Crop uptake and leaching of 15N applied in ruminant slurry with selectively labelled faeces and urine fractions

Two animal slurries either labelled with ¹⁵N in the urine or in the faeces fraction, were produced by feeding a sheep with unlabelled and ¹⁵N-labelled hay and collecting faeces and urine separately. The slurries were applied (12 g total N ^-2) to a coarse sand and a sandy loam soil confined in lysimeters and growing spring barley (Hordeum vulgare L). Reference lysimeters without slurry were supplied with¹⁵ NH₄ ¹⁵NO³ corresponding to the inorganic N applied with the slurries (6 g N m^-2). In the second year, all lysimeters received unlabelled mineral fertilizer (6 g N m^-2) and grew spring barley. N harvested in the two crops (grain + straw) and the loss of nitrate by leaching were determined. ¹⁵N in the urine fraction was less available for crop uptake than mineral fertilizer ¹⁵N. The first barley crop on the sandy loam removed 49% of the ¹⁵N applied in mineral fertilizer and 36% of that applied with urine. The availability of fertilizer ¹⁵N (36%) and urine¹⁵ N (32%) differed less on the coarse sand. Of the¹⁵ N added with the faeces fraction, 12–14% was taken up by the barley crop on the two soils. N mineralized from faeces compensated for the reduced availability of urine N providing a similar or higher crop N uptake in manured lysimeters compared with mineral fertilized ones.About half of the total N uptake in the first crop originated from the N applied either as slurry or mineral fertilizer. The remaining N was derived from the soil N pool. Substantially smaller but similar proportions of¹⁵ N from faeces, urine and fertilizer were found in the second crop. The similar recoveries indicated a slow mineralization rate of the residual faeces N since more faeces was left in the soil after the first crop.More N was lost by leaching from manured lysimeters but as a percentage of N applied, losses were similar to those from mineral fertilizer. During the first and second winter, 3–5% and 1–3%, respectively, of the ¹⁵N in slurry and mineral fertilizer was leached as nitrate. Thus slurry N applied in spring just before sowing did not appear to be more prone to loss by nitrate leaching than N given in mineral fertilizer. Slurry N accounted for a higher proportion of the N leached, however, because more N was added in this treatment.     

Glyceria maxima for wastewater nutrient removal and forage production

Potential biomass yield, nutrient uptake capacity and forage quality of Glyceria maxima was tested in four field lysimeters (I–IV) receiving municipal wastewater. Wastewater was applied during 3 years at different frequencies in order to achieve maximum N-reduction in the system. Accordingly lysimeters III and IV were ponded for part of the week. Harvesting (in June and August) progressively favoured competing terrestrial species in the non-ponded lysimeters (I and II). In year 3 the other species accounted for 65–70% of the harvested biomass in these lysimeters. In the ponded lysimeters (III and IV), reducing conditions in the soil probably prevented terrestrial species from establishing. The total annual yields of the single-species stands in III and IV were 870–1165 gm⁻². Energy, protein, P, K and Ca content of the harvested biomass indicated a high nutritional value. NO₃⁻N concentrations in the harvested biomass varied from 0·08 to 0·32% of dry wt, but did not generally exceed 0·2%.Maximum N and P removed with the harvested biomass was 32 and 4·8 gm⁻² respectively. The relative removal of nutrients in lysimeters III and IV varied from 26 to 55% of the amount of N applied and from 12 to 28% of P applied, which illustrated the possibilities of adjusting the load for optimization of nutrient re-use rather than disposal.     

