Seasonal regulation of nitrification in a rooted macrophyte (Vallisneria spiralis L.) meadow under eutrophic conditions

Variable oxygen release from the root of macrophytes growing in ammonium-rich organic substrates can stimulate the process of nitrification. To verify this hypothesis, we performed seasonal measurements of potential nitrification activity in sediments with and without the perennial submersed plant Vallisneria spiralis L. (Hydrocharitaceae). Pore water and sediment features were simultaneously considered in order to provide insights into the regulation of the process. Results demonstrated a significant effect of season and plant presence on potential nitrification activity, with higher rates in winter and lower rates in summer. Vegetated sediment displayed lower pore water ammonium, but always higher potential nitrification activity compared to the unvegetated substrate, regardless the season. Nitrification activity was strongly correlated with pore water redox status, which were affected by both season and plant presence. Along its annual cycle V. spiralis promoted more oxidized conditions in the rhizosphere likely due to elevated radial oxygen loss and the consequent maintenance of a larger nitrifying community. These outcomes confirm the results of a limited number of studies that demonstrated how sediment biogeochemistry may be controlled by plant-released oxygen also in organic-rich systems.     

Water movement induced variations in growth regulation and metabolism of freshwater macrophyte Vallisneria spiralis L. in early growth stages

Exposure to water movement has been observed to alter various processes in plants. This study attempts to discuss consequences of water movement induced stress on young Vallisneria spiralis plants, since young plants are more vulnerable to damage and their survival is a key factor in successful colonization. Water movement was generated using oscillating grid setups at three different frequencies (1, 2, and 4 Hz). A no-turbulence condition was used as the control. Variations in growth and the alterations in indole acetic acid (IAA), H₂O₂, peroxidase activity (POD), and carbon metabolism of the plants were measured. Growth of young V. spiralis plants suppressed when exposed to higher turbulences. IAA concentration decreased with increasing turbulence, while H₂O₂ concentration increased. POD increased in response to the increase in H₂O_2. The increase in soluble carbohydrates may compensate for the increasing energy demand for respiration under stress. There were evidences for damage to photosynthetic machinery hence reducing carbon assimilation and degrading non-structural carbohydrate storages. Observed variations in the growth, growth regulation and metabolism show that mechanical stress due to turbulence induces significant alterations in plant physiology. Plant stress responses varied with the degree of turbulence they have been exposed.     

Habitat selection in spatially heterogeneous environments: a test of foraging behaviour in the clonal submerged macrophyte Vallisneria spiralis

1. To test whether clonal macrophytes can select favourable habitats in heterogeneous environments, clonal fragments of the stoloniferous submerged macrophyte Vallisneria spiralis were subjected to conditions in which light intensity and substratum nutrients were patchily distributed. The allocation of biomass accumulation and ramet production of clones to the different patches was examined. 2. The proportion of both biomass and ramet number of clones allocated to rich patches was significantly higher than in poor patches. The greatest values of both clone and leaf biomass were produced in the heterogeneous light treatment, in which clones originally grew from light‐rich to light‐poor patches, while clones produced the most offspring ramets in the treatments with heterogeneous substratum nutrients. Similarly, root biomass had the highest values in nutrient‐rich patches when clones grew from nutrient‐rich to nutrient‐poor patches. 3. The quality of patches in which parent ramets established significantly influenced the foraging pattern. When previously established in rich patches, a higher proportion of biomass was allocated to rich patches, whereas a higher proportion of ramet number was allocated to rich patches when previously established in poor patches. 4. Results demonstrate that the clonal macrophyte V. spiralis can exhibit foraging in submerged heterogeneous environments: when established under resource‐rich conditions V. spiralis remained in favourable patches, whereas if established in adverse conditions it could escape by allocating more ramets to favourable patches.     

Effects of white clover (Trifolium repens L.) on plant and soil nitrogen and soil organic matter in mixtures with perennial ryegrass (Lolium perenne L.)

