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Towards Integrated Understanding of the Rhizosphere Phenomenon as Ecological Driver: Can Rhizoculture Improve Agricultural and Forestry Systems?

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Agriculture and forestry traditionally focus on improving plant growth traits based on an anthropocentric point of view. This paradigm has led to global problems associated to soil overexploitation such as soil losses, reductions of the C stock in soils, and the generalized use of fertilizers, which particularly increases the costs of production and pollution treatment. This view may also have limited our understanding of mutualistic symbioses of plants and microorganisms assuming that the main role of non-photosynthetic symbionts is to mobilize the nutrients that are necessary for plant growth and development, and being plants the dominant agents of the symbiotic relationship. In response to these issues, this chapter offers an alternative approach taking advantage of the “rhizo-centric” point of view, where non-photosynthetic partners are the main protagonists in play; and secondly, it builds a multidisciplinary body of knowledge that could be called “rhizoculture”, which includes techniques focussing on the intensification of the development and activity of roots, mycorrhizae, and other symbiotic and free living rhizosphere organisms. In short, rhizoculture may lead to decrease plant production dependence on fertilization and provides other benefits to agriculture, forestry, and the environment. Within this conceptual framework, the first objective of this book chapter is to explore whether there is a “paradox of calcium salts” (i.e., Ca 2+ and its salts are simultaneously nutrients, promoters, and stressors for the host plants) that would explain a dominance of mycorrhizal fungi over plants based on inducing a Ca(pH)–mediated chlorosis to the host plants. If this paradigm shifting hypothesis were finally fully verified, it would provide conceptual bases to reconsider our current technologies in agriculture and forestry by introducing the “rhizocultural” approach, based on the management of roots (introducing alternative cultural practices), Ca 2+ salts (using liming and other techniques), rock-eating mycorrhizae, organic matter, and the soil microbiome (increasing the presence of symbiotic microorganisms against saprophytes), N and P contents (by aquaculture and smart recycling of organic waste), and the physical properties of the soil (by the activity of soil symbiotic microorganisms and soil fauna, such as ants, termites and earthworms). The development of such new technological approaches in rhizoculture would significantly decrease the high cost and associated pollution of the application of fertilizers and phytochemicals; as well as it would increase soil C stocks, improve the resilience of agricultural and forest systems to environmental disturbances, such as climate change, and enhance food production and security.

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The arbuscular mycorrhiza (AM) is among the most ubiquitous symbioses in the world. A meta-analysis of 759 articles (1978-2012) was conducted to test whether ecologically important host plant traits (N-fixation and C-fixation pathway) affect the response of the plant to mycorrhizal colonization. We found that the effect of N-fixation on mycorrhizal growth response (MGR) depended on whether the plant was woody or a forb. N-fixing forbs had a higher MGR than non-N-fixing forbs, but the reverse was true for woody plants. Moreover, C4-grasses had significantly higher MGR than C3-grasses, but no significant difference was found between C3 and C4 forbs, or between C3 and C4 woody species. Overall, woody species had higher MGR than any other functional group. These results demonstrate that MGR does depend on host functional characteristics, but neither N-fixation capacity nor C-fixation pathway are apparently fundamental controllers of MGR. Instead, it would appear possible that these traits influence MGR only insofar as they influence more fundamental functions such as P demand and P supply.
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Publisher Summary This chapter reviews the cycling of nutrients within pasture soils. The major emphasis is on the central role of the grazing animal in influencing soil fertility, particularly in the dung and urine patches. In addition, the progress is reviewed that has been made in modeling the cycling of nutrients in pasture systems. Nutrients are partitioned differently between dung and urine, with K being excreted mainly in urine; P, Ca, and Mg are excreted principally in dung and N and S are excreted in both forms. Nutrients returned in excreta can be in inorganic and organic forms, depending on the particular nutrient being considered. For some nutrients (e.g., P and S) significant mineralization of ingested organic forms occurs during passage through the animal, whereas much of the N is excreted in the readily available organic urea form. From the present knowledge of the major nutrient inputs and losses for the grazed pasture system and an understanding of the pathways of nutrient flux within the system and some key measurements, simple mass balance nutrient models for various pasture systems can be constructed. Such simple models have been used to calculate site-specific maintenance fertilizer requirements of pastures based on the amount of nutrient required to replace losses in the soil (e.g., through fixation and leaching) and losses by animal transfer and in animal products.
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Ectomycorrhizal fungal (EMF) communities are extremely species-rich in most forest ecosystems (Horton and Bruns 2001). The number of EMF species can often reach more than 100 at even a small landscape scale (Walker et al. 2005; Ishida et al. 2007). The structure of EMF communities in developed forests resembles those of highly diverse plant and animal communities (Magurran 2004). A variety of disturbances can create bare ground where a biological legacy is absent and primary succession begins (Walker and del Moral 2003). Such disturbances include volcanic activity, glacier retreat, sand dune movement, and mining. Specifically, intensive volcanic activity can abruptly create vast barren deserts completely covered by sterile substrates. These deserts are relatively large and homogeneous habitats relative to those created by other gradually occurring disturbances. Such homogeneous habitats would be ideal for testing fundamental ecological models that could be applied to other ecosystems. Indeed, many ecologists have been attracted to volcanic sites for these reasons (see Walker and del Moral 2003). Until recently, mycorrhizal fungi (especially EMF) have been ignored in studies of volcanic succession. However, data obtained from studies in volcanic systems may lead to a breakthrough in EMF ecology. In this chapter, I focus on EMF in volcanic deserts and use several basic ecological models to provide broadly applicable insights into EMF ecology.
