Impact of arbuscular mycorrhiza on soils Nitrification is a key process in the cycling of nitrogen (N) in ecosystems and may be influenced by the presence of arbuscular mycorrhizal fungi (AMF) in the roots of plants. An EU-funded project tested this hypothesis. Climate Change and Environment © Thinkstock Soil nitrification is of interest to scientists as it affects the availability of two sources of inorganic N to plants: ammonium (NH4+) and nitrate (NO3-). The form of N available to plants can be affected by two ubiquitous groups of soil organisms: ammonium oxidizers (AOs) and AMF. Nitrification is the oxidation of ammonium compounds in organic material into nitrites and nitrates by soil bacteria, which can affect plants' physiology and their competitive interactions. Furthermore, NO3- is known to trigger de-nitrification, whereby microorganisms ultimately produce molecular N. The aim of the EU-funded project AM-NITRIFICATION was to investigate the interactions between AMF and AO soils with high N availability. Experiments were conducted to determine if competition between AMF and AOs represents a mechanism for explaining any negative effects of AMF on nitrification. Large compartmentalised growth chambers known as mesocosms were used to isolate the effects of fungal hyphae from the combined effects of plant roots and hyphae. Results did not reveal either a negative effect of AMF on nitrification potential rates or a significant interaction between AMF and mesocosm compartment. The composition of the bacterial community was studied, but no evidence was found that AMF suppressed any individual AO bacterial taxa. However, it was found that the abundance of two AO bacterial taxa was higher in AMF treatments. Although the soil used in the experiments was moderately fertile, the availability of NH4+ was low by the time the test plants were harvested. The reason was that the plants had immobilised most of the available nutrients. This could also have been the reason why the ratio of Nitrospira to Nitrosomonas taxa was high. It was concluded that further experiments are required to ascertain the conditions under which competition could be a major factor in the detrimental effects of AMF on nitrification. Another possible mechanism could be allelopathy, whereby an organism produces compounds that restrict the growth of other species. As well as having ecological and physiological applications, AM-NITRIFICATION will have benefits for agriculture by helping to minimise nitrate leaching and N loss from crops. The project's results can also be used to support the argument for application of AMF in agriculture. Keywords Arbuscular mycorrhiza, soils, nitrification, ecosystems, fungi
