Soil redox potential and plant species identity likely affect root-associated fungi in salt marshes.

Root-associated fungi (RAF) are important for plant performance because they form beneficial, neutral, or pathogenic relationships with their hosts, thus regulating nutrient uptake, stress tolerance, and overall plant fitness. At the transition from marine to terrestrial ecosystems, salt marshes are shaped by strong environmental gradients and provide vital ecosystem services such as climate regulation and coastal protection, yet RAF remain poorly understood in these systems. Therefore, we examined the community structure and abundance along this ecotone of RAF in general, and arbuscular mycorrhizal fungi (AMF) in particular, using microscopy to assess AMF abundance in plant roots, high-throughput sequencing to determine fungal community composition, and measured abiotic soil conditions. The observed shift in fungal community from marine-dominated zones at low elevations to those at the terrestrial end at higher elevations corresponded to a pronounced increase of AMF, from 0.04 ± 0.17% to 14.05 ± 8.61% in arbuscular colonization of roots and from 0.85% ± 2.55% to 19.75% ± 11.77% in relative (root DNA-derived) amplicon abundance of Glomeromycota. It mainly correlated to an increase of the soil redox potential, a proxy for oxygen availability (RDA, R2 = 0.05, p = 0.004). Within similar environmental conditions of a zone, host species identity became an additional strong biotic filter for the RAF community. Our findings offer new insights into the distribution of fungal communities along the marine-terrestrial ecotone and highlight the need for further research to understand mechanisms of their community assembly, their function and responses to ongoing global change.