To increase our insight into the above- and belowground N flows in grass and grass-clover swards relations between crop and soil parameters were studied in a cutting trial with perennial ryegrass (Lolium perenne) monocultures and ryegrass–white clover (Trifolium repens) mixtures. The effects of clover cultivar on herbage yield, the amount of clover-derived nitrogen, apparent N transfer to companion grass, dynamics of N and organic matter in the soil were estimated.The grass monocultures had very low DM yields (<2.1 t ha^-1) and a low N concentration in the harvested herbage. During 1992–1995 the annual herbage DM yield in the mixtures ranged from 7.0 to 14.3 t ha^-1, the white clover DM yield from 2.4 to 11.2 t ha^-1 and the mean annual clover content in the herbage DM harvested from 34 to 78%. Mixtures with the large-leaved clover cv. Alice yielded significantly more herbage and clover DM and had a higher clover content than mixtures with small/medium-leaved cvs. Gwenda and Retor. Grass cultivar did not consistently affect yield, botanical composition or soil characteristics.The apparent N₂ fixation was very high, ranging from 150 to 545 kg N ha^-1 in the different mixtures. For each tonne of clover DM in the harvested herbage 49 to 63 kg N was harvested, while the apparent N transfer from clover to grass varied between 55 and 113 kg N ha^-1 year^-1.The net N mineralization rate was lower under monocultures than under mixtures. The C mineralization and the amounts of C and N in active soil organic matter fractions were similar for monocultures and mixtures, but the C:N ratio of the active soil organic matter fractions were higher under grass than under mixtures. This explains the lower N mineralization under grass.     

Initial differentiation of vertical soil organic matter distribution and composition under juvenile beech (Fagus sylvatica L.) trees

In a lysimeter experiment with juvenile beech trees (Fagus sylvatica L.) we studied the development of depth gradients of soil organic matter (SOM) composition and distribution after soil disturbance. The sampling scheme applied to the given soil layers (0–2 cm, 2–5 cm, 5–10 cm and 10–20 cm) was crucial to study the subtle reformation of SOM properties with depth in the artificially filled lysimeters. Due to the combination of physical SOM fractionation with the application of ¹⁵N-labelled beech litter and ¹³C-CPMAS NMR spectroscopy we were able to obtain a detailed view on vertical differentiation of SOM properties. Four years after soil disturbance a significant decrease of the mass of particulate OM (POM) with depth could be found. A clear depth distribution was also shown for carbon (C) and nitrogen (N) within the SOM fractions related to bulk soil. The mineral fractions <63 µm clearly dominated C storage (between 47 to 60% of bulk soil C) and N storage (between 68 to 86% of bulk soil N). A drastic increase in aliphatic C structures concomitant to decreasing O/N-alkyl C was detected with depth, increasing from free POM to occluded POM. Only a slight depth gradient was observed for ¹³C but a clear vertical incorporation of ¹⁵N from the applied labelled beech litter was demonstrated probably resulting from faunal and fungal incorporation. We clearly demonstrated a significant reformation of a SOM depth profile within a very short time of soil evolution. One important finding of this study is that especially in soils with reforming SOM depth gradients after land-use changes selective sampling of whole soil horizons can bias predictions of C and N dynamics as it overlooks a potential development of gradients of SOM properties on smaller scales.     

Comparative study of nitrogen fixation and carbon metabolism in two chick-pea (Cicer arietinum L.) cultivars under salt stress

