MODIFIED METHOD FOR PREDICTING PHOSPHATE DESORPTION KINETICS AND DYNAMICS FROM SOILS AND ITS RELATION WITH PLANT GROWTH

Abstract

Phosphorus (P) limits plant growth particularly in very strongly acidic soils. P availability to plant is a functional concept rather than measurable quantity. This research work was intended to synthesize nanocomposite material that can monitor soil P desorption kinetics and dynamics and relate to P uptake by maize (Zea mays L.). The binary oxide Fe-Al composite system, both in a crystalline and amorphous form was synthesized by sol-gel evaporation method and characterized by XRD, FTIR, SEM-EDX. In the preliminary study, leakage of the as-synthesized nanocomposite in hydrous form (HFAO and HAFAO) placed in dialysis membrane tube (DMT) in contact with de-ionized water was checked using qualitative and quantitative tests. Performance of the nanocomposites suspensions in contact with KH2PO4 (10-100 ppm) aqueous solution and 1 g soil suspension was evaluated and data fitness to isothermal and kinetic models was checked.The single system hydrated ferric oxide filled in dialysis membrane tubes (DMT-HFO) was used as a benchmark. The long -term (1-42 days) phosphate desorption kinetics and dynamics study of soil samples collected from six sites ( Bako, Bishoftu, Gununo, Guto-gida, Hagre-selam and Mechara) were carried out by extracting P with DMT-HFAO and DMT-HFO while shaking 1 g of soil suspension for intervals of 1, 7, 14, 21, 28 and 42 days. The remaining P in soils was fractionated successively as subsequent P-fractions by using 0.5 M NaHCO3, 0.1M NaOH, 1M HCl, c/hot HCl and c/H2SO4+H2O2 for extraction different P pools. DMT-HFAO kinetics and dynamics data of soils equilibrated for a period from 1 to 42 days was evaluated against maize yield grown in glass housee for 56 days in RCD design. Two component first order model was employed to estimate the cumulative P desorbed and P desorption indices and how these parameters are related to maize yield.The XRD data revealed crystalline form with crystallite size of 21.4 nm. The FTIR functional group data showed sorption of phosphate on the synthesized Fe-Al binary oxide in crystalline as well as amorphous forms. SEM-EDX micrograph revealed heterogeneous nature of the nanosorbent.The qualitative test result showed no leakage of the as-synthesized nanocomposite in crystalline form. However, the quantitatve test showed amorphous oxide leaked out through DMT. For the aqueous system, the sorption capacity of the nanocrystalline binary suspension (DMT-HFAO) wasfound to be 360% of DMT-HFO used as a benchmark where as the amorphous congener was also approximately 300% of DMT-HFO during equilibration of 24 h. For the phosphate in soil solution of the selected soil samples of Gununo and Bishoftu, the DMT-HFAO system on average sorbed roughly 500% P compared with a single system, hydrous ferric oxide (DMT-HFO), in 168 h. All the three sorbent, DMT-HFAO, DMT-HAFAO and DMT-HFO fitted to both Freundlich and Langmuir models with better fit of Freundlich to the data than Langmuir. From Langmuir model the maximum sorption capacity of DMT-HFAO was 15.9 mg/g while that of DMT-HFO was 4.2 mg/g. Using Freundlich model kF of DMT-HFAO = 3.6 and that of DMT-HFO=0.44. This clearly showed binary nanosorbent, DMT-HFAO has remarkable sorption capacity as compared to the reference DMT-HFO. For both DMT-HFAO and DMT-HFO, the kinetic models fitted the data better in the order: First order (R2=0.983-0.998)> xvii pseudo first order (R2=0.954-0.998) > Elovich (R2=0.929-0.989) > pseudo second order (R2=0.286-0.874). The DMT-HAFAO data fitted fairly well (R2=0.772-0.999) to pseudo second order but not fairly well for other models. This could be due to leakage of the sorbent through dialysis membrane that limited its potential for phosphate sorption capacity in this study. For the desorption experiment 24-168 h carried out under soil suspension of Bishoftu and Gununo, the data fitted fairly well with first order kinetics for both sorbents (R2=0.946-0.998), the sorption capacity of DMT-HFAO being greater than DMT-HFO. The soil data fitted intra-particle model fairly well for both sorbents (R2= 0.98-0.992) with intra-particle rate constants, kp, following the order: DMT-HFAO>DMT-HAFAO>DMT-HFO. In soil phosphate desorption study (1-42 days) by the modified method DMT-HFAO and DMT-HFO, the amount of Pi extracted by the modified system DMT-HFAO and DMT-HFO are significantly different (p0.05) at all levels and extraction time for all soils in this study.The amount of cumulative P extracted by DMT-HFAO was greater than the amount extracted by DMT-HFO during course of extraction of 1-42 days. This indicated the modified method has more sorption capacity from slow labile pools than the reference. The modified system DMT-HFAO extracted 2.9-13.4% of P total while the reference DMT-HFO extracted 0.2-3.5%. The cumulative P-extracted in 42 days using this technique significantly correlated (p<0.01) to maize plant uptake (r=0.777**) and shoot dry matter yield (r=0.788**) for soils considered in this study. The correlation obtained using this modified technique was found to be better when compared with DMT-HFO, the correlation of which is significant (p<0.01) to P uptake (r=0.664**) and dry matter yield (r=0.512*). This indicated that DMT-HFAO is superior in mimicking plant mode of action as compared to a single oxide system used as benchmark to study long-term P desorption. With assumption that the rate constants of P release of slow labile pool P do not change over time and continuous cropping it can be predicted that this pool will be depleted in about 300-461 days for the soils in this study. This is a very important finding in recommendation of fertilizers application. However, this prediction is on the basis of glass house pot experiment and needs to be tested under field condition in the future work. Our findings provide some insights into the importance of this technique for long-term P desorption studies although further experiments are required at field and green house levels for a wide range of soils to verify the universality of this modified method.