Plant-mycorrhizal interactions mediate ecosystem nutrient cycling across three tropical forests: Case studies along a gradient of ectomycorrhizal abundance
The relationship between plants and mycorrhizal fungi has profound consequences for ecosystem nutrient cycling. Ectomycorrhizal (ECM) fungi can produce extracellular enzymes to degrade complex soil organic matter, whereas arbuscular mycorrhizal (AM) fungi mostly acquire nutrients in inorganic form. Current conceptual models posit that these different modes of fungal resource acquisition affect soil carbon cycling and storage. However, it is not clear whether nutrient cycling dynamics exhibit a quantitative relationship with the relative abundance of ECM vs. AM fungi. Tropical forests vary dramatically in the relative abundances and diversities of both types of fungi, making them excellent systems to study the ecosystem-level consequences of plant-mycorrhizal symbioses. Here, we discuss changes in fungal community composition and element cycling along gradients of ECM abundance in three tropical forests in Guyana, Malaysia, and Costa Rica.
Our sites span a broad range of climate, edaphic conditions, and land use, ranging from intact, monodominant ECM forest to highly degraded palm plantations. In general, forests with a high abundance of ECM fungi exhibited slower decomposition and decreased soil enzyme activity, consistent with the hypothesis that ECM outcompete free-living fungi for soil nutrients. However, while the diversity of saprotrophic fungi was significantly lower in the monodominant ECM forest in Guyana, there was a positive relationship between ECM and saprotroph diversity in the Costa Rican sites. Forests regenerating from pasture in Costa Rica contained diverse ectomycorrhizal communities, but heavily degraded soils in Malaysian palm plantations exhibited dramatic decreases in ECM species richness. Taken as a whole, these patterns suggest that mycorrhizal type has a strong influence on ecosystem processes across tropical forests that vary substantially in climate, soil type, and plant community composition. However, the diversity and abundance of mycorrhizal fungal communities are strongly dependent upon land-use history. Tropical forest responses to anthropogenic change are likely to be mediated by the resilience of native fungal communities.