Improving Phosphorus Nutrition of Upland Rice through Native Arbuscular Mycorrhiza (AM)

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Upland rice (Oryza sativa L.) is mostly grown under rainfed, drought prone ecology in aerobic soil condition and is direct seeded. The soils are generally poor in organic matter content, water holding capacity and acidic in nature. A majority of uplands under rice cultivation in India consists of red Alfisols and lateritic soils. These render upland rice poor in acquisition of less mobile nutrients of which phosphorus (P) is most important. This is because, beside formation of poorly soluble complexes of P with iron and aluminum in acidic soil [1], even available P forms remain adsorbed on soil particles making it inaccessible to plant from beyond P depletion zone which is created around effective root absorptive area [2]. This adsorbed P is very slowly diffusible unlike nitrogen and potassium and hence, remained inaccessible unless intercepted by roots. The external mycelia network of arbuscular mycorrhizal fungi (AMF) which is connected to colonized (AMF) plant through roots, serves as extended root surface up to 8 cm beyond root zone [3]. This mycelia network of AMF promotes acquisition of P from outside P depletion zone by intercepting the adsorbed P which is otherwise (under no AM association) not accessible to plant. The soil environment (aerobic condition) of uplands is favorable for AMF activities. Moreover, due to comparatively poor adoption of intensive agriculture in upland ecology, the soil system is less disturbed maintaining the soil microflora diversity, including that of AMF, almost intact [4]. The native AMF flora of upland ecology has been found to be efficient and upland rice is generally responsive to AM [5]. This makes the researchers to think of addressing the constraint of poor P nutrition of upland rice through exploiting native AMF flora. Use of native flora has some added advantages like: (1) these are more adaptive to the target ecology and do not face rejection by the native flora, (2) have less negative ecological consequences in terms of invasive species introduction as unintended contaminants [6].