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A study of organo-mineral complexes in some gleysolic soils : their isolation and the mineralization of nitrogen, sulphur and phosphorus

University of British Columbia

A detailed study of the factors affecting the efficiency of ultrasonic dispersion of five Gleysolic soils by the Biosonik BP-III probe type ultrasonic vibrator was conducted. Various soil/water ratios, lengths of vibration time and intensity settings were investigated and it was found that a soil suspension (1:10 or 1:5 soil/water ratio) not exceeding a total volume of 125 ml could be effectively dispersed by vibration for 20 minutes at a probe setting of 80. The efficiency of soil dispersion was reduced by 30-40 percent unless there was a "cooling off" period for the probe of 25-30 minutes between consecutive dispersions. The condition of the probe tip exerted a major influence on the effectiveness of dispersion and severe pitting which developed after about 70 hours of use, reduced the output of energy from the probe by nearly 70 percent. The heating effects of sonication was found to be a useful index of the energy output of the probe, and a technique for comparing the energy delivered to a suspension, which was equally effective and simpler than radiation pressure measurements, is described. A major finding which emerged from this study was that relatively large amounts of Fe, Al, Si, and C were solubilized during the vibration of soils in water, and that this effect could be suppressed by dispersing soils in a dilute electrolyte solution, notably 0.01 M CaCl₂. A fractionation scheme for the isolation in bulk of organo-mineral complexes was developed based on ultrasonic dispersion of the soil and a system of continuous flow centrifugation. The method reduces considerably the time involved in obtaining various particle-size fractions, and gives quantitative separation of the size groups, (an average of 98 percent of the soil material was recovered). The amounts of C, N, S, and P in the soils and their particle-size fractions (2-50μm, l-2μm, 0.2-lμm, and <0.2μm) were determined. There was an increase in the contents of C, N, S, and P and a narrowing of the C/N, C/S, and C/P(organic) ratios with decreasing particle size. An aerobic incubation technique was developed, based on a Factorial Experiment designed to determine the effects of moisture, sand, inoculum, and nutrients plus their interactions, on the mineralization of N, S, and P from organo-mineral complexes. The percent of organic C, N, and S in the particle-size separates of the five soils were found to be positively and significantly correlated with ammonium-N, nitrate-N, total-N, and sulphate mineralized during eight weeks of incubation, but not with phosphate. Total-P and organic-P were also positively correlated with N and S mineralized, but negatively correlated with phosphate. The percentage of total N, S, and P mineralized after six weeks of incubation ranged between 3.6-18.6 percent for N; 8.3-21 percent for S and 0.2-60.8 percent for P. The rate of mineralization as well as the percent of the total N, S, and P mineralized was generally highest in the <0.2μm fractions. On the basis of the rates and amounts of N, S, and P mineralized from the particle-size fractions, (especially the <0.2μm fraction) it was concluded that the bonding of mineral and organic colloids in these soils was unlikely to be directly responsible for the observed biological stability of the organic moiety, however, the aggregation of such complexes could occlude relatively large amounts of organic matter thus rendering it inaccessible to soil microbes. A colorimetric method for the determination of total and ammonium-N in soils and soil extracts was developed during this study, The method is based on the reaction of ammonia with phenol and hypochlorite in alkaline solution to form a blue complex with an absorbance maximum at 625 nm. The proposed method has the advantage over other standard methods of being quite simple yet precise and better suited to routine laboratory analysis. The distribution of exchangeable and "organically" bound Mn, Cu, and Zn, in the various soil size fractions was determined and their possible role in organo-mineral complexes discussed.

Text

2010-01-25

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http://hdl.handle.net/2429/19133

Soil Science

University of British Columbia

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