Interactions between cover crops and soil microorganisms increase phosphorus availability in conservation agriculture

Plant and Soil - An essential task of agricultural systems is to improve internal phosphorus (P) recycling. Cover crops and tillage reduction can increase sustainability, but it is not known...     

Interactions between rhizosphere microorganisms under iron limitation

Bacillus polymyxa, Pseudomonas cepacia and Pseudomonas fluorescens are present in the rhizosphere of many crop plants. Little is known about microbial interactions in the rhizosphere. We investigated the type of interaction between these species under iron limitation. We found that, in mixed batch cultures, P. cepacia stimulates the growth of B. polymyxa and this stimulation can be observed also in low iron medium. Cell-free supernatants of cultures of P. fluorescens with various amounts of the siderophore pyoverdine also stimulate the growth of B. polymyxa. In this case we observed a positive correlation between pyoverdine concentration and growth stimulation. Purified pyoverdine also affects positively the growth of B. polymyxa.     

Competition for nitrogen between plants and soil microorganisms

Experiments suggest that plants and soil microorganisms are both limited by inorganic nitrogen, even on relatively fertile sites. Consequently, plants and soil microorganisms may compete for nitrogen. While past research has focused on competition for inorganic nitrogen, recent studies have found that plants/mycorrhizae in a wide range of ecosystems can use organic nitrogen. A new view of competitive interactions between plants and soil microorganisms is necessary in ecosystem where plant uptake of organic nitrogen is observed.     

Interactions between arbuscular mycorrhizal fungi and soil bacteria

The soil environment is interesting and complicated. There are so many interactions taking place in the soil, which determine the properties of soil as a medium for the growth and activities of plants and soil microorganisms. The soil fungi, arbuscular mycorrhiza (AM), are in mutual and beneficial symbiosis with most of the terrestrial plants. AM fungi are continuously interactive with a wide range of soil microorganisms including nonbacterial soil microorganisms, plant growth promoting rhizobacteria, mycorrhiza helper bacteria and deleterious bacteria. Their interactions can have important implications in agriculture. There are some interesting interactions between the AM fungi and soil bacteria including the binding of soil bacteria to the fungal spore, the injection of molecules by bacteria into the fungal spore, the production of volatiles by bacteria and the degradation of fungal cellular wall. Such mechanisms can affect the expression of genes in AM fungi and hence their performance and ecosystem productivity. Hence, consideration of such interactive behavior is of significance. In this review, some of the most important findings regarding the interactions between AM fungi and soil bacteria with some new insights for future research are presented.     

Interactions between biological processes,cultivation and soil structure

Summary An inherent (autochthonous) biomass is characteristic of a soil while the input of substrates for plant roots or crop residues promotes the transient (zymogenous) biomass. However successful micro-organisms will show aspects of both types of ecological strategy. The biomass generated from plant residue substrates can include toxin-producing and pathogenic species but also beneficial organisms such as N-fixers and polysaccharide-producers. Rhizosphere activity can, depending on soil, plant and microbial species, stabilize or destabilize soils. Microbial activity should be considered in soil management and it may be possible to manipulate the soil population balance towards beneficial organisms.Keynote address     

Interactions between atmospheric CO2 enrichment and soil fauna

We have reviewed the responses of soil fauna to increased concentrations of atmospheric CO₂ and the consequent climate change. These will affect several attributes of animal populations and communities including their density, biomass, diversity, activity, rates of consumption, life history parameters and migration ability. Changes in the quality and quantity of litter and global warming are the main factors which are expected to modify soil fauna. Although changes have been observed in several attributes of the soil fauna as a consequence of increased concentrations of atmospheric CO₂, no general trend which might allow to the prediction of a general pattern of response has been identified. Because of the complexity of the biological mechanisms and the synergetic action of several factors, the few resulting responses reported in the literature are inconclusive. However, some aspects of the situation deserve more attention. These include the consequences of (1) changes in the food resources for soil fauna in the litter layer and in the rhizosphere, (2) the consumption of low quality litter by the macrofauna, (3) the change in life span in response to temperature elevation, (4) the enhancement of earthworm burrowing activity and (5) the changes in community composition arising because of specific differential resistance to adverse conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interactions of chromium with microorganisms and plants

Chromium is a highly toxic non-essential metal for microorganisms and plants. Due to its widespread industrial use, chromium (Cr) has become a serious pollutant in diverse environmental settings. The hexavalent form of the metal, Cr(VI), is considered a more toxic species than the relatively innocuous and less mobile Cr(III) form. The presence of Cr in the environment has selected microbial and plant variants able to tolerate high levels of Cr compounds. The diverse Cr-resistance mechanisms displayed by microorganisms, and probably by plants, include biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate efflux. Some of these systems have been proposed as potential biotechnological tools for the bioremediation of Cr pollution. In this review we summarize the interactions of bacteria, algae, fungi and plants with Cr and its compounds.     

