Plant growth promoting bacteria (PGPR) associations range in degree of bacterial proximity to the root and intimacy of association. In general, these can be separated into extracellular PGPR (ePGPR), existing in the rhizosphere, on the rhizoplane or in the spaces between cells of the root cortex, and intracellular PGPR (iPGPR), which exist inside root cells, generally in specialized nodular structures. The latter includes rhizobia and Frankia species, both of which fix nitrogen in symbiosis with higher plants. There has been considerable development in understanding signaling mechanisms of rhizobia (iPGPR) during the establishment of the rhizobia–legume symbiosis, and this may serve as a model of knowledge regarding cross-talk and plant growth promoting mechanisms. We provide a detailed review of this process, including plant-to-bacteria signal molecules, followed by bacterial perception and consequent production of bacteria-to-plant signals. A history of PGPR discovery is also provided, indicating progress in understanding each of the PGPR groups. Recent advances in understanding plant growth responses to microbial signals are reviewed, along with the research areas that require attention. Based on new understandings of signaling mechanisms in the iPGPR (rhizobia) and recent findings with ePGPR we are able to speculate regarding general patterns of signaling in the ePGPR.

Intracellular and extracellular PGPR: commonalities and distinctions in the plant–bacterium signaling processes

Summary Biodiversity is responsible for the provision of many ecosystem services; human well-being is based on these services, and consequently on biodiversity. In soil, earthworms represent the largest component of the animal biomass and are commonly termed ‘ecosystem engineers’. This review considers the contribution of earthworms to ecosystem services through pedogenesis, development of soil structure, water regulation, nutrient cycling, primary production, climate regulation, pollution remediation and cultural services. Although there has been much research into the role of earthworms in soil ecology, this review demonstrates substantial gaps in our knowledge related in particular to difficulties in identifying the effects of species, land use and climate. The review aims to assist people involved in all aspects of land management, including conservation, agriculture, mining or other industries, to obtain a broad knowledge of earthworms and ecosystem services.

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A review of earthworm impact on soil function and ecosystem services

Regulation of soil organic matter dynamics and microbial activityin the drilosphere and the role of interactionswith other edaphic functional domains

History shows that Caribbean coastal ecosys- tems were severely degraded long before ecologists began to study them. Large vertebrates such as the green turtle, hawksbill turtle, manatee and extinct Caribbean monk seal were decimated by about 1800 in the central and northern Caribbean, and by 1990 elsewhere. Subsistence over-fishing subsequently decimated reef fish populations. Local fisheries accounted for a small fraction of the fish consumed on Caribbean islands by about the mid nine- teenth century when human populations were less than one fifth their numbers today. Herbivores and predators were reduced to very small fishes and sea urchins by the 1950s when intensive scientific investigations began. These small consumers, most notably Diadema antillarum, were apparently always very abundant; contrary to speculation that their abundance had increased many-fold due to overfishing. Studying grazing and predation on reefs today is like trying to understand the ecology of the Serengeti by studying the termites and the locusts while ignoring the elephants and the wildebeeste. Green turtles, hawksbill turtles and manatees were almost certainly comparably important keystone species on reefs and seagrass beds. Small fishes and invertebrates feed very di⁄erently from turtles and manatees and could and can not compensate for their loss, despite their great abund- ance long before overfishing began. Loss of megaverte- brates dramatically reduced and qualitatively changed grazing and excavation of seagrasses, predation on sponges, loss of production to adjacent ecosystems, and the structure of food chains. Megavertebrates are critical for reef conservation and, unlike land, there are no coral reef livestock to take their place.

Reefs since Columbus

Publisher Summary The chapter presents a discussion on both rhizosphere and rhizoplane. Rhizosphere is defined as the soil adjacent to roots with a different physical, chemical, and biological environment from the bulk soil. The rhizoplane is defined as the actual root surface or soil interface. The chapter describes plant responses, the nature and dynamics of rhizosphere populations and their determinants, the management of the plant, and management of microorganisms. The chapter also explores the impact of soil conditions and agronomic practices on the rhizoshere microflora and suggests areas that require more study in order to achieve effective management. The chapter also illustrates different methods that are used by agronomists, plant pathologists, and microbiologists in order to modify the rhizosphere. In most cases, when microbiologists and plant pathologists needs to change the rhizosphere in order to improve plant growth they often think in terms of introducing microorganisms that possess the desired characteristics, be they antibiotic or siderophore production, or the ability to stimulate plant growth.

