Ectomycorrhizal fungi in association with Pinus sylvestris seedlings promote soil aggregation and soil water repellency

Research on fungal effects on soil aggregation has been heavily biased towards arbuscular mycorrhiza. Even though ectomycorrhizal fungi are thought to be as important as arbuscular mycorrhizal fungi and saprotrophic fungi in contributing to soil structure, there are few experimental studies on this topic. Here we quantified how nine ectomycorrhizal fungi in association with Pinus sylvestris seedlings affected soil aggregation and soil water repellency (SWR) of a sandy loamy soil. Water-stable aggregates (>0.25 mm diameter) increased by 6–12% when plants were associated with Laccaria bicolor, Laccaria laccata, Lactarius theiogalus, Paxillus involutus and Suillus bovinus. Mean weight diameter (MWD) of soil aggregates also increased, primarily in the 2–4 mm diameter size class. However, Suillus granulatus increased water-stable aggregates but not MWD, conversely Rhizopogon roseolus and Suillus luteus increased MWD but not water-stable aggregates. We also found Lt. theiogalus, R. roseolus and S. luteus promoted SWR. Furthermore, hyphal length was weakly correlated with MWD (R = 0.27, P < 0.05), especially with aggregate mass in the 2–4 mm size class (R = 0.32, P < 0.05). However, we could not identify clear soil effects (soil pH, soil protein content) serving as explanation for either soil aggregation or SWR. Thus, we conclude that interactions between fungi and soil structure are a species-dependent processes based on yet to be characterized fungal traits. Our results have added further evidence from direct experimentation that ectomycorrhizal fungi can contribute to soil aggregation and SWR.