Establishing arbuscular mycorrhiza-free soil: A comparison of six methods and their effects on nutrient mobilization

In order to study decomposer–mycorrhiza interactions, mycorrhiza-free treatments are often compared with reinoculated treatments. However, methods to achieve mycorrhiza-free soil, such as chloroform fumigation and autoclaving, cause strong side effects by increasing nutrient availability and changing microbial activity. We investigated the effectiveness of soil heating (60, 80, 100 and 120 °C), autoclaving and chloroform fumigation in eliminating arbuscular mycorrhizal (AM) fungi and also determined the impacts of the tested methods on mobilization of ammonium, nitrate and phosphorus as well as on microbial activity. Heating at 60 °C and chloroform fumigation reduced colonization of plant roots (Plantago lanceolata) by AM fungi to less than 1%, while all other treatments decreased colonisation to less than 0.2%. All six methods affected plant growth and plant tissue nutrient concentrations. This in part was due to mobilization of ammonium, nitrate and phosphorus in particular in autoclaved soil and soil heated at 100 and 120 °C. Heating soil at 60 °C also increased plant growth and shoot nutrient concentration, but had least side effect on nutrient mobilization and microbial activity, suggesting that moderate heating is preferable to other methods when setting up experiments investigating mycorrhiza–decomposer interactions.