Article
Organic farming is based on the concept of working 'with nature' instead of against it; however, compared with conventional farming, organic farming reportedly has lower productivity. Ideally, the goal should be to narrow this yield gap. In this review, we specifically discuss the feasibility of new breeding techniques (NBTs) for rewilding, a process involving the reintroduction of properties from the wild relatives of crops, as a method to close the productivity gap. The most efficient methods of rewilding are based on modern biotechnology techniques, which have yet to be embraced by the organic farming movement. Thus, the question arises of whether the adoption of such methods is feasible, not only from a technological perspective, but also from conceptual, socioeconomic, ethical, and regulatory perspectives. Copyright © 2015 Elsevier Ltd. All rights reserved.
Article
Acid deposition induced losses of calcium (Ca) from northeastern forests have had negative effects on forest health for decades, including the mobilization of potentially phytotoxic aluminum (Al) from soils. To evaluate the impact of changes in Ca and Al availability on sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia Ehrh.) growth and forest composition following a major ice storm in 1998, we measured xylem annual increment, foliar cation concentrations, American beech root sprouting, and tree mortality at the Hubbard Brook Experimental Forest (Thornton, New Hampshire) in control plots and in plots amended with Ca or Al (treated plots) beginning in 1995. Dominant sugar maple trees were unaffected by the treatment, but nondominant sugar maple tree growth responded positively to Ca treatment. Although plots were mainly composed of sugar maple, American beech experienced the greatest growth on Al-treated plots. Increases in tree mortality on Al-treated plots may have released surviving American beech and increased their growth. The Al tolerance of American beech and the Ca:Al sensitivity of sugar maple contributed to divergent growth patterns that influenced stand productivity and composition. Given that acidic inputs are expected to continue, the growth dynamics associated with Al treatment may have direct relevance to future conditions in native forests. © 2015 National Research Council of Canada. All rights reserved.
Article
(1) Roots collected from three contrasting types of permanent grassland in the Pennines were consistently infected by vesicular-arbuscular (VA) mycorrhizal fungi. (2) About half of the root material with cortex was infected and there was little variation in infection with soil depth. Seasonal changes in infection were small. The greatest total weight of root and total weight of infected root occurred in summer, although the highest levels of infection were in winter. (3) Spores of the VA fungi were not common and it is likely that many of them are non-sporing forms. (4) Top dressings of phosphate fertilisers (250 kg P ha$^{-1}$) decreased mycorrhizal infection during the first year of application, but this effect declined after 2 years at two of the sites. Lime, nitrogen and potassium dressings did not appreciably affect the amount of mycorrhizal infection.
Article
Organic farming aims to stimulate soil fertility by avoiding the use of synthetic fertiliser inputs, relying instead on locally available natural resources. It is regarded by many as a sustainable alternative to conventional farming because it ensures higher biodiversity, restricts environmental pollution, prevents land degradation and is easy to apply for smallholder and subsistence farmers. Although widely practiced and studied in temperate regions, little is known about the potential overall benefits of organic farming in the tropics and subtropics. This paper addresses this gap by undertaking an analysis of the differences between organic and conventional agriculture in the tropics and sub-tropics based on an extensive literature review including 88 papers with 458 data pairs. The comparison is based on three main indicators: yield, gross margin and soil organic carbon (SOC). The differences between the organic and conventional systems for each of these main indicators is represented by the ratio of the value of the indicator in the organic system divided by the corresponding value in the conventional system. This was initially calculated for each data pair individually, and grouped by a variety of explanatory factors, such as precipitation, human development level, soil texture, crop type, organic input type, time after conversion and certification. The results demonstrate that under organic management, yields were on average 26% higher, gross margins 51% higher and soil organic carbon 53% higher than under conventional management. The highest yield increases in organic cropping systems were achieved in the least developed countries, in arid regions and on coarse soils. For gross margins, certification was the main reason for differences between organic and conventional systems. Certified farmers, mostly located in developed countries, receive significantly higher prices. Furthermore, organic farming in the driest regions results in higher profits than in other regions. Even though soil organic carbon was significantly higher overall under organic management, the results do not show significant differences when grouped by the explanatory factors. They do however suggest that the highest carbon sequestration potential occurs in systems that had a high level of inputs, in regions with 1 000-1 500 mm of rainfall and on clayey soils.