The influence of Bacillus subtilis RB14-C on the development of Rhizoctonia solani and indigenous microorganisms in the soil

The effect of soil inoculation with an antagonistic strain Bacillus subtilis RB14-C on the development of Rhizoctonia solani and changes occurring in soil and rhizosphere microbial communities were studied. RB14-C was added to the soil as a water suspension of the cells or as a broth culture. Application of cell suspensions to non-planted soil reduced the number of culturable bacteria. The density of R. solani and the number of filamentous fungi were not significantly affected by RB14-C. A similar effect was observed in the rhizosphere of tomato plants growns in bacterized soil. Broth cultures of RB14-C suppressed R. solani 1 d after inoculation, but after 3 d there was no difference in the pathogen density between soil amended with broth culture and control soil. In microcosm studies, cell suspensions of RB14-C also did not inhibit growth of R. solani on filters buried in soil. However, an inhibitory effect was obtained when a broth culture of the bacterium was used. The effect of RB14-C on fungal biomass was also estimated by measurement of ergosterol concentration in soil. It was found that ergosterol was mostly derived from R. solani and that there were no significant differences in its content between untreated soil and soil treated with RB14-C. The results suggest that suppression of Rhizoctonia damping-off by B. subtilis RB14-C probably is not related to the reduction of the pathogen population in the soil.     

Interactions between soil habitat and geographic range location affect plant fitness

Populations are often found on different habitats at different geographic locations. This habitat shift may be due to biased dispersal, physiological tolerances or biotic interactions. To explore how fitness of the native plant Chamaecrista fasciculata depends on habitat within, at and beyond its range edge, we planted seeds from five populations in two soil substrates at these geographic locations. We found that with reduced competition, lifetime fitness was always greater or equivalent in one habitat type, loam soils, though early-season survival was greater on sand soils. At the range edge, natural populations are typically found on sand soil habitats, which are also less competitive environments. Early-season survival and fitness differed among source populations, and when transplanted beyond the range edge, range edge populations had greater fitness than interior populations. Our results indicate that even when the optimal soil substrate for a species does not change with geographic range location, the realized niche of a species may be restricted to sub-optimal habitats at the range edge because of the combined effects of differences in abiotic and biotic effects (e.g. competitors) between substrates.     

Interactions between biochar and mycorrhizal fungi in a water-stressed agricultural soil

Biochar may alleviate plant water stress in association with arbuscular mycorrhizal (AM) fungi but research has not been conclusive. Therefore, a glasshouse experiment was conducted to understand how interactions between AM fungi and plants respond to biochar application under water-stressed conditions. A twin chamber pot system was used to determine whether a woody biochar increased root colonisation by a natural AM fungal population in a pasture soil (‘field’ chamber) and whether this was associated with increased growth of extraradical AM fungal hyphae detected by plants growing in an adjacent (‘bait’) chamber containing irradiated soil. The two chambers were separated by a mesh that excluded roots. Subterranean clover was grown with and without water stress and harvested after 35, 49 and 63 days from each chamber. When biochar was applied to the field chamber under water-stressed conditions, shoot mass increased in parallel with mycorrhizal colonisation, extraradical hyphal length and shoot phosphorus concentration. AM fungal colonisation of roots in the bait chamber indicated an increase in extraradical mycorrhizal hyphae in the field chamber. Biochar had little effect on AM fungi or plant growth under well-watered conditions. The biochar-induced increase in mycorrhizal colonisation was associated with increased growth of extraradical AM fungal hyphae in the pasture soil under water-stressed conditions.     

Interaction between the fungicide metalaxyl and soil microorganisms on survival of Phytophthora cinnamomi

In soil leachates the fungicide, metalaxyl, eliminated sporangia production by Phytophthora cinnamomi as well as enhancing hyphal lysis. There was a marked interaction between lytic soil microorganisms, fungicide application and hyphal lysis.     

农田土壤动物和微生物与生物化学动态关系的研究

研究了农田土壤动物和微生物的季节动态与土壤生物化学特征之间的关系 ,并利用灰色理论与方法进行分析。结果表明 ,土壤生物和土壤生物化学特征均有明显的季节性变化。而土壤动物和微生物的季节动态与土壤生物化学特征之间也具有明显的相关性。其中关系最大的生物化学特征是过氧化氢酶、脱氢酶、无机磷转化作用和枯枝落叶分解速率。并建立了 5个灰色数学模型 [GM(1,5 ) ]。     

Interactions of plant species mediated plant competition for inorganic nitrogen with soil microorganisms in an alpine meadow

The aim of the present work was to evaluate the effects of regulated deficit irrigation (RDI) applied in the post-harvest stage of peach trees. The 3-year trial was carried out in Italy (N 40°20′, E 16°48′) on mature peach plants (cv “Springcrest”) trained to transverse Y. From bud break to harvest, irrigation was carried out by applying 100% ET_c, while from harvest to early autumn, plants were separated into three groups and subjected to different irrigation treatments (100, 57 and 34% ET_c). The decrease in soil water content caused a reduction in the values of tissue water potential and gas exchange both in 57% ET_c and 34% ET_c treatments. RDI determined the reduction in the growth of waterspouts and lateral shoots but did not influence the growth of fruiting shoots. During the trial, no significant reductions in crop yield and quality were observed in the 57% ET_c treatment, whereas about 1,100, 1,800 and 2,500 m³ ha⁻¹ of water were saved in the first, the second and the third year, respectively. In the second year of the trial, the use of RDI in the post-harvest stage determined carbohydrate and nitrogen accumulation in roots, branches, shoots and floral buds. The results demonstrate that, under scarce water supply conditions, a clear benefit can be obtained through the use of RDI during the post-harvest stage. This confirms the possibility to reduce the irrigation water by applying RDI during phenological stages less sensitive to water deficit without negatively affecting peach growth and yield.