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Phosphorus nutrition of chickpea under dry topsoil conditions as in the High Barind Tract of Bangladesh

Kabir, Enamul (2012) Phosphorus nutrition of chickpea under dry topsoil conditions as in the High Barind Tract of Bangladesh. PhD thesis, Murdoch University.

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In many parts of the world, notably in South Asia, crop intensification is resulting in more crops being grown on stored soil moisture, under which conditions the topsoil commonly dries out during crop growth. In the Mediterranean climatic region also, topsoils dry out particularly during the later part of the growing season. Implications of the drying of topsoils for crop nutrition in general are poorly understood. Crop intensification in South Asia is also leading to increased mechanisation and the emergence of minimum tillage sowing of crops. The key research question for the present thesis is availability of phosphorus (P) for chickpea grown with stored residual soil water in the context where placement of P fertilizer with the seed is accomplished by mechanised planters. This study investigated the P nutrition of chickpea considering uptake from topsoil and subsoil and factors affecting P availability, distribution and remobilization throughout the growing season when P fertilizer was supplied with the seed or below the seed (in the subsoil) under well-watered and dry topsoil conditions. Short-term (up to 12 or 24 days) glasshouse studies assessed the risk of toxicity for seed emergence and early growth of chickpea when P fertilizers were placed with seed in well-watered soil. In sand, seed-placed P fertilizers (diammonium phosphate and triple superphosphate) depressed chickpea germination and seedling growth while seed-placed P was safe in sandy clay loam soil at rates equivalent to 20 kg P/ha even in the scenario where the seed was sown in wide rows (up to 60 cm) that results in higher effective P fertiliser concentration around seed. In the field when chickpea was sown in four soils (sandy clay loam- clay loam texture) with a drying surface, the seed-placed P fertilizer at 20 kg P/ha as triple superphosphate had no suppressive effect in the early growth stage of chickpea but the grain yield improvement was very small (~10% over the nil P). Under a drying topsoil, chickpea accumulated P until late pod filling stage irrespective of P fertilizer treatments. From the accumulated P, plants remobilized a substantial amount of P (52% of total in vegetative shoot parts) which contributed the equivalent of 69% of the total pod P. The remobilization of P from the vegetative parts was not sufficient for the pod P requirement but rather concurrent P uptake from the soil was needed to complete the P requirements of the pods. These results suggested that continual P uptake of chickpea depended on uptake from the subsoil P where moisture was available after drying of the topsoil. To assess the contribution of subsoil P to P uptake by chickpea under dry topsoil condition, a glasshouse study was setup by supplying P levels in the subsoil (10-30 cm). This study also showed that chickpea continued to accumulate P until late podding stage when the topsoil was completely dried; the pod P content was contributed by both remobilized P (70%) and concurrent uptake of P (30 %), but the level of P in the dry topsoil had no effect on total P content of the plant or its pods. Fractionation of P in rhizosphere soil showed that chickpea depleted sparingly-soluble 0.1 M NaOH-extractable inorganic P (NaOH Pi) in addition to labile P fractions. The drawdown of depleted P fractions was greater in the subsoil than the topsoil. The response of carboxylate exudation of chickpea under dry topsoil condition was investigated in the final glasshouse study. Under dry topsoil condition, chickpea exuded substantially higher amounts of total carboxylates in the well-watered subsoil compared to the dry topsoil. Malonate was the principal form of carboxylate followed by malate and citrate. The depletion of sparingly-soluble P forms (NaOH Pi from both the P-supplied topsoil and subsoil and the 1 M HCl extractable inorganic P from the subsoil without added P) suggests a link between the carboxylates excreted in the subsoil rhizosphere and the depletion of sparingly soluble P fractions. In conclusion, chickpea continued to take up P during the whole period of dry matter accumulation including during pod filling. Under conditions where the topsoil dried out mostly before flowering, the post-flowering P uptake is most likely to have been acquired from the subsoil. In this study, substantial root growth and carboxylate exudation by chickpea into the moist subsoil has been demonstrated suggesting a possible mechanism for mobilisation of subsoil P reserves for uptake during pod filling. The seed-placed P fertilizer had limited positive effect on chickpea grain yield in the field possibly due to shallow depth of fertilizer placement into topsoil which was dry when the plant’s P demand was high. Subsoil placement of P fertilizer showed promise for improving P uptake and grain yield of chickpea under dry topsoil condition. Further studies are required under different soils and environmental conditions to assess the contribution of subsoil P to the P nutrition of chickpea.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental Science
Supervisor(s): Bell, Richard and Johansen, Chris
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