Extraradical arbuscular mycorrhizal fungal hyphae in an organic tropical montane forest soil

•Predominance of arbuscular mycorrhizae in a highly organic tropical forest soil.•AM hyphae colonize decomposing leaves in the organic layer to a great extent.•More than half of extraradical AM hyphae are located at the root surface.•Standard hyphal extraction methods are inefficient in this highly organic soil.

Previous research from the tropics indicates that AMF may be well adapted to organic soils and even represent the dominant mycorrhizal form, though the extraradical part of the symbiosis was omitted as in most other tropical studies. Our study aims at characterizing the extraradical part of arbuscular mycorrhizal fungi (AMF) in a highly organic tropical montane forest soil in Southern Ecuador. Based on recent studies on the interaction of AM fungal hyphae and litter we hypothesized that within the organic layer AM hyphae grow in close contact with decomposing material. To test this idea, AM fungal hyphal distribution in the organic layer was determined by directly staining roots and decomposing leaves and extracting hyphae from the remaining particulate organic material. AM and non-AM fungal hyphae were analyzed, as well as root colonization patterns. Our results showed that AMF indeed represented the dominant mycorrhizal form with an average root colonization of 43%. The extraradical AM hyphal length ranged from 2 to 34 m g−1 soil with a mean of 10.4 m g−1 soil (equals 3.1 m cm−3 soil), and therefore exceeded root length about 13-fold. As hypothesized, 29% of AM extraradical hyphae were closely attached to decomposing leaves. These hyphae were mainly located at the leaf surface, though in some parts leaf veins and inner leaf tissues were colonized. More than half of AM hyphal biomass was detected on the root surface, a pattern potentially driven by the predominant Paris-type AMF. Non-AM fungal hyphae colonized decomposing material to a significantly greater extent, though hyphal length attached to roots was equal. This study supports the adaptation of AMF to highly organic soils in the tropics and the existence of a widespread extraradical mycelium, which is not readily detectable by standard methods. The close association with decomposing leaves most likely improves direct nutrient uptake from decomposed material and points to a potential indirect contribution of AMF to the decomposition process.

Graphical abstractSketch of the organic layer in a tropical montane forest soil, consisting of roots, decomposing leaves and particulate organic material. The ramification of arbuscular mycorrhizal extraradical hyphae is illustrated. Bar represents the mean, error bar s.e.m. (linear-mixed effect model (P < 0.05) including the respective Sample ID as random effect). Figure optionsDownload full-size imageDownload as PowerPoint slide