Contributions to nitrogen leaching and pasture uptake by autumn-applied dairy effluent and ammonium fertilizer labeled with 15N isotope

The objective of this study was to compare the N leaching loss and pasture N uptake from autumn-applied dairy shed effluent and ammonium fertilizer (NH₄Cl) labeled with ¹⁵N, using intact soil lysimeters (80 cm diameter, 120 cm depth). The soil used was a sandy loam, and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The DSE and NH₄Cl were applied twice annually in autumn (May) and late spring (November), each at 200 kg N ha^-1. The N applied in May 1996 was labeled with ¹⁵N. The lysimeters were either spray or flood irrigated during the summer. The autumn-applied DSE resulted in lower N leaching losses compared with NH₄Cl. However, the N applied in the autumn had a higher potential for leaching than N applied in late spring. Between 4.5–8.1% of the ¹⁵N-labeled mineral N in the DSE and 15.1–18.8% of the ¹⁵N-labeled NH₄Cl applied in the autumn were leached within a year of application. Of the annual N leaching losses in the DSE treatments (16.0–26.9 kg N ha^-1), a fifth (20.3–22.9%) was from the mineral N fraction of the DSE applied in the autumn, with the remaining larger proportion from the organic fraction of the DSE, soil N and N applied in spring. In the NH₄Cl treatments, more than half (53.8–64.8%) of the annual N leaching loss (55.9–57.6 kg N ha^-1) was derived from the autumn-applied NH₄Cl. DSE was as effective as NH₄Cl in stimulating pasture production. Since only 4.4–4.5% of the annual herbage N uptake in the DSE treatment and 12.3–13.3% in the NH₄Cl treatment were derived from the autumn-applied mineral N, large proportions of the annual herbage N uptake must have been derived from the N applied in spring, the organic N fraction in the DSE, soil N and N fixed by clover. The recoveries of ¹⁵N in the herbage were similar between the DSE and the NH₄Cl treatments, but those in the leachate were over 50% less from the DSE than from the NH₄Cl treatment. The lower leaching loss of ¹⁵N in the DSE treatment was attributed to the stimulated microbial activities and increased immobilization following the application of DSE. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen uptake and assimilation in Enteromorpha intestinalis (L.) Link (Chlorophyta): using N to determine preference during simultaneous pulses of nitrate and ammonium

We investigated the ability of Enteromorpha intestinalis (L.) Link to take up pulses of different species of nitrogen simultaneously, as this would be an important mechanism to enhance bloom ability in estuaries. Uptake rates and preference for NH₄⁺ or NO₃⁻ following 1, 3, 6, 9, 12 or 24 h of exposure to either ¹⁵NH₄NO₃ or NH₄¹⁵NO₃ were determined by disappearance of N from the medium. Differences in assimilation rates for NH₄⁺ or NO₃⁻ were quantified by the accumulation of NH₄⁺, NO₃⁻, and atom % ¹⁵N in the algal tissue. NH₄⁺ concentration was reduced more quickly than water NO₃⁻ concentration. Water column NH₄⁺ concentration after the longest time interval was reduced from 300 to 50 μM. Water NO₃⁻ was reduced from 300 to 150 μM. The presence of ¹⁵N or ¹⁴N had no effect on uptake of either NH₄⁺ or NO₃⁻. ¹⁵N was removed from the water at an almost identical rate and magnitude as ¹⁴N. Differences in accumulation of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ in the tissue reflected disappearance from the water; ¹⁵N from NH₄⁺ accumulated faster and reached an atom % twice that of ¹⁵N from NO₃⁻. This outcome suggested that when NH₄⁺ and NO₃⁻ were supplied in equal concentrations, more NH₄⁺ was taken up and assimilated. The ability to take up high concentrations of NH₄⁺, and NO₃⁻ simultaneously is important for bloom-forming species of estuarine macroalgae subject to multiple nutrient species from various sources.     

