N2-fixing bacteria in the rhizosphere: Quantification and hormonal effects on root development

The paper summarizes the results of a series of experiments on enumeration of N₂-fixing bacteria (diazotrophs) and hormonal effects of Azospirillum on root development. Numbers of N₂-fixing and N-heterotrophic bacteria were determined on the root (rhizoplane plus “inner” root surface) and in the rhizosphere soil (0–3 mm from the root surface) of Arrhenatherum elatius, other forage grasses and some herbaceous plant species. Pot experiments involved freshly collected soil from an unfertilized grassland area containing its natural population of N₂-fixing bacteria. The MPN (most probable number) of diazotrophs in relation to the MPN of the total bacterial population was always lower on the root than in the rhizosphere soil, suggesting that diazotrophs were not selectively advantaged at the root surface. Supply of mineral nitrogen (NH₄NO₃) decreased the proportion of N₂-fixing bacteria at the rhizoplane as well as in the rhizosphere soil. Similar results were obtained when N was supplied via the leaves. The data suggest that N₂-fixing bacteria in the rhizosphere are poor competitors once they loose their competitive advantage of binding dinitrogen. Correspondingly, the increase in the MPN of the diazotrophs found during plant development was interpreted as a result of decreased available combined N in the rhizosphere. The proportion of N₂-fixing bacteria relative to the total number of bacteria was generally below 1%. Considering the potential amount of substrate released from the roots and the substrate requirement of the bacterial population, N₂-fixation was considered insignificant for plant growth under the given conditions. For the investigations on possible beneficial effects on plant development by bacterial hormones, Azospirillum brasilense was chosen because evidence suggests that amongst the soil bacteria releasing hormones, especially IAA, certain strains of this species are more important than other bacteria. Application of A. brasilense Cd (ATCC 29710) onto the roots of young wheat plants grown in soil increased the number of lateral roots, the total root length and the number of root hairs. Similar results were obtained after application of IAA. This suggests that IAA is an important factor responsible for the effects observed after inoculation with A. brasilense. The increase in root surface may improve acquisition of nutrients and enhance growth of plants. Another hormonal effect of A. brasilense was an increase in nodulation of Medicago sativa grown on agar. Again pure IAA resulted in a similar increase in nodule number. Increases in nodule number were only in part associated with a change in root morphology. Therefore an effect of IAA on the plant immanent regulation system for nodulation is likely.     

Effects of increasing fire frequency on black carbon and organic matter in Podzols of Siberian Scots pine forests

Fires in boreal forests frequently convert organic matter in the organic layer to black carbon, but we know little of how changing fire frequency alters the amount, composition and distribution of black carbon and organic matter within soils, or affects podzolization. We compared black carbon and organic matter (organic carbon and nitrogen) in soils of three Siberian Scots pine forests with frequent, moderately frequent and infrequent fires. Black carbon did not significantly contribute to the storage of organic matter, most likely because it is consumed by intense fires. We found 99% of black carbon in the organic layer; maximum stocks were 72 g m^?2. Less intense fires consumed only parts of the organic layer and converted some organic matter to black carbon (> 5 g m^?2), whereas more intense fires consumed almost the entire organic layer. In the upper 0.25 m of the mineral soil, black carbon stocks were 0.1 g m^?2 in the infrequent fire regime. After fire, organic carbon and nitrogen in the organic layer accumulated with an estimated rate of 14.4 g C m^?2 year^?1 or 0.241 g N m^?2 year^?1. Maximum stocks 140 years after fire were 2190 g organic C m^?2 and 40 g N m^?2, with no differences among fire regimes. With increasing fire frequency, stocks of organic carbon increased from 600 to 1100 g m^?2 (0–0.25 m). Stocks of nitrogen in the mineral soil were similar among the regimes (0.04 g m^?2). We found that greater intensities of fire reduce amounts of organic matter in the organic layer but that the greater frequencies may slightly increase amounts in the mineral soil.     

The use of 8-hydroxyquinoline for the isolation and prequalification of plant growth-stimulating rhizosphere pseudomonads

Summary The application of 8-hydroxyquinoline (8OHQ) in a low-iron medium is demonstrated to be an important selective agent providing a simple and non-laborious method in the search for plant growth-stimulating and antagonistic fluorescent pseudomonads. Eleven isolates obtained from low-iron plates containing 120 ppm 8OHQ possessed similar or even better antagonistic properties in vitro than the reference strains Pseudomonas fluorescens WCS 374 and pseudobactin-producing strain B10 with plant growth-stimulating properties. One out of four of the best in vitro antagonists showed significant growth stimulation of radish (Raphanus sativus), comparable to that of the plant growth-stimulating reference strains. Iron supply abolished their antagonism in vitro, indicating that siderophores were primarily involved. The selective mode of action of 8OHQ was demonstrated to be iron stress induced by the strong chelation of available iron.     

