Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Organic acids in the rhizosphere: a critical review

The rhizosphere is the zone of soil immediately surrounding plant roots that is modified by root activity In this critical zone, plants perceive and respond to their environment As a consequence of normal growth and development, a large range of organic and inorganic substances are exchanged between the root and soil, which inevitably leads to changes in the biochemical and physical properties of the rhizosphere Plants also modify their rhizosphere in response to certain environmental signals and stresses Organic anions are commonly detected in this region, and their exudation from plant roots has now been associated with nutrient deficiencies and inorganic ion stresses This review summarizes recent developments in the understanding of the function, mechanism, and regulation of organic anion exudation from roots The benefits that plants derive from the presence of organic anions in the rhizosphere are described and the potential for biotechnology to increase organic anion exudation is highlighted

Function and mechanism of organic anion exudation from plant roots

Phosphorus is an important plant nutrient which is in short supply in many agricultural soils Because much of the soluble phosphate (P) applied to soils as fertilizer is “fixed” by the soil and rendered less available to plants, the long-term application of P fertilizers has resulted in an accumulation of total soil P, most of which is poorly soluble Many soil fungi, predominantly of the genera Aspergillus and Penicillium , have been shown to possess the ability to solubilize sparingly soluble phosphates in vitro by secreting inorganic or organic acids Growth promotion and increased uptake of P by plants inoculated with P-solubilizing fungi have also been reported by many investigators

Growth Promotion of Plants Inoculated with Phosphate-Solubilizing Fungi

Tannins make up a significant portion of forest carbon pools and foliage and bark may contain up to 40% tannin. Like many other plant secondary compounds, tannins were believed to function primarily as herbivore deterrents. However, recent evidence casts doubts on their universal effectiveness against herbivory. Alternatively, tannins may play an important role in plant–plant and plant–litter–soil interactions. The convergent evolution of tannin-rich plant communities on highly acidic and infertile soils throughout the world, and the intraspecific variation in tannin concentrations along edaphic gradients suggests that tannins can affect nutrient cycles. This paper reviews nutrient dynamics in forest ecosystems in relation to tannins. Tannins comprise a complex class of organic compounds whose concentration and chemistry differ greatly both among and within plant species. Because the function and reactivity of tannins are strongly controlled by their chemical structure, the effects of tannins on forest ecosystem processes are expected to vary widely. Tannins can affect nutrient cycling by hindering decomposition rates, complexing proteins, inducing toxicity to microbial populations and inhibiting enzyme activities. As a result, tannins may reduce nutrient losses in infertile ecosystems and may alter N cycling to enhance the level of organic versus mineral N forms. The ecological consequences of elevated tannin levels may include allelopathic responses, changes in soil quality and reduced ecosystem productivity. These effects may alter or control successional pathways. While a great deal of research has addressed tannins and their role in nutrient dynamics, there are many facets of tannin biogeochemistry that are not known. This lack of information hinders a complete synthesis of tannin effects on forest ecosystem processes and nutrient cycling. Areas of study that would help clarify the role of tannins in forest ecosystems include improved characterization and quantification techniques, enhanced understanding of structure-activity relationships, investigation of the fate of tannins in soil, further determination of the influence of environmental factors on plant tannin production and decomposition, and additional information on the effects of tannins on soil organisms.

Tannins in nutrient dynamics of forest ecosystems- a review

The ecological activities of plant phenolics are diverse and highly variable. Although some variation is attributable to differences in concentration, structure, and evolutionary history of association with target organisms, much of it is unexplained, making it difficult to predict when and where phenolics will be active. I suggest that our understanding is limited by a failure to appreciate the importance of oxidative activation and the conditions that influence it. I summarize examples of oxidative activation of phenolics in ecological interactions, and argue that physicochemical conditions of the environment that control phenolic oxidation generate variation in ecological activity. Finally, I suggest that measurements of oxidative conditions can improve our predictions of phenolic activity and that experiments must be designed with conditions appropriate to the biochemical mode of phenolic action.

Phenolics in ecological interactions: The importance of oxidation.

Aluminum has long been recognized as a major limiting factor for root growth in acid subsoils, but little has been done to delineate toxic and nontoxic forms of soil-solution Al. In an effort to determine if the presence of organic acids in soil solutions affected Al phytotoxicity, short-term, split-root experiments were conducted with cotton (Gossypium hirsutum L.) taproots as the growth indicator. Based on pure solution experiments, short-chain, carboxylic acids can be divided into three groups as Al detoxifiers: (i) strong (citric, oxalic, tartaric), (ii) moderate (malic, malonic, salicylic), and (iii) weak (succinic, lactic, formic, acetic, phthalic). The Al detoxifying capacities of these acids were positively correlated with the relative position of OH/COOH groups on their main C chain, positions that favored the formation of stable 5- or 6-bond ring structures with Al. In addition, analyses of soil solutions from several eluviated acid horizons (E, EB, BE) revealed the presence of several organic acids whose concentrations were generally higher in forested than in cultivated soils. Based on these concentrations, total solution Al (as measured by ICAP) was partitioned into monomeric Al (Al³⁺ + hydroxy-Al species) and complexed Al (Al-organic acid complexes). The latter accounted for 93 and 76% of the total solution Al concentrations of the two acid subsoils into which elongation rate of cotton taproot was studied. Root growth was significantly correlated with monomeric Al but not with total Al in soil solutions.

G. R. Craddock

Papers

Effect of Organic Acids on Aluminum Toxicity in Subsoils