorals bearing barnacles accumulated significantly more ¹⁴C and ³²P than those with no barnacles. The possibility that the hydrocorals recycle substances excreted by the barnacles is discussed in the context of the oligotrophic environment of the Red Sea.     

Effect of salinity and pH on cobalt biosorption by the estuarine microalgaChlorella saliva

Summary Accumulation of cobalt (⁶⁰Co) by the estuarine microalgaChlorella salina has been characterized. At cobalt concentrations ranging over 3.125–100 M, a significant amount of cobalt was bound within 1 min. This was metabolism-independent and unaffected by incubation in light or dark conditions. This initial rapid phase of biosorption was followed by a slower phase of uptake which was apparently active and inhibited by incubation in the dark, or by the uncoupler dinitrophenol and the respiratory and photosynthetic inhibitor potassium cyanide in the light. For cells suspended in 10 mM Taps pH 8, cobalt biosorption followed a Freundlich adsorption isotherm. However, in the presence of 0.5 M NaCl, biosorption deviated from the Freundlich model because of competition by Na⁺. Cobalt biosorption was decreased by increasing concentrations of Na⁺, decreasing pH and the presence of Cs⁺, Li⁺, Rb⁺, Zn²⁺. Mn²⁺ and Sr²⁺ (added as chlorides). This was a result of competition between Co²⁺ and the other cations, including H⁺, for available binding sites on the cell wall and was confirmed by increased desorption of cobalt by solutions of low pH or high salinity. Increasing cell density resulted in increased removal of cobalt from solution but decreased the specific amount of cobalt taken up by the cells.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

Zooplankton grazing effects on 14C-based phytoplankton production measurements: a theoretical study

A model constructed to describe carbon dynamics of phytoplanktongrowth during ¹⁴C-tracer incubations gives the range of errorin estimating specific growth rates and productivity rates causedby zooplankton grazing during the incubation. Error increaseswith increasing incubation times and higher specific growthrates. The range of these errors can be given as a functionof the specific growth rate calculated from measurements. Atthe low calculated specific growth rates of the oligotrophicPacific Ocean, 0.2 d^–1, errors are 16%. Similar argumentssuggest that bacterial uptake of excreted organics would notcause large errors at low oligotrophic ocean growth rates. Thereare, however, other possible ways that ¹⁴C-based productivityestimates could be wrong.     

Assimilation of nod gene inducer 14C-naringenin and the incorporation of labelled carbon atoms into the acyl side chain of a host-specific Nod factor produced by Rhizobium leguminosarum bv. viciae

The fate of ¹⁴C-naringenin during its specific activation of nod genes in Rhizobium leguminosarum bv. viciae was examined. After incubation with either strain RBL5560 or its pSym-cured derivative in a medium supplemented with ¹⁴C-naringenin at nod gene-inducing concentrations of 2 nM (ca. 12.5 kBq) plus cold acetate (0.5 M), a radiocarbon inventory for the cells and supernatant extracts was obtained. The level of ¹⁴C-label incorporation was also determined in the fractionated cellular components. Using ¹⁴C-acetate at 0.5 M (1036 kBq) and cold naringenin (2 nM) in incubations with strain RBL5560 as a separate treatment, the Nod metabolites were detected by thin layer and high performance liquid chromatographic methods and the data provided the basis for identification of the Nod factors from the supernatant obtained from ¹⁴C-naringenin treatments. Subsequent radio-biochemical and chemical analyses revealed that RBL5560 cells assimilated ¹⁴C-naringenin during the activation of nod genes. Our analysis also showed that labelled carbon atoms from the ¹⁴C-naringenin were incorporated into the acyl moiety of a lipo-oligosaccharide Nod factor, NodRlv IV, present in the culture supernatants of RBL5560. The pSym-cured derivative failed to synthesize any Nod metabolites in a ¹⁴C-naringenin supplemented medium. The tracing of flavonoid-derived carbon atoms to the acyl chain of a host-specific Nod factor, a moiety that defines host specificity for this Rhizobium, adds a new dimension to the signalling function of flavonoids in legume-Rhizobium interactions.Abbreviations Ac acyl chain - ca calculated approximately - dpm disintegrations per minute - HPLC High Performance Liquid Chromatography - pSym symbiotic plasmid - R. Rhizobium - TLC Thin Layer Chromatography     

Efficient CO2 fixation by surface Prochlorococcus in the Atlantic Ocean

Nearly half of the Earth''s surface is covered by the ocean populated by the most abundant photosynthetic organisms on the planet—Prochlorococcus cyanobacteria. However, in the oligotrophic open ocean, the majority of their cells in the top half of the photic layer have levels of photosynthetic pigmentation barely detectable by flow cytometry, suggesting low efficiency of CO₂ fixation compared with other phytoplankton living in the same waters. To test the latter assumption, CO₂ fixation rates of flow cytometrically sorted ¹⁴C-labelled phytoplankton cells were directly compared in surface waters of the open Atlantic Ocean (30°S to 30°N). CO₂ fixation rates of Prochlorococcus are at least 1.5–2.0 times higher than CO₂ fixation rates of the smallest plastidic protists and Synechococcus cyanobacteria when normalised to photosynthetic pigmentation assessed using cellular red autofluorescence. Therefore, our data indicate that in oligotrophic oceanic surface waters, pigment minimisation allows Prochlorococcus cells to harvest plentiful sunlight more effectively than other phytoplankton.     

Azoxystrobin and soil interactions: degradation and impact on soil bacterial and fungal communities

Aims: To provide an independent assessment of azoxystrobin effects on nontarget soil bacteria and fungi and generate some baseline information on azoxystrobin’s persistence in soil. Methods and Results: Plate based assay showed that azoxystrobin exhibited differential toxicity upon cultured fungi at different application rates. While ¹⁴C labelled isotopes experiments showed that less than 1% of azoxystrobin was mineralized, degradation studies revealed over 60% azoxystrobin breakdown over 21 days. PCR DGGE analysis of 16S and 18S rRNA genes from different soil microcosms showed that azoxystrobin had some effects on fungal community after 21 days (up to 84 days) of incubation in either light or dark soil microcosms. Light incubations increased fungal diversity while dark incubations reduced fungal diversity. Bacterial diversity was unaffected. Conclusions: Significant biotic breakdown of parent azoxystrobin occurred within 21 days even in the absence of light. Azoxystrobin under certain conditions can reduce fungal soil diversity. Significance and Impact of the Study: One of the few independent assessments of azoxystrobin (a widely used strobilurins fungicide) effects on soil fungi when used at the recommended rate. Azoxystrobin and metabolites may persist after 21 days and affect soil fungi.