A comparison of the effects of subsoiling on plant uptake and leaching losses of sulphur and nitrogen from a simulated urine patch

Synthetic cow urine labelled with ³⁵S and ¹⁵N was applied to large, undisturbed, monolith lysimeters sampled from subsoiled and non-subsoiled areas of a grass/clover pasture. For one year following the urine application, the lysimeters were subjected to a combination of natural rainfall, simulated rainfall and simulated flood irrigations. Drainage from the lysimeters was sampled regularly and monthly (approx.) pasture cuts taken. At the end of the year, the lysimeters were destructively sampled in 50 mm depth increments for soil analysis. Leachates, plant samples and soil samples were analysed for ³⁵S and ¹⁵N.There were no significant differences in plant uptake of ³⁵S and ¹⁵N between the subsoiled and nonsubsoiled lysimeters. Initially grass showed a higher degree of labelling than clover. Total amounts of ³⁵S and ¹⁵N leached from the subsoiled lysimeters were approximately twice that leached from the nonsubsoiled ones. Leaching patterns differed substantially between the two nutrients.Total recoveries of ³⁵S (in plants, leachates and soil extracts) accounted for 82% of the applied ³⁵S for the subsoiled lysimeters and 72% for non-subsoiled ones. The unrecovered ³⁵S is considered to have been incorporated into soil organic matter. Total recoveries of ¹⁵N (in plants, soil and leachates) were similar to those for ³⁵S, but unrecovered ¹⁵N is attributed to loss by denitrification.     

The fate of15N-labelled organic nitrogen in sheep manure applied to soils of different texture under field conditions

The fate of nitrogen from¹⁵N-labelled sheep manure and ammonium sulfate in small lysimeters and plots in the field was studied during two growth seasons. In April 1991,¹⁵N-labelled sheep faeces (87 kg N ha^–1) plus unlabelled (NH₄)₂SO₄ (90 kg N ha^–1), and (¹⁵NH₄)₂SO₄ (90 kg N ha^–1) were each applied to three soils; soil 1 (100% soil + 0% quartz sand), soil 2 (50% soil + 50% quartz sand) and soil 3 (25% soil + 75% quartz sand). The lysimeters were cropped with spring barley (Hordeum vulgare L.) and undersown ryegrass (Lolium perenne L.). The barley crop recovered 16–17% of the labelled manure N and 56% of the labelled (NH₄)₂SO₄-N. After 18 months 30% of the labelled manure N and 65% of the labelled (NH₄)₂SO₄-N were accumulated in barley, the succeeding ryegrass crop and in leachate collected below 45 cm of soil, irrespective of the soil-sand mixture. Calculating the barley uptake of manure N by difference of N uptake between manured and unmanured soils, indicated that 4%, 10% and 14% of the applied manure N was recovered in barley grown on soil-sand mixtures with 16%, 8% and 4% clay, respectively. The results indicated that the mineralization of labelled manure N was similar in the three soil-sand mixtures, but that the manure caused a higher immobilization of unlabelled ammonium-N in the soil with the highest clay content. Some of the immobilized N apparently was remineralized during the autumn and the subsequent growth season. After 18 months, 11–19% of the labelled manure N was found in the subsoil (10–45 cm) of the lysimeters, most of this labelled N probably transported to depth as organic forms by leaching or through the activities of soil fauna. In unplanted soils 67–74% of the labelled manure N was recovered in organic form in the 0–10 cm soil layer after 4 months, declining to 55–64% after 18 months. The lowest recovery of labelled N in top-soil was found in the soil-sand mixture with the lowest clay content. The mass balance of¹⁵N showed that the total recovery of labelled N was close to 100%. Thus, no significant gaseous losses of labelled N occurred during the experiment.     

