Possibilities for dispersal in annual and perennial grasses in a savanna in Botswana

Dispersal of 11 dominant grass species in the savanna of southern Botswana was investigated. The dispersal is autochorous, anemochorous and epizoochorous independent of the life-cycle (annuals, perennials). Dispersal distances were estimated experimentally. Anemochorous species with a plume-like spikelet such as Chloris virgata and Enneapogon cenchroides have a low rate of descent (<1 m s^-1) and a low Reynold number (100–110). Nevertheless they can only be transported up to 13 m from the originating infructescence at a wind velocity of 10 m s^-1. Therefore, the majority of the disseminules remained near the parent plant. By analysing the seed pool under the canopy of trees of Dichrostachys cinerea epizoochorous species such as Tragus berteronianus were dominant at the cattle resting sites under trees. The results are discussed in relation to the three seed dispersal hypotheses and the model of selective interaction of dispersal, dormancy, and seed site as adaptions to variable environments.     

Daily and seasonal variation in water relations of macchia shrubs and trees in France (Montpellier) and Turkey (Antalya)

Based upon two different research studies in the mediterranean regions of France and Turkey, drought resistance strategies were investigated in a broad group of species. The diurnal and seasonal patterns of the water relations of different lifeforms from the thermo-mediterranean to submediterranean lifezones were compared. Three sites near Montpellier, in Southern France, and five sites near Antalya, Turkey were used for this comparison. Xylem pressure potential and relative stomatal aperture were the key water relations parameters collected in France while these parameters as well as osmotic potential and leaf conductance were studied in Turkey.From the 26 different study species investigated in France, 7 distinct types of stomatal control were observed, with the deciduous lifeforms showing the least control, the sclerophyllous and coniferous evergreens the greatest control and the malacophyllous shrublets intermediate levels of control. Predawn water potential values provided a means of classifying species according to their temporal and spatial utilization of site water reserves. The comparison of turgor potentials (difference between water and osmotic potentials) gave an insight into leaf adaptations to site moisture. Species with high predawn water potentials generally maintain positive turgor even at midday during the summer, whereas species with low predawn values were frequently at zero turgor even at predawn. Phlomis grandiflora was the most extreme species with mid-summer predawns and midday water potentials of –6 MPa and osmotic potentials never more negative than –2.4 MPa.     

Plant distribution in relation to the length of the growing season in a snow-bed in the Taisetsu Mountains,northern Japan

The distribution pattern of plants was studied in an alpine snow-bed in six plots along a snow-melting gradient. Each plot consisted of two habitats with respect to the microtopography; the flat habitat and the mound habitat. The number of species per plot decreased with the shortened snow-free period. In the flat habitat, the dominant growth forms changed from the early exposed plots to the late exposed ones as follows; lichens evergreen and deciduous shrubs forbs graminoids bryophytes. In the mound habitat, evergreen and deciduous shrubs prevailed widely along the gradient because of the ability to exploit new habitat by creeping over exposed rocks. For shrubs, the existence of mounds contributed to the expansion of the distribution ranges. Forbs and graminoids shifted their distribution modes to the late exposed plots where shrubs decreased in cover. Deciduous shrubs and forbs completely disappeared in the latest exposed plot.     

Identification and study of growth and nitrogenase activity of nitrogen fixing cyanobacteria from tropical soil

Identification of cyanobacteria species has been performed on samples coming from two different harvest areas. The most important fixing belongs to Scytonema genus. The other genus identified are Nostoc and Lyngbia. Moreover, these cells are living closely with non-fixing cyanobacteria as well as with bacteria. The growth of cells as well as nitrogenase activity has been studied on a semi-axenic strain of Scytonema, a nitrogen fixing cyanobacterium, isolated from soil crusts. The cell growth is relatively show in liquid medium depleted in combined nitrogen. The growth rate increases when nitrates are supplied to cells. A release of ammonium is observed in medium during cell culture. This release exhibits several maxima and minima during cell growth. The heterocyst cells disappear within four days when filaments are growing in nitrates supplied medium. On the contrary, the heterocyst frequency increases up to more 5% in a nitrogen depleted medium. The heterocyst frequency reaches a maxima after 4 days of culture, then decreases later on. Nitrogenase activity changes during cells growth too. The maximum activity is observed after 5 to 6 days of culture to decrease after even though the cells are still in their exponential phase of growth. Nitrogenase activity increases with light intensity, what indicate a possible relation between photosynthetic and nitrogenase activities.     

Indica-Japonica differentiation of rice cultivars viewed from variations in key characters and isozymes,with special reference to landraces from the Himalayan hilly areas

Summary To assess the extent of differentiation into the Indica and Japonica types of Asian rice in its diversity center, we investigated landraces collected from the hilly areas of Nepal, Sikkim and Assam. We examined variations in four key characters and six isozyme loci known to be diagnostic for classifying Indica and Japonica types, and compared the results with those from a control set of rice cultivars representing the whole of Asia. The hill cultivars showed a high level of genetic diversity in key characters as well as in isozymes. A marked feature found in their character variation was the occurrence of various atypical cultivars that were intermediate between the Indica and Japonica types. With respect to isozymes, however, the hill cultivars could be classified mostly as either Indicas or Japonicas, although the patterns of allelic association were more random than in the control cultivars. Indica-Japonica variation in key characters and in isozymes corresponded well with each other in the controls, but not in the hill cultivars. This means that nonrandom association in characters as well as in genes (gametic disequilibrium) is not fully developed in the diversity center. Populations of hill cultivars were highly polymorphic genetically, but did not show a trend to Indica-Japonica differentiation within their populations. The process of Indica-Japonica differentiation is discussed in view of these observations.     

