Ammonium fixation and adsorption experiments were conducted to study the effect of potassium on ammonium fixation into interlayer of vermiculite and ammonium adsorption at the surface of vermiculite both in the binary (NH-4～(+)-Ca～(2+)) and ternary (NH-4～(+)-K～(+)-Ca～(2+)) systems. In the ammonium fixation experiment, 5 mmol NH-4～(+) kg～(-1) was added alone, or after, before or simultaneously with 5 mmol K～(+) kg～(-1) to the vermiculite (vermiculite:solution = 1:1), and the incubation was conducted for 3 days under constant 20℃. In the adsorption experiment, after addition of 10 mmol Ca～(2+) L～(-1) as matrix ions, either NH-4～(+) alone with a concentration series from 0.1 to 5.0 mmol NH-4～(+) L～(-1) (binary exchange system of NH-4～(+) and Ca～(2+) or NH-4～(+) together with 5 mmol K～(+) L～(-1) (ternary exchange system of NH-4 ～(+), K～(+) and Ca～(2+) was added to vermiculite at a vermiculite to solution ratio of l:10 for 24-h equilibrium. The results of the fixation experiment showed that the presence of K～(+) increased NH-4～(+) fixation for all the treatments, regardless of the addition orders. For ammonium adsorption a linear relationship between activity ratios of NH-4～(+) to Ca～(2+) and Mg～(2+) in the equilibrium solutions (ARNH-4) and increases of exchangeable NH-4～(+) at the surface of vermiculite after 24- h equilibrium (△Ex-NH-4) was shown for almost the whole concentration ranges tested. Compared with the binary system, the slope of the curve of the ternary system was steeper, indicating a more intensive adsorption of NH-4～(+) in the presence of K～(+). It was demonstrated that K ～(+) did not reduce the ammounium fixation into the interlayer and ammonium adsororption at the surface of vermiculite, which indicated that, under the experimental conditions of this study, K～(+) did neither occupy the sites for NH-4～(+)-fixation in the interlayer nor the sites for NH-4～(+)-adsorption at the surface of vermiculite.
FENG KE, WANG XIAOZHI, TANG YAN and WANG XIAOLI Agricultural College, Yangzhou University, Yangzhou 225009 (China)
The effects of different structures of 2:1 layer minerals, layer charge location, and changes of structure and charge during the weathering process on the fixation and release of interlayer cations are reviewed. It could be concluded that the fixation capacity is determined by the total amount of interlayer charge originating from both octahedral and tetrahedral sheets. The relationship between interlayer cation fixation and octahedral structure of the secondary minerals may be different from that of the primary minerals. The oxidation and reduction of cations with variable valence can greatly influence the cation-fixation capacity.
FENG KE, WANG XIAOLI, WANG XIAOZHI and TANG YANAgricultural College, Yangzhou University, Yangzhou 225009 (China)
Two soils, one consisting of 1:1 clay minerals at pH 4.5 and the other containing 2:1 clay minerals at pH 7.0, were used to estimate the conversion of added NH+4 under different C/N ratios (glucose as the C source) and the addition of potassium. Under lower C/N ratios (0:1 and 5:1), a large part of the added NH4+ in the acid soil was held in the forms of either exchangeable or water soluble NH4+ for a relatively long time and under higher C/N ratio (50:1), a large amount of the added NH4+ was directly immobilized by microorganisms. In the second soil containing appreciable 2:1 type clay minerals a large part of the added NH+4 at first quickly entered the interlayer of the minerals under both lower and higher C/N ratios. In second condition, however, owing to microbial assimilation stimulated by glucose the newly fixed NH4+c ould be completely released in further incubation because of a large concentration gradient between external NH4+ and fixed NH4+ in the mineral interlayer caused by heterotrophic microorganisms, which imply the fixed NH4+ to be available to plants. The results also showed that if a large amount of K+ with carbon source together was added to soil, the higher K+ concentration of soil solution could impede the release of fixed NH4+ , even if there was a lot of carbon source.