Two cultivars of Cicer arietinum with differential tolerance to salinity have been compared by analysing growth, photosynthesis, nodulation, nitrogenase activity, and carbon metabolism in the nodule cytosol. The aim was to help elucidate the relationships between, on the one hand, sucrose and malate metabolism in nodules and, on the other, the inhibition of nitrogen fixation under salt stress. Chick-pea cultivars Pedrosillano (sensitive) and ILC1919 (tolerant) inoculated with Mesorhizobium ciceri strain Ch-191 were grown in a controlled environmental chamber and were treated with salt (0, 50, 75, and 100 mM NaCl) from sowing to harvest time (28 d). Plant growth and photosynthesis were more affected by salt in Pedrosillano than in ILC1919. Also the effect of salt on nodulation and nitrogen fixation was much more pronounced in Pedrosillano. The increase in nodular mass in ILC1919 can partially counteract the inhibition of nitrogenase activity. The enzymes of sucrose breakdown were inhibited by NaCl, but in ILC1919 a rise in alkaline invertase was observed with salinity, which could compensate for the lack of the sucrose synthase hydrolytic activity. The activity of PEPC was stimulated by salt in ILC1919. Also, this cultivar showed higher malate concentrations in root nodules.     

Diversity of fungi in organic soils under a moorland--Scots pine (Pinus sylvestris L.) gradient

The conservation and regeneration of native Scots pine (Pinus sylvestris L.) woodlands is being actively encouraged by conservation agencies in the UK because of their high biodiversity value. In the present study, the consequences of regeneration on terrestrial fungal communities was determined in three parallel transects running from open moorland, through an intermediate zone showing seedling colonization, into a mature Scots pine forest at Abernethy Forest, Cairngorm, Scotland. Soil cores were taken at 18 m intervals along each 180 m transect, and the diversity of the soil fungal community was investigated by DGGE and sequence analysis of ITS fragments PCR-amplified from DNA extracted from soil. Analysis of DGGE profiles generated for two of the three transects indicates a clear shift in the community from the moorland region of the transects to the forest region. Whereas a few bands were present at all sampling points across the transects, the majority of bands were unique to either the moorland or forest samples. FASTA database searches of ITS sequence data generated from excised DGGE bands revealed the closest species match for each band. In some cases, the similarity of ITS sequences to database sequences was poor, but the remaining sequences were most closely related to ITS sequences of both mycorrhizal and non-mycorrhizal fungi. The data are discussed in relation to the effect of native pine woodland expansion on the soil fungal community.     

Carbon and nitrogen metabolism in a barley (Hordeum vulgare L.) mutant with impaired chloroplast dicarboxylate transport

A mutant line, RPr79/2, of barley (Hordeum vulgare L. cv. Maris Mink) has been isolated that has an apparent defect in photorespiratory nitrogen metabolism. The metabolism of ¹⁴C-labelled glutamine, glutamate and 2-oxoglutarate indicates that the mutant has a greatly reduced ability to synthesise glutamate, especially in air, although in-vitro enzyme analysis indicates the presence of wild-type activities of glutamine synthetase (EC 6.3.1.2) glutamate synthase (EC 1.4.7.1 and EC 1.4.1.14) and glutamate dehydrogenase (EC 1.4.1.2). Several characteristics of RPr79/2 are very similar to those described for glutamate-synthase-deficient barley and Arabidopsis thaliana mutants, including the pattern of labelling following fixation of ¹⁴CO₂, and the rapid rise in glutamine content and fall in glutamate in leaves on transfer to air. The CO₂-fixation rate in RPr79/2 declines much more slowly on transfer from 1% O₂ to air than do the rates in glutamate-synthase-deficient plants, and RPr79/2 plants do not die in air unless the temperature and irradiance are high. Analysis of (glutamine+NH₃+2-oxoglutarate)-dependent O₂ evolution by isolated chloroplasts shows that chloroplasts from RPr79/2 require a fivefold greater concentration of 2-oxoglutarate than does the wild-type for maximum activity. The levels of 2-oxoglutarate in illuminated leaves of RPr79/2 in air are sevenfold higher than in Maris Mink. It is suggested that RPr79/2 is defective in chloroplast dicarboxylate transport.     

Estimating C inputs retained as soil organic matter from corn (Zea Mays L.)

