We found very different levels of activity of peptidase in forest soils depending on the substrates used for forest soils in temperate soil. Some N-cycling enzymes (urease, arginine deaminase, alanine aminopeptidase, lysyl-alanine aminopeptidase) showed increased activities whereas others (N-acetyl-glucosaminidase, protease, leucine aminopeptidase, alanyl-alanyl-phenine aminopeptidase activities) decreased under reduced N treatment. Therefore, I want to recommend using some alternative substrates. (please see: Enowashu E., Poll C., Lamersdorf N., Kandeler E. (2009) Microbial enzyme activities under reduced nitrogen deposition in a spruce forest soil. Applied Soil Ecology doi:10.1016/j.apsoil.2009.05.003.

Which substrate did you use?