Blood earth.

The article focuses on research published in the June 2015 issue of "Biotropica" concerning the ecological advantage of hollow trees. It states ecologist Christian Voigt and colleagues tested soils around hollow trees, which are often used as roosting sites for bats, and at ten artificial bat roosts in Costa Rica. It mentions they tested for a nitrogen isotope which is more common in bat guano than forest soils and found the trees are absorbing nitrogen from bat guano.