Leaching and crop recovery of15N from ammonium sulphate and labelled maize (Zea mays) material in lysimeters at a site in Zimbabwe

The fate of¹⁵N-ammonium sulphate fertilizer that was applied to four lysimeters in the 1990/91 summer was studied over three consecutive growing seasons during which either maize or wheat was grown. Aboveground portions of¹⁵N-labelled maize plants from the first harvest were applied to four other lysimeters at 5 t ha^–1. Two lysimeters in each of the sets of four were assigned a low and a high moisture treatment using irrigation. In both moisture treatments, plant recovery of fertilizer-¹⁵N in the first season was 27% and a further 2% was recovered by plants during the next two seasons. During the second and third seasons, total recovery of¹⁵N by aboveground plant portions from lysimeters that received¹⁵N-labelled maize material was equivalent to 2.5% of applied fertilizer-¹⁵N. This corresponded to ca. 18% recovery of the¹⁵N added in maize material. Leaching of fertilizer-N over the three growing seasons did not exceed 0.3% in total. During the first season, a maximum of 0.25 kg N ha^–1, equivalent to 0.25% of the applied fertilizer-N, was leached in the high moisture treatment. This represented 1.8% of the nitrate load in leachates. Less than 0.002% of the applied fertilizer-N was leached in the low moisture treatment during the first season.     

Distribution of15N in the soil-plant system during a four-year field lysimeter study with barley (Hordeum distichum L.) and perennial meadow fescue (Festuca pratensis Huds.)

An annual cereal, barley, and a perennial grass ley, meadow fescue, were grown in field lysimeters in Sweden and fertilized with 12 and 20g Ca(NO₃)₂-N m⁻² yr⁻¹, respectively. Isotope-labeled (¹⁵N) fertilizer was added during year 1 of the study, whereafter similar amounts of unlabeled N were added during years 2 and 3. The grass ley lysimeters were ploughed after the growing season of year 3 and sown with barley during year 4. The barley harvest in year 1 removed 59% of the added fertilizer N, while the fertilizer N export by two meadow fescue harvests in year 1 was 65%. The labeled N export decreased rapidly after year 1, especially in the barley, but increased slightly after ploughing of the grass ley. The microbial biomass, measured with the chloroform fumigation method, incorporated a maximum of 1.4–1.7% of the labeled N during the first seven weeks after application. Later on, the incorporation stabilized at less than 1% in both cropping systems. The susceptibility of the residual labeled N to mineralization was evaluated three years after application by means of long-term laboratory incubations. The curves of cumulative mineralized N were described by a two-component first-order regression model that differentiated between an available and a more recalcitrant fraction of potentially mineralizable N. There was no difference in the amounts of potentially mineralizable N between the cropping systems. The labeled N comprised 5 and 2% of the amounts of potentially mineralizable N in the available and more recalcitrant fraction, respectively. The mineralization rate constants for the labeled N were almost twice as high as for the total potentially mineralizable N. The available fraction of the total potentially mineralizable N was 12%, while twice that proportion of the labeled N was available. It was concluded that the short-term ley did not differ from the annual crop with respect to the early disposition of the fertilizer N and the behaviour of the residual organic N.     

Influence of Belonolaimus longicaudatus on Nitrate Leaching in Turf

Experiments were conducted to quantify the effects of the sting nematode (Belonolaimus longicaudatus) on root reductions and quantity of nitrate (NO₃ ⁻) leached from ‘Tifdwarf’ bermudagrass in lysimeters. Forty lysimeters were planted with ‘Tifdwarf’ bermudagrass, of which 20 were inoculated with B. longicaudatus and 20 were noninoculated. Root length was compared between treatments at six, 12, and 18 weeks after initiation of the experiments. Turf was fertilized every three weeks, and leaching events were simulated at 21 and 42-day intervals in trial one and trial two, respectively. Leachate was collected, and the quantity of NO₃ ⁻ leached was compared between treatments. Root reductions were observed in lysimeters inoculated with B. longicaudatus at all evaluation dates. Quantity of NO₃ ⁻ leached was greater in inoculated lysimeters at the 18-week evaluation during both trials. This study indicates that nematode damage to turf roots limits root vigor and N uptake, thereby increasing nitrate leaching, adding to water quality concerns.     

