1. Acton, DF, Padbury, GA (1993) A conceptual framework for soil quality assessment and monitoring. In: Acton DF (ed) A Program to Assess and Monitor Soil Quality in Canada. Soil Quality Evaluation Program Summary. Center for Land and Biological Resource Research, Agriculture Canada, Ottawa, ON, pp 2–7.
2. Acton, DF, Gregorich, LJ (1995) Health of our soils. Centre de recherches sur les terres et les ressources biologiques, Direction générale de la recherche, Agriculture et Agroalimentaire Canada. Ottawa, ON.
3. Ajwa, HA, Dell, CJ, Rice, CW (1999) Changes in enzymes activities and microbial biomass of tallgrass prairies soil as related to burning and nitrogen fertilization. Soil Biol Biochem 31:769–777.
4. Alkorta, I, Aizpurua, A, Riga, P, Albizu, I, Amézaga, I, Garbisu, C (2003a) Soil enzyme activities as biological indicators of soil health. Rev Environ Health 18:65–73.
5. Alkorta, I, Amezaga, I, Albizu, I, Aizpurua, A, Onaindia, M, Buchner, V, Garbisu, C (2003b) Molecular microbial biodiversity assessment: a biological indicator of soil health. Rev Environ Health 18:131–151.