Carbohydrate decomposition in beech litter as influenced by aluminium, iron and manganese oxides

Although carbohydrates are one of the most degradable groups of soil organic matter compounds, they can accumulate in soil horizons containing strongly humified organic matter. It is therefore assumed that stabilization reactions exist, which may be related to the presence of particular mineral compounds in soils. An incubation experiment was made to investigate the influence of selected oxides (ferrihydrite, aluminium hydroxide, birnessite) on the decomposition of polysaccharides in beech litter. pH changes and the dynamics of total organic carbon, cellulosic and non-cellulosic polysaccharides, and potential cellulase activity were recorded. Carbon loss during incubation was promoted by 10% in the presence of birnessite, which acts as an oxidizing agent. Polysaccharide decomposition was hardly affected by birnessite. On the other hand, ferrihydrite and aluminium hydroxide reduced the decomposition of bulk organic matter by 12 and 65%, respectively. These mineral phases resulted in a reduction of carbohydrate decomposition by 15 and 50%, respectively. In the presence of aluminium hydroxide, there was no significant cellulose decomposition after 90 d. Nevertheless, throughout the experiment, potential cellulase activity was higher in this treatment than in the presence of birnessite. Therefore we assume that organic matter stabilization is caused by toxic effects, or by adsorption to the oxides or hydroxides, especially to aluminium hydroxide, or by both. This process tends to lower the microbial availability of organic matter and therefore to inhibit the decomposition.