Kinetics of NH(4) Assimilation in Zea mays: Preliminary Studies with a Glutamate Dehydrogenase (GDH1) Null Mutant

In higher plants it is now generally considered that glutamate dehydrogenase (GDH) plays only a small or negligible role in ammonia assimilation. To test this specific point, comparative studies of ¹⁵NH₄⁺ assimilation were undertaken with a GDH1-null mutant of Zea mays and a related (but not strictly isogenic) GDH1-positive wild type from which this mutant was derived. The kinetics of ¹⁵NH₄⁺ assimilation into free amino acids and total reduced nitrogen were monitored in both roots and shoots of 2-week-old seedlings supplied with 5 millimolar 99% (¹⁵NH₄)₂SO₄ via the aerated root medium in hydroponic culture over a 24-h period. The GDH1-null mutant, with a 10- to 15-fold lower total root GDH activity in comparison to the wild type, was found to exhibit a 40 to 50% lower rate of ¹⁵NH₄⁺ assimilation into total reduced nitrogen. Observed rates of root ammonium assimilation were 5.9 and 3.1 micromoles per hour per gram fresh weight for the wild type and mutant, respectively. The lower rate of ¹⁵NH₄⁺ assimilation in the mutant was associated with lower rates of labeling of several free amino acids (including glutamate, glutamine-amino N, aspartate, asparagine-amino N, and alanine) in both roots and shoots of the mutant in comparison to the wild type. Qualitatively, these labeling kinetics appear consistent with a reduced flux of ¹⁵N via glutamate in the GDH1-null mutant. However, the responses of the two genotypes to the potent inhibitor of glutamine synthetase, methionine sulfoximine, and differences in morphology of the two genotypes (particularly a lower shoot:root ratio in the GDH1-null mutant) urge caution in concluding that GDH1 is solely responsible for these differences in ammonia assimilation rate.     

Relationship between N(2)-Fixing Efficiency and Natural N Enrichment of Soybean Nodules

Non-nodular tissue of soybean (Glycine max L. Merrill) plants grown hydroponically in the absence of added N have a ¹⁵N abundance close to that of atmospheric N₂. In contrast, nodules are usually enriched in ¹⁵N. In this paper, we report measurements of the ¹⁵N abundance of foliar tissue and nodules of soybeans inoculated with 11 variably efficient strains of Rhizobum japonicum and grown hydroponically with no added N. The efficiency of the 11 symbioses varied over a wide range as judged by a 16-fold difference in N content. The degree of ¹⁵N enrichment of nodules was closely correlated with N₂-fixing efficiency (milligrams N fixed per milligram N in the nodules).

These results confirm prior preliminary data based on six variably efficient R. japonicum strains. The strong correlation between ^NN enrichment of soybean nodules and N₂-fixing efficiency is consistent with the hypothesis that new nodule tissue is synthesized from a pool of recently fixed N within the same nodule.

     

Photosynthetic Induction in a C(4) Dicot, Flaveria trinervia: I. Initial Products of CO(2) Assimilation and Levels of Whole Leaf C(4) Metabolites

Labeling patterns from ¹⁴CO₂ pulses to leaves and whole leaf metabolite contents were examined during photosynthetic induction in Flaveria trinervia, a C₄ dicot of the NADP-malic enzyme subgroup. During the first one to two minutes of illumination, malate was the primary initial product of ¹⁴CO₂ assimiltion (about 77% of total ¹⁴C incorporated). After about 5 minutes of illumination, the proportion of initial label to aspartate increased from 16 to 66%, and then gradually declined during the following 7 to 10 minutes of illumination. Nutrition experiments showed that the increase in ¹⁴CO₂ partitioning to aspartate was delayed about 2.5 minutes in plants grown with limiting N, and was highly dampened in plants previously treated 10 to 12 days with ammonia as the sole N source. Measurements of C₄ leaf metabolites revealed several transients in metabolite pools during the first few minutes of illumination, and subsequently, more gradual adjustments in pool sizes. These include a large initial decrease in malate (about 1.6 micromoles per milligram chlorophyll) and a small initial decrease in pyruvate. There was a transient increase in alanine levels after 1 minute of illumination, which was followed by a gradual, prolonged decrease during the remainder of the induction period. Total leaf aspartate decreased initially, but temporarily doubled in amount between 5 and 10 minutes of illumination (after its surge as a primary product). These results are discussed in terms of a plausible sequence of metabolic events which lead to the formation of the intercellular metabolite gradients required in C₄ photosynthesis.     