The rhizosphere and its management to improve plant growth

Casts of 13 earthworm species with differing ecological strategies were collected from more than 60 sites and examined for the presence of propagules of vesicular arbuscular mycorrhizal (VAM) fungi. Intact spores of VAM fungi were found in all but one collection. VAM root fragments were also present in some cast samples. The diversity of VAM spore types found in casts was similar to that in adjacent, uningested soils. However, the relative concentration of spores (No. g−1 dry wt) in casts was higher than in the general soil for the two earthworm species (Pontoscolex corethrurus and Diplotrema heteropora) for which this was assessed. There was a strong seasonality in the number of spores recovered from casts of D heteropora and this reflected seasonal patterns of spore production in soil. Glasshouse experiments demonstrated that spores recovered from casts of P. corethrurus and D. heternpora maintained viability and initiated mycorrhizal infection on Sorghum bicolor. Similarly, some VAM root fragments found in casts were also able to infect S. bicolor. Spores of ectomycorrhizal fungi and fragments of ectomycorrhizal roots were observed at low frequencies in some cast samples from sites dominated by sclerophyllous vegetation. The infectivity of these propagules was not determined. The ubiquity and ecological significance of short distance transport of VAM propagules by earthworms is discussed in relation to the ecological strategics of earthworms.

Earthworms as vectors of viable propagules of mycorrhizal fungi

Effects of a tropical cyclone on littoral and sub-littoral biotic communities and on a population of Dugongs (Dugong dugon (Müller))

A glasshouse experiment was used to investigate the transfer of infective Frankia propagules in soil by the endogeic earthworm Pontoscolex corethrurus Seedlings of Casuarina equisetifolia were inoculated, either with a crushed nodule suspension of Frankia, or with casts of P corethrurus that had been raised in a sterilized soil through which crushed nodules of C equisetifolia had been thoroughly mixed Two concentrations of cast material were applied: casts from 3 earthworms and casts from 11 earthworms Uninoculated plants grew poorly and did not nodulate All seedlings inoculated directly with the crushed nodule suspension formed nodules; shoot dry weights of these seedlings 10 weeks after inoculation were 70% higher than those of the uninoculated plants All plants inoculated with casts from 11 earthworms formed nodules Shoot and nodule dry weights of these seedlings were similar to those of the plants that were directly inoculated with the crushed nodules Decreasing the number of earthworm casts applied to 3 per plant substantially reduced plant growth compared to the higher cast concentration Nodulution was erratic in this treatment with only 3 of the 7 plants forming nodules Shoot dry weights of plants inoculated wih 3 casts were similar to those of the uninoculated treatment The possible ecological significance of these findings is discussed in relation to the ecology of earthworms in soil

Transmission of infective Frankia (Actinomycetales) propagules in casts of the endogeic earthworm Pontoscolex corethrurus (Oligochaeta : Glossoscolecidae)

Populations of Dugongs (Mammalia: Sirenia): Aerial survey over the inshore waters of tropical Australia

Closure is a critical phase of a mining operation since this is when the environmental effects of mining and
rehabilitation works will be most stringently judged. Since the time scales required for biological community
development and soil formation are long in comparison to mining operation life cycles - and to human life spans - it is normally only possible to demonstrate the early trajectories of these processes. However, favourable and predictable closure outcomes can usually be achieved by making early decisions on postmining land use, ensuring close integration of mining with incremental rehabilitation works and adequate monitoring. This assumes that a successful system of rehabilitation is available for the materials and environments involved and takes no account of threats or stressors. This paper defines the stages of early post-rehabilitation ecosystem and soil development on highly weathered soil materials at the Alcan Gove Mine using information derived from studies of selected sites from a 26-year chronosequence of sites and local native forest sites. Specifically, we seek to determine the stage at which a high probability of successful ongoing development may be credibly assumed in the absence of further management inputs of energy and materials and in relationship to potential external threats.

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Alister V. Spain

Papers

Earthworms as vectors of viable propagules of mycorrhizal fungi

Effects of a tropical cyclone on littoral and sub-littoral biotic communities and on a population of Dugongs (Dugong dugon (Müller))

Transmission of infective Frankia (Actinomycetales) propagules in casts of the endogeic earthworm Pontoscolex corethrurus (Oligochaeta : Glossoscolecidae)

Populations of Dugongs (Mammalia: Sirenia): Aerial survey over the inshore waters of tropical Australia

Mine Closure and Ecosystem Development ⎯ Alcan Gove Bauxite Mine, Northern Territory, Australia