Fungi are major drivers of carbon and nutrient cycling in terrestrial ecosystems. In boreal and temperate forests, symbiotic ectomycorrhizal (EM) and free-living saprotrophic fungi compete for limiting resources held in soil organic matter. The consequence of these interactions can result in significant suppression of organic matter decomposition, phenomenon known as the ‘Gadgil effect’. While this is an often-cited phenomenon, the direction and magnitude of the ‘Gadgil effect’ varies dramatically among study systems, appearing strongest in Pinus-dominated forest systems and less so in forests where other EM hosts are dominant.
To explicitly test the effects of host and associated EM fungal communities on organic matter decomposition, while holding climate and underlying edaphic variables constant, we setup a field experiment across three sites that represented a host gradient (Pinus-dominated, Quercus-dominated, and mixed) at Cedar Creek Ecological Science Reserve in Minnesota, USA. Within each site, we compared litter mass loss rates, carbon and nitrogen mineralization rates and fungal community structure in treatment plots with and without EM fungi (via trenching).
Corroborating findings from previous studies, we found a strong suppression of organic matter decomposition at the Pinus-dominated site. Conversely, we observed evidence of EM fungi priming organic matter decomposition in the Quercus-dominated site. The effect of EM fungi on organic matter decomposition rates in the mixed host site was highly variable. Collectively, these results support the hypothesis that hosts and their associated EM communities modulate the direction and magnitude of the ‘Gadgil effect’. We emphasize that appreciating variation in EM community structure and functional diversity will be key in developing testable hypotheses and attaining mechanistic understanding of EM-mediated C storage in soil organic matter in forest ecosystems.