Abstract
Iron toxicity is one of the main edaphic constraints that hamper rice production in West African savanna and forest lowlands. Although chemical reduction processes of various types of pedogenic iron oxides could not be underestimated, the bulk of these processes can be ascribed to the specific activity of Iron-Reducing Bacteria (IRB). The reducing conditions of waterlogged lowland soils boost iron toxicity through the reduction of almost all iron into ferrous form (Fe2+), which can cause disorder in rice plant and crop yield losses. Aiming to contribute at the improvement of rice yield in Africa, an experiment was developed to evaluate the impact of subsurface drainage on IRB dynamics and activity during rice cultivation. Twelve concrete microplots with a clay-loam soil and a rice variety susceptible to iron toxicity (FKR 19) were used for the experiment. Soil in microplots was drained for 7 days (P1), 14 days (P2), and 21 days (P3), respectively. Control (T) microplots without drainage were prepared similarly. The evolution of IRB populations and the content of ferrous iron in the paddy soil and in soil near rice root were monitored throughout the cultural cycle using MPN and colorimetric methods, respectively. Data obtained were analyzed in relation to drainage frequency, rice growth stage, and rice yield using the Student t test and XLSTAT 7.5.2 statistical software. From the results obtained, the subsurface drainage reduced significantly IRB populations (p = 0.024). However, the drainage did not affect significantly ferrous iron concentration in the soil near rice roots (p = 0.708). The concentration of ferrous iron (p < 0.0001) in soil near rice roots and the number of IRB (p < 0.0001) were significantly higher during the rice tillering and maturity stages. Although no significant difference was observed for rice yield among treatments (p = 0.209), the P2 subsurface drainage showed the highest yield and the lowest concentration of ferrous iron in soil near rice roots.
Highlights
Iron (Fe) toxicity is a nutritional disorder associated with high iron concentrations in the soil solution that affects rice plant in lowland [1] [2]
Densities of iron-reducing bacterial (IRB) enumerated on soil before flooding, when the soil was dried and on soil near rice roots during the rice growth stages, are shown in Figure 2 and expressed as log10
Oxygen is consumed by soil respiration and a large amounts of mineral elements as Fe III and nutriments were released in the soil solution [28]
Summary
Iron (Fe) toxicity is a nutritional disorder associated with high iron concentrations in the soil solution that affects rice plant in lowland [1] [2]. Large concentrations of ferrous Fe in soil solution may occur either when Fe is mobilized in situ by microbial reduction of ferric Fe [3] or when reduced Fe is translocated into valley bottoms by interflow or subsurface flow from adjacent slopes [4]. Iron toxicity is associated with large concentrations of reduced iron (Fe2+) in the soil solution. Fe (III)-compounds are reduced and it results in ferrous ions production. This reduction of Fe(III) is largely controlled by microbial processes, and coupled to the oxidation of organic matter to support growth [8]. I.e. browning and yellowing of leaves [9], iron toxicity can cause a 10% - 100% crop yield losses [5] [12]
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