Microbial weathering of a brown forest soil: Influence of partial sterilization

Distilled water or nutrient medium with no addition (no sterilization), with thymol added (partial sterilization) or with Na merthiolate added (total sterilization) were semi-continuously perfused through 65 g soil columns. Amounts of Fe, Al, Mg solubilized throughout a 6 week period were respectively 15.2, 27.2, 21.0 mg (non-sterile columns); 55.7. 49.3, 45.7 mg (partially sterile columns): 0.4. 2.6, 11.0 mg (totally sterile columns). Partial sterilization also promoted microbial solubilization of Si. Mn. Ca and increased the weathering indices of Al and Fe. Partial sterilization decreased the total number of microorganisms and reduced the number of species. Fungi were eliminated (except for some yeasts). Moreover the metabolism of the resistant species was changed by this treatment. Thymol increased the yield of compounds such as oxalic acid. known to be active complexing agents.Ecological implications of the stimulation of microbial weathering through partial sterilization are discussed.     

Inoculation withGlomus mosseae improves N2 fixation by field-grown soybeans

Summary A field study carried out in a sandy, relatively acid Senegalese soil with a low soluble P content (7 ppm) and low vesicular-arbuscular mycorrhizal (VAM) populations showed that soybean responded toGlomus mosseae inoculation when the soluble P level in the soil had been raised by the addition of 22 kg P ha^–1. In P-fertilized plots, N₂ fixation of soybean, assessed by the A value method, was 109 kg N₂ fixed hat when plants were inoculated withRhizobium alone and it reached 139 kg N₂ fixed ha^–1 when plants were dually inoculated withRhizobium andGlomus mosseae using an alginate bead inoculum. In addition to this N₂ fixation increase (+28%),Glomus mosseae inoculation significantly improved grain yield (+13%) and total N content of grains (+16%). This success was attributed mainly to the low infection potential of the native VAM populations in the experimental site. In treatments without solubleP or with rock phosphate, no effect of VAM inoculation was observed.     

Non-symbiotic nitrogen fixation in the rhizospheres of rice,maize and different tropical grasses

Nitrogenase activity estimated in the rhizospheres of rice, maize and different tropical grasses grown under controlled laboratory conditions was shown to depend upon plant species. High nitrogenase activity (2000–6000 nmoles C₂H₄ h^?1 g^?1 dry root) occurred in rice rhizosphere, this activity being only 10 times lower than that of symbiotic systems; in the rhizosphere of many other grasses grown in a similar way nitrogenase activity was as low as 10 nmoles C₂H₄ h^?1 g^?1 dry root. The influence of soil type on nitrogenase activity was impressive; but the exact nature of the factors implicated could not be established. A rather weak flush of nitrogenase activity in the rhizosphere occurred in the early stage of the plant growth; it was probably due to the exudation of compounds from the seed and lasted 2 or 4 days according to the size of the seed. When the plant entered into its intense photosynthetic phase, the nitrogenase activity gradually increased. When the shoots were severed, nitrogenase activity in the rhizosphere ceased. Nitrogenase activity in the rhizosphere responded greatly to light intensity. Extrapolation of these laboratory findings to the field is discussed.     

Sesbania rostrata green manure and the nitrogen content of rice crop and soil

The effects of four treatments upon the N content of rice crop and soil in 1m² irrigated microplots were compared: (1) PK fertilization + Sesbania rostrata (inoculated stems) ploughed in as green manure when it was 52 days old. (2) PK fertilization + S. rostrata (non-inoculated stems) ploughed in as green manure. (3) PK fertilization + ammonium sulphate (60kg N ha^?1). (4) PK fertilization alone (control).The application of chemical N fertilizer (treatment 3) increased the grain yield by 169 g m^?2 (1.69t ha^?1). whereas incorporating S. rostrata as green manure resulted in a grain yield increase of 372 g m^?2 (3.72t ha^?1). N₂ fixed by S. rostrata was estimated to be at least 26.7 g m^?2 (267kg N ha^?1), one third being transferred to the crop and two thirds to the soil.     

Assessment of N2 fixation by Casuarina equisetifolia inoculated with Frankia ORS021001 using 15N methods

Casuarina equisetifolia seedlings, uninoculated or inoculated with Frankia strain ORS021001 were grown for 4.5 months in pouches, then transplanted into 1 m³ concrete containers forming 1 m² microplots. Trees were harvested 6.5 months later when they were 11 months old. N₂ fixation was measured using three methods of assessment: the direct isotopic method, the A-value method and the difference method. Estimations of N₂ fixation during the 6.5 months following transplantation were respectively 3.27, 2.31 and 3.07g N₂ per tree. From these values it was calculated that about 40–60 kg N₂ would be fixed per hectare in a year at normal densities of 10,000 trees ha^?1. The results of this experiment confirm that Frankia strain ORS021001 can be confidently recommended to inoculate casuarinas in the field. Means to improve nodulation and subsequently N₂ fixation by casuarinas are discussed.     

Nitrogen-fixing potential of Acacia mangium and Acacia auriculiformis seedlings inoculated with Bradyrhizobium and Rhizobium spp.

Summary Two Australian Acacia species, A. mangium and A. auriculiformis were inoculated in vitro with eight strains of Bradyrhizobium spp. and two strains of Rhizobium spp. On the two plant species, only Bradyrhizobium spp. strains formed effective N₂-fixing nodules. A. mangium, which nodulates effectively with a restricted range of Bradyrhizobium spp. strains, is a specific host compared to A. auriculiformis. A. auriculiformis is assumed to be a promiscuous host because it nodulates effectively with a wide range of Bradyrhizobium spp. strains. Nodule efficiency as expressed by the ratio of N₂ fixed to nodule dry weight appeared to be higher in A. auriculiformis (0.44–0.81) than in A. mangium (0.23–0.55).