Arbuscular mycorrhizal fungi (AMF) found at unprecedented depths in heavily degraded tropical pasture

Mycorrhizal fungi are recognized for their benefits to plants, such as enhancement of nutrient uptake, drought tolerance and pest resistance. Heavy erosion of tropical soils can expose deep sub-soils that lack Arbuscular Mycorrhizal Fungi (AMF). In order to restore AMF in degraded tropical pastures the conventional method is to use non-native grasses as hosts. However, if the goal is to restore a native plant community this method adds a step to the process. Therefore, we tested the use of native legume tree seedlings as hosts to restore AMF in degraded tropical pastures in southern Costa Rica. We simulated extreme erosion conditions by removing all organic soils at six sites (0.1 ha. each). Baseline soil samples revealed that live AMF spores were abundant at 1m depth or greater in the tropical pasture. Soil samples were analyzed for live AMF spore abundance per 100g dry soil: spores were present in all six replications and in 45 of 65 samples. This is the first documentation of AMF spores at this depth. Previous studies find AMF primarily in the top 0-20cm of soil. Given our findings, AMF may not need to be restored even in some of the worst tropical pasture conditions. We were not able to draw any conclusions from restoration treatments since seedlings were damaged, in part, by livestock and human vandalism. Although social engagement was a major part of the project, our experience highlights the need for community-wide project support and management by local leaders.