Studies on the Chemistry and Behaviour of Zinc in Soil

[摘要] Chapter 1 includes a survey of dynamic equilibria of trace metals and the forms which exist in soil fractions. The forms of trace metals as influenced by environmental factors are also discussed. The types of studies used regarding trace metal behaviour in soil are also reviewed. The objectives of the thesis are to study the chemistry and behaviour of zinc in soil and also the changes in the distribution of this metal with plant uptake. Chapter 2 lays emphasis on selective extraction procedures for measurement of soil zinc. The removal of zinc held by different mechanisms in the soil is affected by the nature of the extractant. Pairs of extractants used for the exchangeable, specifically sorbed, and organically bound-Zn fractions showed differences in the removal of zinc from the soil. 0.05M calcium chloride is suggested for the removal of exchangeable zinc in preference to M ammonium acetate (pH 7.0), as it released more zinc into solution. Being alike in charge the Ca2+ ion more readily displaced the exchangeable zinc from the soil than the NH4+ ion. Acetic acid (2.5%) is suggested for the extraction of specifically sorbed zinc compared to modified Morgan's solution (ammonium acetate/acetic acid, pH 4.8). It extracted higher amounts of zinc from the soil probably due to its acid nature. Ammonium EDTA (0.05M) pH 7.0 is suggested for removing zinc from the organic fraction in preference to 0.1M sodium pyrophosphate, as the latter also affected the oxide pool. As well as individual extraction of zinc, sequential extraction techniques were also utilized to define the forms of zinc in soils. Two schemes were used, which showed differences in various pools of extractable zinc, especially in the oxalate extractable fractions. They also indicated the effect of a particular extractant on the extractability of the subsequent pool in the sequence. There may also be some risk of contamination, hence the use of individual extraction is suggested. Air-drying can also bring about slight changes in the amount of extractable zinc from the soil. In the air-dried samples the extractable zinc increased slightly with various extractants for different fractions compared to fresh samples. Seasonal variation, or depth of sample, can affect the extractability of zinc in various soil pools. Significantly more zinc was extracted in adsorbed, organically bound, and oxide bound Zn fractions of Dreghorn (Arkleston) soil during the winter season compared to the summer season, but these variations were nonsignificant in the case of calcium chloride extractable zinc. It is suggested that this readily available zinc fraction is not affected by plant uptake or seasonal variations, indicating its zinc content is maintained by equilibria with the other pools. In Dreghorn (Arkleston) subsoil lower amounts of zinc in all fractions were extracted compared to topsoil, suggesting the decrease of metal with depth. Seasonal variations have a lesser effect on the extractability of zinc fractions in subsoil compared to topsoil. Chapter 3 investigates the influence of continuous cropping of ryegrass on the distribution of zinc in soil using a pot experiment, and assesses this by means of specific extractants. Calcium chloride, acetic acid, ammonium EDTA and acid oxalate were used to extract zinc associated with exchange sites, specific sorption sites, organic complexes and oxide materials respectively. It was found that all soil pools showed, directly or indirectly, a response to plant uptake of zinc, except calcium chloride extractable (exchangeable), which remained unchanged. Acetic acid and ammonium EDTA extractable zinc showed some decline with time, suggesting a response to plant uptake and maintaining the exchangeable pool. The oxide fraction, being a big reservoir, showed a greater decline, suggesting that the other soil pools are maintained by equilibrium with this soil fraction. Results showed that zinc in the immediate bioavailable pool was not affected by continuous growth of ryegrass, and it is suggested that equilibrium was maintained by the other soil pools. The amount of zinc taken up by grass is higher in soils with high zinc in readily available pools, rather than high total amounts, suggesting that total zinc is not a good indicator for plant uptake. Higher amounts of zinc accumulation in roots, compared to the tops, in soils with an adequate supply of zinc suggested the intermediate mobility of this metal within the plant.(Abstract shortened by ProQuest.).

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry, Soil sciences