Metabolism of nitric oxide and nitrous oxide during nitrification and denitrification in soil at different incubation conditions

Abstract NO production and consumption rates as well as N₂O accumulation rates were measured in a loamy cambisol which was incubated under different conditions (i.e. soil moisture content, addition of nitrogen fertilizer and/or glucose, aerobic or anaerobic gas phase). Inhibition of nitrification with acetylene allowed us to distinguish between nitrification and denitrification as sources of NO and N₂O. Under aerobic conditions untreated soil showed very low release of NO and N₂O but high consumption of NO. Fertilization with NH₄⁺ or urea stimulated both NO and N₂O production by nitrification. Addition of glucose at high soil moisture contents led to increased N₂ and N₂O production by denitrification, but not to increased NO production rates. Anaerobic conditions, however, stimulated both NO and N₂O production by denitrification. The production of NO and N₂O was further stimulated at low moisture contents and after addition of glucose or NO₃⁻. Anaerobic consumption of NO by denitrification followed Michaelis-Menten kinetics and was stimulated by addition of glucose and NO₃⁻. Aerobic consumption of NO followed first-order kinetics up to mixing ratios of at least 14 ppmv NO, was inhibited by autoclaving but not by acetylene, and decreased with increasing soil moisture content. The high NO-consumption activity and the effects of soil moisture on the apparent rates of anaerobic and aerobic production and consumption of NO suggest that diffusional constraints have an important influence on the release of NO, and may be a reason for the different behaviour of NO release vs N₂O release.     

Degradation of Aroclor 1221 and survival of strains in soil microcosms

Three bacterial strains able to use different aromatic compounds as the sole carbon and energy source were tested for their potential to degrade Aroclor 1221 in soil microcosms when present in mixed culture. Disappearance of polychlorinated biphenyls (PCBs), occurrence of metabolites, release of chloride, and survival of the laboratory-selected strains were investigated under different conditions. In principle, complete mineralization of various congeners of Aroclor 1221, a technical mixture of PCBs, by the mixed culture was possible. The autochthonous microflora negatively affected the degradation due to formation of a toxic compound from 4-chlorobenzoate. 4-Chlorobenzoate was produced by one of the added strains, Pseudomonas sp. JHK, during degradation of 4-chlorobiphenyl. The unknown metabolite of 4-chlorobenzoate led to a rapid decrease in viable counts of the laboratory-selected strains in the soil microcosm.Correspondence to: J. Havel     

Remediation of Clay Soils Contaminated with Potentially Toxic Elements: The Santo Amaro Lead Smelter,Brazil, Case

The remediation of clayey soil contaminated by potentially toxic elements is a challenging task, especially because of the low permeability and strong affinity of such toxic elements, which prevent the effective use of low-cost in situ remediation methods. The clay soil from the region of the former Santo Amaro primary lead smelter has high concentrations of potentially toxic elements, especially Pb, Cd, Sb, and Zn. This study presents a preliminary evaluation of remediation by soil washing and thermal stabilization, which can support clean-up initiatives for this site. The treatment results indicate that soil washing using EDTA can be an effective method to clean up the soil; however, the solid-liquid separation step by filtration is slow. Soil thermal treatment at temperatures higher than 800°C in an oxidant atmosphere results in the formation of ceramic structure because of the high smectite content, which stabilizes the potentially toxic elements. The estimated cost for remediation by soil excavation, replacement, and stabilization, of the critically contaminated site at the Santo Amaro region is estimated at about 20 to 30 million US$.     

Depth‐based differentiation in nitrogen uptake between graminoids and shrubs in an Arctic tundra plant community

Questions

The rapid climate warming in tundra ecosystems can increase nutrient availability in the soil, which may initiate shifts in vegetation composition. The direction in which the vegetation shifts will co‐determine whether Arctic warming is mitigated or accelerated, making the understanding of successional trajectories urgent. One of the key factors influencing the competitive relationships between plant species is their access to nutrients, depending on the depth where they take up most nutrients. However, nutrient uptake at different soil depths by tundra plant species that differ in rooting depth is unclear.

Location

Kytalyk Nature Reserve, northeast Siberia, Russia.

Methods

We injected ¹⁵N to 5 cm, 15 cm and the thaw front of the soil in a moist tussock tundra. The absorption of ¹⁵N by grasses, sedges, deciduous shrubs and evergreen shrubs from the three depths was compared.

Results

The results clearly show a vertical differentiation of N uptake by these plant functional types, corresponding to their rooting strategy. Shallow‐rooting dwarf shrubs were more capable of absorbing nutrients from the upper soil than from deeper soil. Deep‐rooting grasses and sedges were more capable of absorbing nutrients from deeper soil than the dwarf shrubs. The natural ¹⁵N abundances in control plants also indicate that graminoids can absorb more nutrients from the deeper soil than dwarf shrubs.

Conclusions

Our results show that graminoids and shrubs in the Arctic differ in their N uptake strategies, with graminoids profiting from nutrients released at the thaw front, while shrubs mainly forage in upper soil layers. Our results suggest that tundra vegetation will become graminoid‐dominated as permafrost thaw progresses and nutrient availability increases in the deep soil.