Effect of compost addition and crop rotation point upon VAM fungi

Populations of vesicular-arbuscular mycorrhizal (VAM) fungi and mycorrhiza formation were examined in a field experiment studying the agricultural application of composted animal manures. The replicate experiment allowed each crop of a Zea mays > vegetable > small grain rotation to be sampled each of 3 years. Chicken litter/leaf compost and dairy cow manure/leaf compost enhanced spore populations of two VAM fungus species type groups (Glomus etunicatum type and the general Glomus spp. group, including G. mosseae) relative to those found in plots treated with raw dairy cow manure and conventional fertilizer. Populations of other groups were not affected by amendment, due likely to the large amount of P added in composts and manure relative to the conventional fertilizer applied. Crop rotation point had consistent, significant effects, with both lower populations of spores and less mycorrhizal infectivity of soil in plots after the vegetable crop (Spinacea oleraceae and/or Capsicum annuum) relative to maize and small grain (Avena sativa or Triticum aestivum). This was due to the non-mycorrhizal status and very low mycorrhization (approximately 1% root length colonized) of S. oleraceae and C. annuum, respectively. Future agricultural applications of compost and manure to provide N for crops need to consider the effects upon VAM fungi of other nutrients in these amendments.