Below-ground plant-soil interactions affecting adaptations of rice to iron toxicity

Iron toxicity is a major constraint to rice production, particularly in highly-weathered soils of inland valleys in sub-Saharan Africa where the rice area is rapidly expanding. Although there is wide variation in tolerance in the rice germplasm, progress in introgressing tolerance traits into high-yielding germplasm has been slow owing to the complexity of tolerance mechanisms and large genotype by environment effects. We review current understanding of tolerance mechanisms, particularly those involving below-ground plant-soil interactions, which to date have been less studied than above-ground mechanisms. We cover processes in the rhizosphere linked to exclusion of toxic ferrous iron by oxidation, and resulting effects on the mobility of nutrient ions. We also cover the molecular physiology of below-ground processes controlling Fe retention in roots and root-shoot transport, and also plant Fe sensing. We conclude that future breeding programs should be based on well-characterised molecular markers for tolerance traits. To successfully identify such markers, the complex tolerance response should be broken down into its components based on understanding of tolerance mechanisms, and tailored screening methods developed for individual mechanisms.

Performance of Rolled Erosion Control Products (RECPs) as Bioswale Revetments

Vegetated swales, or bioswales, are among the most commonly used type of green infrastructure (GI) for managing stormwater in temperate climate regions. However, performance data on bioswale drainage technology applied to highly weathered soils (low fertility, high acidity, and erosion prone) in tropical and subtropical climates are still limited. Aimed at closing this gap, this research investigated the performance—assessed in terms of vegetation biomass, biodiversity and coverage of swale, the structural integrity of revetments, and erosion control potential—and cost effectiveness of five rolled erosion control products (RECPs) currently available on the market, in combination with herbaceous vegetation as the revetment of drainage swales, in tropical soils. Additionally, the research project evaluated the performance of a new preseeded RECP, the Preseeded Reinforcement Mat, for drainage in areas that are difficult to access. The performances of all six RECPs were generally adequate as bioswale revetments in the conditions investigated, with performance index values ranging from 6 to 10 in a 0 to 10 scale. At the same time, some RECPs were more conducive to the growth of regional herbaceous vegetation species, measured in terms of biodiversity, which ranged from 2 to 14 species in the different bioswales, and some were more cost effective than others, with costs ranging from 19% to 106% of the cost of concrete lined swales.

Below-ground plant-soil interactions affecting adaptations of rice to iron toxicity

Iron toxicity is a major constraint to rice production, particularly in highly-weathered soils of inland valleys in sub-Saharan Africa where the rice area is rapidly expanding. Although there is wide variation in tolerance in the rice germplasm, progress in introgressing tolerance traits into high-yielding germplasm has been slow owing to the complexity of tolerance mechanisms and large genotype by environment effects. We review current understanding of tolerance mechanisms, particularly those involving below-ground plant-soil interactions, which to date have been less studied than above-ground mechanisms. We cover processes in the rhizosphere linked to exclusion of toxic ferrous iron by oxidation, and resulting effects on the mobility of nutrient ions. We also cover the molecular physiology of below-ground processes controlling Fe retention in roots and root-shoot transport, and also plant Fe sensing. We conclude that future breeding programs should be based on well-characterised molecular markers for tolerance traits. To successfully identify such markers, the complex tolerance response should be broken down into its components based on understanding of tolerance mechanisms, and tailored screening methods developed for individual mechanisms.

Warming drives sustained plant phosphorus demand in a humid tropical forest

Phosphorus (P) is often one of the most limiting nutrients in highly weathered soils of humid tropical forests, which may regulate the responses of carbon (C) feedback to climate warming. Based on a 7-year continuous field warming experiment conducted by translocating microcosm forest ecosystems from a high-elevation site to low-elevation sites, we detected changes in the ecosystem P cycle in response to warming. We report that warming drives sustained plant P demand by increasing P uptake and thus decreasing foliar N:P. This increased plant P content is supplied by multiple processes including enhanced plant P resorption, soil P mineralization and dissolution without changing litter P mineralization and leachate P. These findings suggest that warming may alleviate initial P deficiency and/or limitation of plant growth and contribute to sustaining plant C fixation in these tropical forests.