Einfluß unterschiedlicher Düngung auf pH,N, C und die Gehalte an CAL-extrahierbarem K und P im Boden

Influence of different fertilization on pH, N, C and CAL-extractable K and P in the soil The influence of different fertilization (mineral fertilizer, different kinds and quantities of farmyard manure and biocompost, horn meal) on soil properties was studied during 8 years of cultivation. The plots were planted and harvested as practised under local farming conditions. The pH of the non-fertilized plots decreased from 5.84 to 5.69, while it was increased by fertilization with farmyard manure or biocompost. N_t in the soil was not influenced by different fertilization. C_t was increased by high biocompost application, stayed constant on the other fertilized plots but decreased on the nonfertilized plots from 1.08 to 0.99%. Without fertilization, plant available nutrients were diminished from 7.3 to 4.3 mg P(CAL) 100 g^?1 and from 22.5 to 13.9 mg K(CAL) 100 g^?1. However, if the plots were fertilized with mineral or organic fertilizer, the nutrient content remained on the initial level. Storing farmyard manure under roof or covering with straw or polyethylene sheet did not affect the criteria of soil studied.     

Diminution of acetylene reduction during assay of detached soybean nodules

Initial rates of acetylene reduction were observed to decline after about 2 h at a given ratio of detached soybean nodule mass to assay vessel vol. Such decreases occurred despite final concentrations of oxygen at 20–30 per cent. and could not be attributed to depletion of acetylene or to accumulation of hydrogen or carbon dioxide. Nodules held briefly at high nodule mass to container vol. ratio gave evidence of progressive inactivation with holding time when subsequently assayed. A search for possible gaseous inhibitors revealed several unknown products that occurred consistently during prolonged acetylene reduction assay. One product observed during assay gave a peak on the gas chromatogram that was determined by the initial concentration of oxygen, and varied inversely with the ethylene peak. The compound responsible for this peak was identified as ethanol. Ethanol vapor added to the assay system inhibited acetylene reduction after 2 h. A high initial oxygen concn depressed somewhat the inhibitory effect of ethanol.     

Lignin turnover in arable soil and grassland analysed with two different labelling approaches

When modelling the carbon dynamics of temperate soils, soil organic carbon (SOC) is often represented by three kinetic pools, i.e. fast, slow and passive/inert. Lignin is often considered to be relatively resistant to decomposition, thus possibly contributing to the passive SOC pool. One way to assess SOC turnover under natural conditions is to follow the fate of ¹³C-labelled biomass in soils. We used compound-specific isotope analysis to analyse CuO oxidation products of lignin from grassland topsoils and from an arable topsoil that had received a natural (by C3-C4 vegetation change) or an artificial (by fumigation with labelled CO₂) isotopic label for 9–23 years. Results indicate faster apparent turnover for lignin (5–26 years in grassland, 9–38 years in arable soil) compared with bulk SOC (20–26 years in grassland, 51 years in arable soil). Although these calculated lignin turnover times cannot be extrapolated to the whole soil profiles, this paper provides isotopic evidence that lignin in soils is not preferentially preserved, which is a consistent result from both ways of isotopic labelling. It also demonstrates, however, that a considerable proportion of lignin in temperate soils can be stabilized for at least a few decades.     

Differential effects of potassium and molybdenum deficiency and fertilization on nitrogen metabolism in Norway spruce (Picea abies)

A field study on mineral nutrition of 12 year old Norway spruce (Picea abies (L.) Karst.) was carried out on a Lower Triassic region in the Eifel mountains in a plantation where the juvenile trees exhibited marked visual differences on adjacent plots, thus indicating a patchy pattern concerning the fertility status of the soil. This was confirmed by geological, pedological and chemical analyses, including X-ray diffraction analyses of the clay minerals and soil water as well as needle element analyses. A combined K and N deficiency was diagnosed for those trees, which exhibited growth retardation and small, rigid, and yellowish-green needles. Chlorophyll and protein content and glutamine synthetase activity were reduced while triphenyltetrazoliumchloride (TTC) reduction was enhanced in the sick trees as compared to adjacent healthy looking trees of the same age and provenance. A diagnostic fertilization treatment with K and Mo, respectively, yielded the following results. One year after administration of Mo a marked increase in total N content of the needles was observed, due to an increase of the storage pools for soluble (nonprotein) nitrogen, namely the basic amino acids arginine, histidine and lysine and also of ornithine. Yet, no visual recovery of the trees was observed. On the other hand, K fertilization caused an improvement of all biochemical parameters, except protein content and soluble N storage pools. The relationships between Mo dependent nitrate reduction, potassium content and nitrogen metabolism are discussed.     

Die Seefiefer (Sternkiefer, Igelf?hre)

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