Microbial biomass content, basal respiration and enzyme activities during the course of decomposition of leaf litter in a black alder (Alnus glutinosa (L.) Gaertn.) forest

Decomposition of leaf litter from a black alder (Alnus glutinosa (L.) Gaertn.) forest on a fen (peat soil) in Northern Germany was studied at two sites within the forest. The ‘wet’ site was adjacent to a lake and soil was water-saturated throughout most of the year. In contrast, the other site was substantially drier, especially in summer and therefore called the ‘dry’ site. The pH (H2O) value of the topsoils (horizon below litter) at the wet and dry site was 6.0 and 3.5, respectively. The litter consisted mainly of alder leaves and decomposed at different rates at the two sites. After 12 months, about 40% of the initial litter was decomposed at the dry site compared to 90% at the wet site. The water content of the litter varied seasonally, particularly at the dry site. The pH of the litter was similar to or higher than that in the underlying topsoil and decreased at the dry site after 18 months. The total C content in residual material did not change during the course of decomposition. The microbial biomass content was, with some irregularities, similar throughout the first months and then continually declined. Basal respiration decreased from an initial value of approximately 700 to 100 μg CO2 g−1 dry litter h−1 after 17 months. β-Glucosidase activity also continuously decreased during litter decomposition; protease activity decreased initially but then tended to increase. The metabolic quotient declined during the course of litter decomposition from a relatively high value to a value almost typical for soils. This decline indicates an increase in the C utilization efficiency of the microbial community as well as a shift from a zymogenous to an autochthonous microflora. Regression analyses were performed to determine correlations between properties and to evaluate factors controlling the decomposition process.