Microbial Biomass

Abstract

Micro-organisms (bacteria, fungi) contribute to more than 90% of the carbon dioxide evolved during decomposition of forest litter (Schaefer 1991), indicating their role in the element cycling in forest ecosystems. Nitrogen is mostly cycled between primary producers and decomposer biota. The activity and growth of micro-organisms depend on carbon and nutrient supply and the physico-chemical environment because of their high surface area/volume ratio of microbial bodies (Hattori and Hattori 1976; Paul and Clark 1996). Soil acidity has often been shown to reduce microbial biomass (C_mic) and to increase metabolic respiration (qCO₂, ratio between microbial-respiration-C and microbial-biomass-C) in the surface mineral soil which was interpreted as an increased stress on micro-organisms living in acid soils (Anderson and Domsch 1993; Anderson, Chap. 20, this volume). In addition to the chemical stress, soil acidity may change the litter quality, litter amount and their distribution in the soil profile by acid sensitive earthworms. Quality and amount of litter determine the amount of micro-organisms that would live on the organic matter, and is indicated by the fraction of C_mic in organic carbon (C_org) (Anderson and Domsch 1986). Another effect of soil acidity relates to an increase in bacterial respiration with increasing pH. Less is known of microbial biomass and the mentioned relationships in the surface organic layer and deeper soil horizons. Here, we provide depthwise distribution of microbial carbon (C_mic), microbial nitrogen (N_mic), metabolic quotient (qCO₂), the fraction of C_mic in C_org (C_mic-to-C_org ratio) and the fraction of fungal respiration of soil respiration in three beech forest soils, the Göttinger Wald, Zierenberg, and the Solling sites (for site description see Part A). The following questions will be addressed. (1) Does soil acidity reduce the microbial biomass and the qCO₂ value in the acid soil profile at Solling site compared to the less acid soils at Göttinger Wald and Zierenberg sites? (2) Does better substrate quality at Göttinger Wald and Zierenberg sites increase the C_mic-to-C_org ratio? (3) What is the contribution of bacteria and fungi to soil respiration and how does soil acidity affect these contributions?