Multinuclear NMR of cis-dicarbonylbis(dithiocarbamato)iron(II) complexes in chloroform solution

The ¹³C and ¹⁵SN NMR spectra of eleven cis-Fe(S₂CNRR′)₂(CO)₂ complexes, where R and R′ are organic substituents, have been measured at ambient temperature in CDCl₃ (0.08–0.16 M). The ¹³C absorptions for the carbonyl ligands correlate well with the force constants for the CO stretching vibrations in CHCl₃ solution. Each of the parameters (¹³CO absorption and k_cis for CO) correlate well with the aqueous solution pK_a for⁺H₂NRR′, corrected for the phenyl-containing substituents, high pK_a values corresponding to high ¹³CO absorptions and low k_cis CO force constants. [p ]Evidence was found in the ¹³C NMR spectra for hindered rotation about the CN bond in S₂CNC₂ in complexes with higher pK_a(corr) values and in the ¹³C spectra of the corresponding thiuram disulfides. [p ]The ¹⁵N (natural abundance) NMR spectra for each of the complexes was determined. Each revealed a single sharp absorption in a region of the ¹⁵N NMR spectrum which indicates substantial CN double bond character, as one would expect for coordinated dithiocarbamate ligands.     

Expression of OsAMT1 (1.1-1.3) in rice varieties differing in nitrogen accumulation

OsAMT is a high-affinity ammonium transporter responsible for NH ₄ ⁺ uptake by rice plants. To investigate the expression patterns of OsAMT in different genotypes in relation to nitrogen accumulation, we measured the expression of OsAMT1.1, OsAMT1.2, and OsAMT1.3 using Real-Time PCR (RT-PCR) in GD (higher N accumulation) and NG (lower N accumulation) seedlings of the Oryza sativa L. cultivar treated with 0.1 mM NH₄NO₃ and 2 mM NH₄NO₃. We found that the expression level of OsAMT1.1 was significantly higher than those of OsAMT1.2 and OsAMT1.3 in the roots treated with 0.1 mM NH₄NO₃, suggesting that OsAMT1.1 contributed the most to N accumulation among the three genes. In GD root, OsAMT1.1 had significantly higher expression levels when it was up-regulated by 0.1 mM NH₄NO₃ than when down-regulated by 2 mM NH₄NO₃. OsAMT1.1 was mainly found in GD roots treated with 0.1 mM NH₄NO₃. We conclude that the OsAMT1.1 in GD roots, which was significantly up-regulated by low N and down-regulated by high N, was the dominating factor in determining the higher N acquisition in GD than in NG at 0.1 mM NH₄NO₃.     

Nitrogen gas (N2+N2O) flux from urea applied to lowland rice as affected by green manure