In agroecosystems, the annual C inputs to soil are a major factor controlling soil organic matter (SOM) dynamics. However, the ability to predict soil C balance for agroecosystems is limited because of difficulties in estimating C inputs and in particular from the below-ground part. The objective of this paper was to estimate the proportion of corn residue retained as SOM. For that purpose, the results of a ¹³C long-term (15 yr) field study conducted on continuous silage corn and two silage corn rotations along with data from the existing literature were analyzed. The total amount of corn-derived C (0–30 cm) was about 2.5 to 3.0 times higher for the continuous corn treatment (445 g m^-2), compared to the two rotational treatments (175 and 133 g m^-2 for the corn-barley-barley-wheat and corn-underseeded barley hay-hay rotations, respectively). Assuming that the C inputs to the soil from silage-corn was mainly roots and would have been similar across treatments on an annual basis, the total amount of corn-derived C for the two rotational treatments was approximately proportional to the number of years the silage-corn was present in the rotation (4 yr). The results from the current study indicate that about 17% of root-derived C is retained as SOM. This value is higher than those reported in the literature for long-term studies on shoot-derived C (range of 7.7 to 20%, average of 12.2%), which is in agreement with previous studies showing that more C is retained as SOM from roots than from shoots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of ozone on nitrogen metabolism in the leaves of Fagus crenata seedlings under different soil nitrogen loads

To clarify the effects of ozone (O₃) on nitrogen (N) metabolism in the leaves of Fagus crenata seedlings under different N loads, the combined effects of O₃ and N load on N enzyme activity, amino acid profiles and soluble protein concentrations were investigated. The seedlings were grown in potted andisol supplied with N at 0 (N0), 20 (N20) and 50 kg ha⁻¹ year⁻¹ (N50) and were exposed to charcoal-filtered air or O₃ at 1.0, 1.5 and 2.0 times the ambient concentration in open-top chambers from April 2004 to October 2005. The average 24-h concentrations of O₃ during the two growing seasons were 11.8, 42.7, 63.3 and 83.7 ppb, respectively. In July 2005, exposure to O₃ did not significantly affect the concentration of total free amino acids and activities of nitrite reductase and glutamine synthetase in any N treatment. Exposure to O₃ significantly increased the relative content of acidic amino acids in all N treatments while it significantly reduced concentration of total soluble protein (TSP) and ratio of TSP concentration to leaf N concentration in the N50 treatment, but not in the N0 and N20 treatments. Based on the results obtained in the present study, we concluded that exposure to O₃ reduces the allocation of N to soluble protein in addition to O₃-induced degradation of protein in the leaves of seedlings grown under a relatively high N load, with the result that the degree of O₃-induced reduction in the soluble protein was greater under a relatively high N load than under a relatively low N load.     

Carbon and nitrogen metabolism in barley (Hordeum vulgare L.) mutants lacking ferredoxin-dependent glutamate synthase

Five mutant lines of barley (Hordeum vulgare L.), which are only able to grow at elevated levels of CO₂, contain less than 5% of the wild-type activity of ferredoxin-dependent glutamate synthase (EC 1.4.7.1). Two of these lines (RPr 82/1 and RPr 82/9) have been studied in detail. Leaves and roots of both lines contain normal activities of NADH-dependent glutamate synthase (EC 1.4.1.14) and the other enzymes of ammonia assimilation. Under conditions that minimise photorespiration, both mutants fix CO₂ at normal rates; on transfer to air, the rates drop rapidly to 15% of the wild-type. Incorporation of ¹⁴CO₂ into sugar phosphates and glycollate is increased under such conditions, whilst incorporation of radioactivity into serine, glycine, glycerate and sucrose is decreased; continuous exposure to air leads to an accumulation of ¹⁴C in malate. The concentrations of malate, glutamine, asparagine and ammonia are all high in air, whilst aspartate, alanine, glutamate, glycine and serine are low, by comparison with the wild-type parent line (cv. Maris Mink), under the same conditions. The metabolism of [¹⁴C]glutamate and [¹⁴C]glutamine by leaves of the mutants indicates a very much reduced ability to convert glutamine to glutamate. Genetic analysis has shown that the mutation in RPr 82/9 segregates as a single recessive nuclear gene.Abbreviations GDH glutamate dehydrogenase (EC 1.4.1.2) - GS glutamine synthetase (EC 6.3.1.2) - RuBP ribulose 1,5-bisphosphate     

Effects of sulphur nutrition on nitrogen and carbon metabolism in lucerne (Medicago sativa L.)