Article
Forests are a significant pool of terrestrial carbon. A key feature related to forest biomass harvesting and use is the typical time difference between carbon release into and sequestration from the atmosphere. Traditionally, the use of sustainably grown biomass has been considered as carbon neutral in life cycle assessment (LCA) studies. However, various approaches to account for greenhouse gas (GHG) emissions and sinks of forest biomass acquisition and use have also been developed and applied, resulting in different conclusions on climate impacts of forest products. The aim of this study is to summarize, clarify, and assess the suitability of these approaches for LCA. A literature review is carried out, and the results are analyzed through an assessment framework. The different approaches are reviewed through their approach to the definition of reference land‐use situation, consideration of time frame and timing of carbon emissions and sequestration, substitution credits, and indicators applied to measure climate impacts. On the basis of the review, it is concluded that, to account for GHG emissions and the related climate impacts objectively, biomass carbon stored in the products and the timing of sinks and emissions should be taken into account in LCA. The reference situation for forest land use has to be defined appropriately, describing the development in the absence of the studied system. We suggest the use of some climate impact indicator that takes the timing of the emissions and sinks into consideration and enables the use of different time frames. If substitution credits are considered, they need to be transparently presented in the results. Instead of carbon stock values taken from the literature, the use of dynamic forest models is recommended.
Article
Since they were first described in 1829, earthworm calciferous glands have intrigued invertebrate anatomists and physiologists alike. These organs are present in all species of the family Lumbricidae, occurring in a range of morphological forms. A common feature of the glands is that constituent secretory cells produce a concentrated suspension of calcium carbonate. A number of possible biological roles have been suggested for the secretion (i.e. egg formation, pH buffering of the blood and ingested food, excretion and respiration) but the true function has not yet been demonstrated satisfactorily. Here, we investigated the putative respiratory function of these organs by exposing the worms to 13C-labelled CO2 and glucose and measuring tracer incorporation into the body wall, the gland tissues and the calcareous secretion. Our results support the view that these organs provide a mechanism of CO2 regulation in their tissues and that both environmental and metabolic CO2 can be fixed in this way.
Article
While there is a plethora of studies on the effects of invertebrate grazing on mycelia, including several studies on saprotrophic cord-forming basidiomycetes, there is little information on the effects of grazing on mycelial interactions. The study compares the progress and outcomes of interspecific mycelial interactions between Hypholoma fasciculare, Phallus impudicus, Phanaerochaete velutina and Resinicium bicolor when grazed by the collembola Folsomia candida or Protaphorura armata in agar culture and trays (24 × 24 cm) of non-sterile soil. In ungrazed systems results were broadly consistent with previous studies, though there were few instances of deadlock, and a clear transitive (A > B > C) hierarchy could not be discerned. Instead, there was an intransitive hierarchy (i.e., A > B, B > C but C > A). Additionally, in agar culture, there were considerable differences in combative ability of four different strains of H. fasciculare.
Article
Acidic deposition has caused a depletion of calcium (Ca) in the northeastern forest soils. Wollastonite (Ca silicate) was added to watershed 1 (WS1) at the Hubbard Brook Experimental Forest (HBEF) in 1999 to evaluate its effects on various functions of the HBEF ecosystem. The effects of Ca addition on foliar soluble (extractable in 5% HClO4) ions, chlorophyll, polyamines, and amino acids were studied in three hardwood species, namely sugar maple, yellow birch, and American beech. We further analyzed these effects in relation to elevation at Ca-supplemented WS1 and reference WS3 watersheds. Foliar soluble Ca increased significantly in all species at mid and high elevations at Ca-supplemented WS1. This was accompanied by increases in soluble P, chlorophyll, and two amino acids, glutamate and glycine. A decrease in known metabolic indicators of physiological stress (i.e., the amino acids, arginine and γ-aminobutyric acid (GABA), and the diamine, putrescine) was also observed. In general, these changes were species-specific and occurred in an elevation dependent manner. Despite an observed increase in Ca at high elevation for all three species, only sugar maple exhibited a decrease in foliar putrescine at this elevation indicating possible remediation from Ca deficiency. At higher elevations of the reference WS3 site, foliar concentrations of Ca and Mg, as well as Ca:Mn ratios were lower, whereas Al, putrescine, spermidine, and GABA were generally higher. Comparison of metabolic data from these three species reinforces the earlier findings that sugar maple is the most sensitive and American beech the least sensitive species to soil Ca limitation. Furthermore, there was an increase in sensitivity with an increase in elevation.
Article
One of the crucial challenges currently facing the world is “to support sustainable industrial growth”. A possible solution is offered by process intensification (PI), a design approach offering concrete benefits in manufacturing and processing, substantially shrinking equipment size, boosting plant efficiency, saving energy, reducing capital costs, increasing safety, minimizing environmental impact and maximizing the raw materials exploitation.Membrane processes address the goals of PI because they have the potential to replace conventional energy-intensive techniques, to accomplish the selective and efficient transport of specific components, and to improve the performance of reactive processes. On a number of occasions, commercial conventional separation processes in industry were converted to membrane separation processes with significant reductions in cost, energy, and environmental impact.This paper discusses how membrane engineering contributes to realization of the principles of process intensification. An overview of current developments in the field of membrane operations and their place in the intensification of chemical manufacturing and processing is presented. Several cases of successfully commercialized technologies are discussed in detail. Finally, the opportunity to integrate conventional membrane units with innovative membrane systems or into existing industrial processes is also emphasized.