No abstract

The relative impacts of nutrient availability, disturbance intensity, and interspecific competition on species distribution and growth were studied by applying five levels of nutrients and five intensities of clipping to simple artificial communities (of Dactylis glomerata, Holcus lanatus, Lolium perenne, and Trifolium repens) in a factorial design for 16 mo. This was done to test the hypothesis that the effects of interspecific competition would decline along gradients as the amount of nutrients decreased and the degree of disturbance increased. Nutrients and disturbance, alone and in interaction, produced significant effects on percent cover of all species, and all species achieved maximum percent cover in the undisturbed regime. The experimental design permitted us to distinguish between percent cover reductions caused directly by nutrients and disturbance from those caused by interspecific competition. When grown in mixture, interspecific competition produced marked changes in the distribution patterns of percent cover that were in addition to the direct impacts of nutrients and disturbance. The absolute reduction in percent cover due to interspecific competition was greatest in the undisturbed regime, and for many mixtures this competitive effect was significantly reduced or even eliminated in the highly disturbed areas. Along the nutrient gradient the responses were variable and neighbor dependent, but for all species, the absolute reduction in percent cover due to interspecific competition was generally most severe where percent cover in pure stands was highest.When the data were standardized for differences in percent cover in pure stands and competition was assessed as the percent reduction in percent cover between pure and mixed stands, then the effect of interspecific competition on Trifolium declined along both gradients. The three grasses had a reasonably constant competitive effect across the disturbance gradient, but along the nutrient gradient the highest competitive effects were generally achieved where percent cover in pure stands was highest. We argue that this standardized index is less appropriate because it obscures the obvious differences in interspecific competition effects that are clearly operating within these communities.

A greenhouse experiment was performed to evaluate the effects of plant growth-promoting rhizobacteria (PGPR) on nodulation, biological nitrogen fixation (BNF) and growth of the common bean (Phaseolus vulgaris L. cv. Tenderlake). Single and dual inoculation treatments of bean with Rhizobium and/or PGPR were administered to detect possible changes in the levels of and interactions between the phytohormones IAA and cytokinin. Bean plants cv. Tenderlake were grown in pots containing Fluvic Neosol eutrophic (pH 6.5). Fourteen kilogram aliquots of soil contained in 15-l pots were autoclaved. Bean seeds were surface sterilized and inoculated with Rhizobium tropici (CIAT 899-standard strain) alone and in combination with one of the PGPR strains: Bacillus endophyticus (DSM 13796), B. pumilus (DSM 27), B. subtilis (DSM 704), Paenibacillus lautus (DSM 13411), P. macerans (DSM 24), P. polymyxa (DSM 36), P. polymyxa (Loutit L.) or Bacillus sp.(65E180). The experimental design was randomized block design with three replications. Beans co-inoculated with Rhizobium tropici (CIAT899) and Paenibacillus polymyxa (DSM 36) had higher leghemoglobin concentrations, nitrogenase activity and N 2 fixation efficiency and thereby formed associations of greater symbiotic efficiency. Inoculation with Rhizobium and P. polymyxa strain Loutit (L) stimulated nodulation as well as nitrogen fixation. PGPR also stimulated specific-nodulation (number of nodules per gram of root dry weight) increases that translated into higher levels of accumulated nitrogen. The activities of phytohormones depended on their content and interactions with Rhizobium tropici and Paenibacillus and/or Bacillus (PGPR) strains which affect the cytokinin in content in the common bean.

We inoculated lodgepole pine (Pinus contorta var. latifolia (Dougl.) Engelm.) with Paenibacillus polymyxa P2b-2R, a diazotrophic bacterium previously isolated from internal stem tissue of a naturally regenerating pine seedling to evaluate biological nitrogen fixation and seedling growth promotion by this microorganism. Seedlings generated from pine seed inoculated with strain P2b-2R were grown for up to 13 months in a N-limited soil mix containing 0.7 mM available N labeled as Ca((15)NO3)2 to facilitate detection of N2-fixation. Strain P2b-2R developed a persistent endophytic population comprising 10(2)-10(6) cfu g(-1) plant tissue inside pine roots, stems, and needles during the experiment. At the end of the growth period, P2b-2R had reduced seedling mortality by 14 % and (15)N foliar N abundance 79 % and doubled foliar N concentration and seedling biomass compared to controls. Our results suggest that N2-fixation by P. polymyxa enhanced growth of pine seedlings and support the hypothesis that plant-associated diazotrophs capable of endophytic colonization can satisfy a significant proportion of the N required by tree seedlings growing under N-limited conditions.

Laboratory experiments were conducted with the millipede Harpaphe haydeniana haydeniana Wood (Polydesmida: Xystodesmidae) to determine (i) its litter feeding preferences, (ii) rates of leaf litter consumption, (iii) feeding effects on available nitrogen, and (iv) functional microbial diversity. The millipede exhibited a preference for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and, to a lesser extent, Sitka spruce (Picea sitchensis (Bong.) Carrière) litter compared with western hemlock (Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thuja plicata Donn ex D. Don) litter when given a choice. When only one litter type was provided, millipedes consumed considerably more western redcedar than Douglas-fir, Sitka spruce, or western hemlock. Among the six broadleaf species tested, paper birch (Betula papyrifera Marsh.), bigleaf maple (Acer macrophyllum Pursh), vine maple (Acer circinatum Pursh), and red alder (Alnus rubra Bong.) were consumed at much higher rates than swordfern (Polystichum munitum (Kaulf.) Presl.) or salal (Gaultheria shallon Pursh). Daily rates of conifer litter consumption ranged between 10 and 20% of the millipede's fresh biomass and may translate to 36% of the annual litter fall. Our results suggest that transformation of conifer litter into millipede frass can increase rates of litter decomposition and N mineralization, as well as influence microbial activity and diversity in coastal forests.Résumé : Des expériences en laboratoire ont été réalisées avec des millipèdes (Harpaphe haydeniana haydeniana Wood, Polydesmida : Xystodesmidae) en vue de déterminer (i) la litière qu'ils préfèrent comme source de nourriture, (ii) leur taux de consommation de litière de feuilles, (iii) les effets de leur consommation sur l'azote disponible et (iv) la diversité microbienne fonctionnelle. Lorsqu'ils avaient le choix, les millipèdes ont manifesté une préférence pour la litière du douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco) et, dans une moindre mesure, pour celle de l'épinette de Sitka (Picea sitchensis (Bong.) Carrière), comparativement à la litière de la pruche de l'Ouest (Tsuga heterophylla (Raf.) Sarg.) et du thuya géant (Thuja plicata Donn ex D. Don). Lorsqu'on leur fournissait seulement un type de litière, les millipèdes consommaient beaucoup plus de thuya géant que de douglas de Menzies, d'épinette de Sitka ou de pruche de l'Ouest. Parmi les six espèces feuillues testées, le bouleau blanc (Betula papyrifera Marsh.), l'érable à grandes feuilles (Acer macrophyllum Pursh), l'érable circiné (Acer circinatum Pursh) et l'aulne rouge (Alnus rubra Bong.) étaient consommés à un taux beaucoup plus élevé que le polystic épée (Polystichum munitum (Kaulf.) Presl) ou la gaulthérie salal (Gaultheria shallon Pursh). Les taux journaliers de consommation de litière de conifères se situaient entre 10 et 20% de la biomasse fraîche des millipèdes et pouvaient représenter 36% de la chute annuelle de litière. Les résultats obtenus suggèrent que, dans les forêts côtières, la transformation de la litièr...