Studies were conducted with one-year-old plants of the perennial legume, lucerne ( Medicago sativa L.), to determine the effects of various levels of S (0.3, 1.5, and 7.5 m M ) on N₂-fixation, photosynthesis, herbage free amino acid pools and protein amino acids, levels of protein, and activities of key enzymes of leaf nitrogen and carbon metabolism. Sulphur deficiencies in the 0.3 m M S treatment, as determined by N:S ratios, did not appear until the second growth cycle. This treatment was severely S-deficient by the end of the third growth cycle. Sulphur deficiencies did not occur in the other two treatments over three cycles of growth. By the middle of the second growth cycle acetylene reduction rates of plants grown with 0.3 m M S were lower than those of other treatments. At the end of the third growth cycle acetylene reduction rates of the 0.3 m M S treatment were ca . 30–35% that of the other treatments. There was no effect of S-deficiency on photosynthesis, levels of leaf NADP-isocitrate dehydrogenase and glutamine synthetase activity, or concentrations of leaf protein over the course of the experiments. Levels of NAD-glutamate dehydrogenase, NAD-malate dehydrogenase, and glutamate oxaloacetate trans-aminase activity and concentrations of free methionine, arginine, urea, and ammonia increased, with S-deficiency. Boron concentrations were very high (ca. 300 μg/g dry wt.) in S-deficient plants. The mole percent methiomne in leaf protein decreased slightly and chlorophyll concentration decreased markedly with S-deficiency. This study suggests that N₂-fixation is affected early during S-deprivation of lucerne and that subsequent effects of S-deficiency may be due to loss of fixed nitrogen.     

Chilling injury in okra (Hibiscus esculentus L.) in relation to plant development and nitrogen metabolism

Summary Okra plants were exposed to chilling temperatures of 5° or 10°C for various periods up to 144 hours in a controlled environment room. Controls were maintained in a similar room at 25°C. Seedling plants which had developed five true leaves and plants in a preflowering condition with well developed buds were used. Results showed that injury occurred at both 5° and 10°C in plants at both growth stages. Seedling plants exposed at 5°C for 144 hours were killed while the older ones died back to the lower nodes. Eventually growth occurred from the buds at these nodes. In cases where the plants survived there was a delay in flowering and a decrease in the total number of flowers. Increased cold exposure resulted in an increased percentage dry matter. It appeared that increased dessication contributed to injury, but that atmospheric humidity itself had little relationship to injury. The insoluble nitrogen fraction did not show any great change. There was an increase in some amino acids, and it was noted that these were the same as those derived from pyruvate. The soluble protein fraction did not change. It was suggested that this lack of change might be a manifestation of the difference between plants that can be hardened and those that cannot.     

Community metabolism and buffering capacity of nitrogen in a ruppia cirrhosa meadow

