Root and mycorrhizal responses to nutrient additions in a tropical mountain forest and in pastures after forest conversion

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The goal of this project is to enhance the comparatively poor knowledge about arbuscular mycorrhizas (AM) and roots in tropical mountain forest systems in general, and in particular to integrate mycorrhizal responses into ongoing nutrient manipulations (NUMEX in the forest and FERFAST in the pasture) of the Research Unit. Within this framework, we ask questions about the abundance of AM fungi in roots and in the soil (extraradical hyphae), hypothesizing that even low levels of fertilization will cause decreased mycorrhizal fungal abundance. We also hypothesize that there are tradeoffs between extraradical AM hyphal abundance and fine roots, two structures with partially overlapping functions. Using molecular ecology tools (pyrosequencing and terminal restriction fragment length polymorphism analysis) we test for differences in AM fungal assemblages in the soil in response to nutrient manipulations and along an elevation gradient. Integrating both potential abundance and assemblage composition changes of AM fungi, we finally ask the pivotal question if fertilization can favor the occurrence of less mutualistic AM fungal assemblages. This will be accomplished using a greenhouse inoculation approach. One of the mycorrhizal functions examined will be plant growth, but we will also examine the ecosystem process of soil aggregation.