Professor University of California Irvine Irvine, California, United States
Abstract: Globally, it has been suggested that ectomycorrhizal fungi (ECM) promote C storage while arbuscular mycorrhizal fungi (AMF) reduce it by decreasing or promoting the decomposing activity of free-living saprotrophs (SAP). These inter-guild interactions are known as the “Gadgil effect” and “priming effect”, respectively. However, these patterns are inconsistent among studies. They could be associated with the nutrient concentration of the substrate and other environmental factors. We hypothesized that ECM reduce the decomposition of substrates with a high C:N ratio, while AMF promote the decomposition of substrates with a low C:N ratio, by interacting with SAP. We obtained data from 32 articles (77 studies) that reported decomposition rate, substrate C:N ratio, mycorrhizal guild, mean annual or incubation temperature, mean annual precipitation, length of the experiment, absolute latitude and biome. The studies were carried out under field or laboratory conditions and consisted of restricting or allowing the access of mycorrhizal fungi to substrates colonized by SAP. Our meta-analysis showed that neither ECM nor AMF influence decomposition independently. Nevertheless, mycorrhizal fungi promote substrate decomposition by 5.6% overall. Despite these patterns, we observed a large variance among studies in the effect of inter-guild interactions on decomposition (Q=624), corroborating inconsistencies in literature. We examined potential sources of this heterogeneity by using a random forest approach and a meta-regression. We found that the C:N ratio and absolute latitude were the most important moderators affecting decomposition—more than mycorrhizal guild. In particular, interactions between mycorrhizal fungi and SAP decrease decomposition at high C:N ratios and increase it at low C:N ratios. This supports the notion that the Gadgil and priming effects are regulated by substrate nutrient concentration, regardless of the guild involved. Furthermore, decomposition increased at lower latitudes with a positive association with mean annual temperature under field conditions. This result suggests a latitudinal pattern of microbial biodiversity and environmental factors influencing the outcome of the inter-guild interactions. Our results show that mycorrhizal fungi potentially influence soil organic matter dynamics and C storage by interacting with free living saprobes in different biomes. Consequently, they could play an important role in mediating soil responses to climate change.
