Hi Sebastian, The authors in this recent paper ran into the same problem and suggest using artist canvas instead. In this paper they compare it's usefulness for decomp studies to the Shirley material.
Slocum et al. (2009). "Artist canvas as a new standard for the cotton-strip assay". Journal of plant nutrition and soil science (1436-8730), 172 Good luck! David Samuel Johnson The Ecosystems Center Marine Biological Laboratory Woods Hole, MA 02543
