Wahid, P APadmini Amma, K PKAU2018-12-052018-12-051988http://krishikosh.egranth.ac.in/handle/1/5810085249An investigation on the physico-chemical characteristics, reductive transformation of iron and sulphate in kari and karappadam soils of Kuttanad and pokkali soils of Ernakulam District was conducted under flooded conditions during 1986-88 at Radiotracer Laboratory, College of Horticulture, Vellanikkara. Particular emphasis was given to evaluate the changes in soil properties and their influence on the transformation of Fe3+ in anaerobic soils, to examine whether there was preferential reduction of amorphous Fe oxides, to determine the relative extent of chemical versus microbiological reduction of Fe3+ and also to develop a methodology for studying sulphate reduction in anaerobic soils using 35S. For chemical analysis, spectrophotometric, flame- photometric and atomic absorption spectrophotometric methods were adopted. Radioassays of 35S were done using liquid scintillation counting technique. The salient findings from these studies are summarized below: Changes in soil characteristics were monitored at different intervals upto 150 days after flooding the soils. Following flooding, the pH of the soils generally increased in pokkali, karappadam and kari soils but the extent of rise in pH was much less in kari soils. Redox potential (Eh) decreased in all the soils and the decrease was seen evenafter 3 to 5 months. The redox potential of kari soils remained much higher at all intervals upto 5 months than in other soils. Among the three soils, pokkali (4-35 mmho cm-1) and kari (2-10.5 mmho cm-1) were found to be more saline as compared to karappadam soils (less than 3 mmho cm-1) and a slight increase in EC was observed from 5th day of flooding onwards. Analysis of free Fe oxide content indicated that kari soils contained the highest quantities of free Fe followed by karappadam. In all the three soil types the amorphous Fe content was generally higher than crystalline Fe. Transformation of Fe3+ in submerged soils was studied by monitoring NaOAc-extractable Fe2+ concentration. High concentrations of Fe2+ were observed in pokkali (19375ppm) and kari (31250 ppm) soils compared to karappadam (13125 ppm) soils. In pokkali soils Fe2+ concentrations reached peak values within 15 to 30 days where as in karappadam and kari soils highest concentrations were obtained 2 months and in about 1 to 2 months after flooding respectively. Studies on the preferential reduction of amorphous and crystalline forms of Fe in flooded soils revealed that with time there was a more or less linear increase on the concentration of Fe2+ where as a reverse trend was noticed in total free Fe, amorphous Fe and crystalline Fe forms with a decrease in Eh and an increase in pH. The preferential reduction of crystalline Fe (III) oxides over amorphous forms is observed in the present study and the reduction of these Fe (III) forms was more marked below an Eh of 200mV. The phenomenon was attributed to the reductases responsible for the transformation of amorphous and crystalline Fe (III) oxides, which require attainment of certain critical redox levels in flooded soils for their induction as well as for their function. The extent of chemical and microbial reduction of Fe in flooded soils was studied with and without sterilization by gamma irradiation. The results indicated a less pronounced decline in the redox potential and more or less a constant pH as well as Fe2+ content in the irradiated soils as against substantial increase in pH as well as Fe2+ content and a drop in redox potential observed in the unirradiated soils suggesting the participation of microorganisms in the reduction of Fe. The data relating to the changes in the concentration of soil sulphate following flooding revealed that in general there was an increase in sulphate concentrations in all the three types of soils as a result of flooding. Extractable sulphate content showed highest concentrations in kari soils (63,750 ppm) followed by pokkali (33,333 ppm) and karappadam (6666ppm). Peak concentrations were attained within 25-30 days of flooding in pokkali, where as in karappadam and kari soils it attained maxima within 15 days. A method was developed to study the reduction of sulphate in flooded soil to its end product H2S using 35S. The evolution of H235S was detected from 91.5 h onwards. Evolution of H235S steadily increased upto 211.5 h of incubation beyond which there was a slight decrease in the rate of evolution. The total quantity of H235S evolved during 312.5 h of experiment was 1589 cpm. It was also observed that Eh of the flooded soil system decreased very rapidly with a concomitant increase in pH. It was found that reduction of 35SO42- to H235S commenced when the Eh of the flooded soil system dropped to -5mV whereas the transformation of Fe3+ to Fe2+ started at a still higher Eh much earlier to the reduction of 35SO42-.ennullThesis