Fluxes of oxygen, inorganic nitrogen (DIN) and denitrification (isotope pairing) were measured from January 1997 to February 1998 via intact cores incubation in a shallow brackish area within the eutrophic Valli di Comacchio (northern Adriatic coast, Italy). Rates were measured in the light and in the dark in sediments colonized by the rooted macrophyte Ruppia cirrhosa and in adjacent sediments with benthic microalgae. Ruppia biomass (25-414 g DW m^− 2) exhibited a seasonal evolution whilst that of microphytobenthos (12-66 mg chl a m^− 2) was more erratic. Net (NP) and gross (GP) primary productivity was 1.15 and 6.89 mol C m^− 2y^− 1 for bare and 25.4 and 51.7 mol C m^− 2y^− 1 for Ruppia vegetated sediments. Nitrogen pools in Ruppia standing stock varied from 43.6 to 631.4 (annual average 201.2) mmol N m^− 2; the macrophyte N content was correlated with DIN concentration in the water column. Estimated N pool in microphytobenthos was one order of magnitude lower (from 2.4 to 14.5 mmol N m^− 2, annual average 7.2). Theoretical DIN assimilation calculated from NP was 127.8 and 1112.6 mmol N m^− 2y^− 1 whilst that calculated from GP was 765 and 2282 mmol N m^− 2y^− 1 for microphytobenthos and Ruppia respectively. Measured annual fluxes of DIN were 974.6 and − 577 mmol N m^− 2y^− 1 in bare and Ruppia vegetated sediments meaning that the two sites were a source and sink for DIN and that from 25 to 50% of Ruppia annual DIN requirements came from the water column. During the period of this study total denitrification was lower in the macrophyte colonized (92.3 mmol N m^− 2y^− 1) compared to bare sediments (163.3 mmol N m^− 2y^− 1) as a probable consequence of higher competition between denitrifiers and phanerogams. At both sites the ratio between denitrification of water column nitrate (D_W) and denitrification coupled to nitrification (D_N) was >1.6 due to little oxygen penetration in reducing sediments (< 1.2 mm) and scarce nitrification activity. D_W (0-35 µmol N m^− 2h^− 1) was significantly correlated with water column NO₃⁻  (2-16 µM). Theoretical DIN assimilation to denitrification ratio varied from 12.0 to 24.8 for Ruppia vegetated and from 0.8 to 4.7 for unvegetated sediments.At Valle Smarlacca, Ruppia may influence nitrogen cycling by incorporating large DIN pools in biomass which is scattered in surrounding areas and fuels intense bacterial activity. With increasing anthropogenic nutrient input and insignificant organic matter export in the open sea the already severe eutrophic conditions are enhanced and may accelerate the decline of the macrophyte meadow.     

Effects of NaCl on the nitrogen metabolism of the halophyteArthrocnemum fruticosum (L.) Moq., grown in a greenhouse

Summary Arthrocnemum fruticosum (L.)Moq., a halophyte from the shore of the Dead Sea in Jordan was grown in a greenhouse with nutrient solution supplemented with various concentrations of NaCl. It was shown that with increasing salinity the plants became more succulent, mainly due to an accumulation of sodium and water. Sodium was taken up into the roots in equal amounts to chloride, but in the shoots far more sodium than chloride was found, suggesting a control of these ions either in the excretion into the xylem, or in the uptake by the shoot out of the xylem. Ammonium and nitrate in the plants decreased with time on nutrient solution more or less independently of the salt concentration. However, more nitrate appeared again in the plants when they started flowering. After an initial period of adaptation the nitrate reductase activityin vivo was not inhibited by a salinity of up to 2%, but at higher NaCl concentrations a shift of nitrate reductase activity occurred from the roots to the shoots. This was consistent with earlier observations in the field. In the vegetative phase of the plants the nitrate reductase in the roots was influenced by the soil water potential, but in the shoot it was mainly dependent on the supply of nitrate from the roots. High NaCl concentrations delayed flower initiation. During flowering the nitrate reductase was involved in the re-allocation of nitrogenous compounds from the roots to the developing flowers, and it became effectively independent from salinity.     

Cobalt stress affects nitrogen metabolism,photosynthesis and antioxidant system in chickpea (Cicer arietinum L.)