A field study was conducted on a clay soil (Andaqueptic Haplaquoll) in the Philippines to directly measure the evolution of (N₂+N₂O)−¹⁵N from 98 atom %¹⁵N-labeled urea broadcast at 29 kg N ha⁻¹ into 0.05-m-deep floodwater at 15 days after transplanting (DT) rice. The flux of (N₂+N₂O)−¹⁵N during the 19 days following urea application never exceeded 28 g N ha⁻¹ day⁻¹. The total recovery of (N₂+N₂O)−¹⁵N evolved from the field was only 0.51% of the applied N, whereas total gaseous¹⁵N loss estimated from unrecovered¹⁵N in the¹⁵N balance was 41% of the applied N. Floodwater (nitrate+nitrite)−N in the 5 days following urea application never exceeded 0.14 g N m⁻³ or 0.3% of the applied N. Prior cropping of cowpea [Vigna unguiculata (L.) Walp.] to flowering with subsequent incorporation of the green manure (dry matter=2.5 Mg ha⁻¹, C/N=15) at 15 days before rice transplanting had no effect on fate of urea applied to rice at 15 DT. The recovery of (N₂+N₂O)−¹⁵N and total¹⁵N loss during the 19 days following urea application were 0.46 and 40%, respectively. Direct recovery of evolved (N₂+N₂O)−¹⁵N and total¹⁵N loss from 27 kg applied nitrate-N ha⁻¹ were 20% and 53% during the same 19-day period. The failure of directly-recovered (N₂+N₂O)−¹⁵N to match total¹⁵N loss from added nitrate-¹⁵N might be due to entrapment of denitrification end products in soil or transport of gaseous end products to the atmosphere through rice plants. The rapid conversion of added nitrate-N to (N₂+N₂O)−N, the apparently sufficient water soluble soil organic C for denitrification (101 μg C g⁻¹ in the top 0.15-m soil layer), and the low floodwater nitrate following urea application suggested that denitrification loss from urea was controlled by supply of nitrate rather than by availability of organic C.     

Effect of Feeding Millet (Sorghum vulgarie) Protein at Various Protein Levels on Adrenal Lipids of Rats

A significant increase in adrenal weight, total lipids, cholesterol phospholipids and glycerides (mono-and triglycerides) was observed in rats fed millet at 5, 10 and 15 % protein levels respectively for a period of six weeks as compared to rats fed casein at 10 per cent level. Increases in cholesterol were in both its free and esterified fraction. Adrenal phosphatidyl etha-nolamine was increased in all millet fed rats whereas phosphatidyl choline increased in M–15 % and decreased in M–5 % groups. Other phospholipid fractions viz. monophosphatidyl inositol, lysophosphatidyl ethanolamine, sphingomyeline, phosphatidic acid and polyglycerophosphatide also showed significant alterations in rats fed millet protein as compared to control. Incorporation of acetate–l–¹⁴C into adrenal lipids was lower and that of glucose–U–¹⁴C, palmitate–l–¹⁴C and NaH₂³²PO₄ was higher than the control.     

Urea dynamics during Lake Taihu cyanobacterial blooms in China

Lake Taihu, the third largest freshwater lake in China, suffers from harmful cyanobacteria blooms caused by Microcystis spp., which do not fix nitrogen (N). Reduced N (i.e., NH₄⁺, urea and other labile organic N compounds) is an important factor affecting the growth of Microcystis. As the world use of urea as fertilizer has escalated during the past decades, an understanding of how urea cycling relates to blooms of Microcystis is critical to predicting, controlling and alleviating the problem. In this study, the cycling rates of urea-N in Lake Taihu ranged from non-detectable to 1.37 μmol N L⁻¹ h⁻¹ for regeneration, and from 0.042 μmol N L⁻¹ h⁻¹ to 2.27 μmol N L⁻¹ h⁻¹ for potential urea-N removal. The fate of urea-N differed between light and dark incubations. Increased ¹⁵NH₄⁺ accumulated and higher quantities of the removed urea-¹⁵N remained in the ¹⁵NH₄⁺ form were detected in the dark than in the light. A follow-up incubation experiment with ¹⁵N-urea confirmed that Microcystis can grow on urea but its effects on urea dynamics were minor, indicating that Microcystis was not the major factor causing the observed fates of urea under different light conditions in Lake Taihu. Bacterial community composition and predicted functional gene data suggested that heterotrophic bacteria metabolized urea, even though Microcystis spp. was the dominant bloom organism.     