Effects of Tree Species on Root Exudation and Mineralization of Organic Acids in a Tropical Forest

Aims Root exudation of organic acids is one of strategies for tropical trees to facilitate nutrient uptake from the highly weathered soils. However, paradoxical relationship remains that root exudation also stimulates microbial activities to consume organic acids in the rhizosphere (root-soil interface). Plant-specific root exudation might shape different rhizosphere carbon (C) cycles between tree species. We test whether root exudation and rhizosphere C fluxes of organic acids and sugars differ between dominant dipterocarp trees and pioneer trees (Macaranga spp.). Methods We measured (1) root exudation from mature trees, (2) soil solution concentrations of organic acids and monosaccharides, and (3) mineralization kinetics of 14C-radiolabelled substrates in the rhizosphere and bulk soils of the Dipterocarp and Macaranga trees. Results Malate was a dominant organic acid exuded from Dipterocarp roots, while monosaccharides were dominant exudates of pioneer Macaranga trees. Malate exudation rates by Dipterocarp roots were greater compared to Macaranga roots. Organic acid exudation increased with increasing root surface area and with decreasing soil pH. Microbial activities of malate mineralization were enhanced in the rhizosphere both under Dipterocarp and Macaranga trees, but the C fluxes of malate mineralization by rhizosphere microbes far exceeded root exudation due to microbial malate production in the rhizosphere of Dipterocarp trees. Conclusion Tree species develop different strategies to increase malate concentration in rhizosphere soil directly through root exudation or indirectly through rhizosphere microbial activities to increase malate production, which is favorable for phosphorus solubilization, aluminum detoxification, and lignin degradation in acidic soils.

What controls microbial growth in tropical soils? The role of carbon and phosphorus.

<p>Tropical forest ecosystems are important components of global carbon (C) and nutrient cycles. Many tropical rainforests grow on old and highly weathered soils depleted in phosphorus (P) and other rock-derived nutrients. While plants in such forests are usually P limited, it remains unclear if heterotrophic microbial communities are also limited by P or rather by C or energy. Elemental limitations of microorganisms in soil are often approached by measurements of changes in respiration rates or microbial biomass in response to additions of nutrients or carbon. However, it has been argued lately, that microbial growth rather than respiration or biomass should be used to assess microbial limitations.</p><p> </p><p>In this study we asked the question whether the growth of heterotrophic microbial communities in tropical soil is limited by available P or by C. We sampled soils along a topographic gradient (plateau, slope, bottom) differing in soil texture and total and available P concentrations from a highly weathered site in French Guiana. We incubated these soils in the laboratory with cellulose as a C source, phosphate (pH adjusted) and with a combination of both. We determined microbial growth by measuring the incorporation of <sup>18</sup>O from labelled water into microbial DNA.</p><p> </p><p>In general, plateau soils were higher in microbial C, while bottom soils were higher in microbial P, leading to increased microbial C:P ratios in plateau soils compared to bottom soils. Microbial C, N and P did not respond to the addition of cellulose. Microbial P on the other hand was significantly increased by P additions, with no interactive effect between cellulose and P. Although microbial C was significantly higher in plateau soils, respiration rates were similar to those of bottom soils. This led to similar mass specific respiration rates in plateau and slope soils, with bottom soils being significantly higher. Moreover, we found that C and P addition increased mass specific respiration rates and both nutrient additions showed a positive interactive effect. Gross microbial growth rates were stimulated by P additions but were unresponsive to C additions alone. However, the addition of carbon further stimulated the effect of P on growth.</p><p> </p><p>The observed interactive effect of C and P additions on gross microbial growth rates suggests a co-limitation of microorganisms by C and P in highly weathered soils. We argue that co-limitation bears significant ecological advantages for microorganisms as it minimizes the investments in acquiring nutrients for growth.We further conclude that microorganisms in tropical soils are highly efficient in taking up and storing P from the environment. In our experiment, microbial P almost doubled in the six days after P addition, while microbial C was not enhanced. This also means that the microbes were not homeostatic with regard to their C:P ratios. Finally, our study demonstrates the importance of investigating gross microbial growth rates, rather than respiration or biomass, for inferring nutrient limitations.</p>

Effects of tree species on root exudation and mineralization of organic acids in a tropical forest

<p>Tropical forests can develop by roots foraging nutrients in the highly weathered soils. In rhizosphere, soil volume affected by roots, tree species modify carbon (C) and nutrient cycles directly through root exudation and indirectly through increased microbial activity. We test whether root exudation and rhizosphere C fluxes of organic acids and sugars differ between dominant dipterocarp trees and pioneer trees (Macaranga gigantea). To quantify the C fluxes of organic acids in the rhizosphere soils, we measured in situ root exudation from mature trees, concentrations of monosaccharides and organic acids (acetate, oxalate, malate, and citrate) in the rhizosphere and bulk soil fractions, and mineralization kinetics of <sup>14</sup>C-radiolabelled substrates. Organic acid exudation increases with increasing root surface area. Dipterocarp roots release greater amounts of malate, while monosaccharides are dominant exudates of pioneer trees. Microbial activities of malate mineralization increase in the rhizosphere soil both under dipterocarp and pioneer trees. The greater C fluxes of malate mineralization, compared to root exudation, suggests rhizosphere microbes are another malate producer under dipterocarp trees. Both root exudation composition and rhizosphere microbes increase malate production with increasing phosphorus demands and with increasing soil acidity.</p>