Extracellular enzyme activity assay as indicator of soil microbial functional diversity and activity Niels Bohse Hendriksen, Anne Winding. Department of Environmental Science, Aarhus University, 4000 Roskilde, Denmark Soils provide numerous essential ecosystem services such as carbon cycling, recycling of nutrients and waste, soil remediation, plant growth support and regulation of above ground biodiversity, resilience, and soil suppressiveness. As such, soil ecosystem services are beneficial and vital for human life and at the same time threatened by anthropogenic activities. Increasing awareness of the threats to soil and the ecosystem services has fostered the need for a thorough understanding of soil functions and activities. Soils are very species-rich and show great functional heterogeneity. While molecular analysis of gene expression is developing, simple and inexpensive indicators of soil microbial functions is still needed. In soil, enzymes originate from a variety of organisms, notably fungi and bacteria and especially hydrolytic extracellular enzymes are of pivotal importance for decomposition of organic substrates and biogeochemical cycling. Their activity will reflect the functional diversity and activity of the microorganisms involved in decomposition processes. Their activity has been measured by the use of fluorogenic model substrates e.g. methylumbelliferyl (MUF) substrates for a number of enzymes involved in the degradation of polysacharides as cellulose, hemicellulose and chitin, while degradation of proteins has been followed by amino-methyl-coumaric substrates (AMC). Based on these fluorogenic substrates the Extracellular Enzyme Activity assay was optimized as a microwell based standardized assay for the activity of enzymes involved in degradation of polysaccharides and proteins. Using specific MUF and AMC substrates on soils from across Europe and based on literature data the experimental conditions of extraction of enzymes from soils, use of buffer and pH, substrate concentration, temperature and the necessary controls to include were optimized and standardized. This has resulted in an optimized standard operating procedure of the assay which will be tested in a soil transect across Europe as an indicator of soil functions.