Dinitrogen Fixation by Cultures of Frankia sp CpIl Demonstrated by N(2) Incorporation

The filamentous bacterium Frankia of the Actinomycetales, isolated from the nitrogen-fixing root nodules of certain woody plants, has shown nitrogenase activity in culture, using the acetylene reduction method. In the present work, nitrogenase activity in pure cultures of Frankia sp. CpIl is confirmed using mass spectrometric measurements of ¹⁵N₂ incorporation. After addition of carrier NH₄⁺ to digested cultures, those exposed to ¹⁵N₂ (25 atom%) had a ¹⁵N content of 3.16 atom% compared to 0.354 atom% ¹⁵N in the controls.     

Platinum(II) triammine antitumour complexes: structure-activity relationship with guanosine 5′-monophosphate (5′-GMP)

The multinuclear (¹H, ¹⁵N, ³¹P and ¹⁹⁵Pt) NMR spectroscopies, ES-MS and HPLC have been employed to investigate the structure-activity relationship for the reactions between guanosine 5′-monophosphate (5′-GMP) and the platinum(II)-triamine complexes of the general formulation cis-[Pt(NH₃)₂(Am)Cl]NO₃ (where Am represents a substituted pyridine). The order of reaction rate of the reactions was found to be: 3-phpy > 4-phpy > py > 4-mepy > 3-mepy > 2-mepy. The two basic factors, steric and electronic, were attributed to the order of the binding rate constants. A possible mechanism of the reaction of cis-[Pt(NH₃)₂(Am)Cl]⁺ with 5′-GMP suggested that the reactions proceed via direct nucleophilic attack and no loss of ammonia. cis-[Pt(NH₃)₂(Am)Cl]⁺ binds to the N7 nitrogen of the guanine residue of 5′-GMP to form a coordinate bond with the Pt metal centre. This mechanism is apparently different from that of cisplatin. The pK_a value of cis-[Pt(NH₃)₂(4-mepy)(H₂O)](NO₃)₂ (5.63) has been determined at 298 K by the use of distortionless enhancement by polarization transfer (DEPT) ¹⁵N NMR spectroscopy and compared to the pK_a value of cis-[PtCl(H₂O)(NH₃)₂]⁺.     

增强UV-B辐射和氮对谷子叶光合色素及非酶促保护物质的影响

以谷子(Setaria italica(L)Beauv.)为对象,从拔节期开始持续给予低氮(1.875 mmol/L)和高氮(15 mmol/L)两种氮供应条件并从抽穗期开始进行26 d两种强度(4.29、7.12 kJ·m-2·d-1)的增强UV-B辐射处理,研究了谷子叶中光合色素含量、类黄酮含量和苯丙氨酸解氨酶(PAL)活性的变化.结果表明:与高氮供应条件相比,低氮供应条件明显降低了谷子叶中光合色素含量但提高了类胡萝卜素/叶绿素含量比值;在开花期中段和灌浆期中段,高氮供应条件下谷子叶中光合色素含量对增强UV-B辐射比低氮供应条件下的谷子更敏感.从灌浆期开始到处理结束,两种影响因子对谷子叶中类黄酮含量均有显著影响,增强UV-B辐射导致谷子叶中类黄酮含量逐渐升高,且相同增强UV-B辐射强度下低氮供应条件下的谷子叶中类黄酮含量明显高于高氮供应条件下的谷子.谷子叶中PAL活性对两种影响因子的响应较类黄酮含量更加敏感,低氮供应条件使谷子叶中PAL活性明显提高.结合上述指标的相关性分析结果可知,低氮供应条件加强了处于繁殖期主要阶段的谷子叶中类黄酮的积累,并使谷子叶中的类胡萝卜素/叶绿素含量比值明显提高,进而有助于维持谷子叶中光合色素含量在增强UV-B辐射条件下的相对稳定性,对植株抵抗UV-B辐射伤害有利.