Abstract

Plants of chickpea were exposed to varied levels of cobalt (Co) and sampled at the 60-day stage. Cobalt at concentration <100 µM significantly increased the number of nodules, their dry mass, leghemoglobin concentration and the activity of nitrogenase. Similarly, the activities of glutamate dehydrogenase, glutamine synthetase and glutamate synthase also exhibited an increase in the presence of Co <100 µM, in nodules and leaves, respectively. The various photosynthetic attributes in leaves and the activity of antioxidative enzymes both in nodules and leaves were inhibited by Co in a concentration-dependent manner. However, the lipid peroxidation and the content of proline exhibited a significant increase in response to Co and were at a maximum in the plants exposed to 250 µM concentration of cobalt. Since most of the parameters showed a significant increase in response to 50 µM cobalt, this concentration may be regarded as a threshold concentration.     

Effect of Vallisneria americana (L.) on community structure and ecosystem function in lake mesocosms

Submerged aquatic vegetation is known as a key structural component and regulator in ecosystems. In this mesocosm study, we examine community- and system-level responses to the presence of Vallisneria americana (L), a deep-rooted macrophyte. Phytoplankton, bacteria and filamentous algal biomasses were significantly lowered in the presence of V. americana. In addition, mesocosms with macrophytes had significantly reduced porewater phosphate and iron, water column dissolved organic carbon and total suspended solids, but elevated sediment redox. All mesocosms were net autotrophic (gross primary production/respiration >1). Compared to the macrophyte treatments, the control mesocosms had lower diel net primary production (NPP) midway through the experiment (d 16), but at the end of the experiment (d 36), the controls had the higher values, presumably due to increased filamentous algae. NPP and NPP/R were constant in the macrophyte treatments, whereas NPP/R increased significantly from middle to end of the experiment in the controls. We show that community and system-level responses to the presence of V. americana have significant consequences on system structure and function.     

Invertebrate communities associated with a native (Vallisneria americana) and an alien (Trapa natans) macrophyte in a large river

1. We used a corer and a Downing box sampler to sample macroinvertebrates living on and beneath the introduced Trapa natans and the native Vallisneria americana in the freshwater tidal Hudson River, New York. 2. Densities of macroinvertebrates were higher in Trapa than in Vallisneria, and higher in the interior of plant beds than at their edges. These effects were largely a result of high plant biomass in Trapa beds and in bed interiors (the plants have similar surface area per unit mass). 3. The composition of both epiphytic and benthic macroinvertebrates differed distinctly between Trapa and Vallisneria, and also seasonally. 4. These compositional differences were not easily interpretable as rising from possible differences in oxygen concentrations, fish predation, or water circulation in the two macrophytes. 5. Sida crystallina (Cladocera) collected from Trapa contained more haemoglobin than those collected from Vallisneria. 6. The replacement of Vallisneria by Trapa in the Hudson probably increased system‐wide biodiversity and food for fish, although macroinvertebrates in Trapa beds may not be readily available to fish because of low oxygen concentration there.     

缺磷条件下低分子量有机酸对大豆氮积累和结瘤固氮的影响

采用砂培方法在温室条件下研究了低分子量有机酸柠檬酸、草酸、苹果酸及3种酸的混合物对大豆植株氮素积累、结瘤和固氮的影响.结果表明：低分子量有机酸对大豆植株氮素积累有显著的抑制作用,使大豆地上部各时期氮素积累量的降低幅度分别为：苗期17.6%～44.9%,花期29.8%～88.4%,鼓粒期9.18%～69.6%,成熟期2.21%～41.7%;低分子量有机酸对大豆根瘤生长和固氮能力也有显著影响,表现为使根瘤数量、根瘤固氮酶活性和豆血红蛋白含量显著降低,降低幅度分别为11.4%～59.6%,80.5%～91.7%和11.9%～59.9%,从而使大豆的固氮效率降低,最终导致大豆的固氮量较对照显著降低(降低幅度9.71%～64.5%).低分子量有机酸对大豆氮积累、根瘤生长和固氮能力的抑制作用随浓度的增加而增加.3种有机酸中,草酸的抑制作用相对大于柠檬酸和苹果酸,3种有机酸混合后,抑制作用加强.