Soil fauna, a.k.a. earthworms, beetles, springtails, millipedes, centipedes and other creepy crawlies, do a soil good. They facilitate aeration, pull nutrients from the surface down into soil strata, and break down organic matter and boost plant growth. Some even eat soil-borne plant pathogens.
In a rain garden they may positively affect pollution removal, water infiltration and remove contaminants such as soil-borne animal or plant pathogens. They may also have negative effects by making nutrients and pollutants more mobile and thus more likely to leach.
Practically no research is considering their effects in rain garden stormwater treatment. But could earthworms, springtails, centipedes and other soil fauna be included in a rain garden work to optimize functionality of the feature? That’s the gist of a literature review published by Andrew Mehring and Lisa Levin from the University of California San Diego.
Based on studies in Washington, D.C. and Southeastern Australia, we know rain garden soils are teeming with life: earthworms, potworms, springtails, mites, fly larva, beetles (adults and larva), millipedes, centipedes, isopods, ants, spiders and snails.
In terrestrial systems, all this soil life provides ecosystems services. Earthworms tunnel, creating networks for infiltration that have been shown to increase infiltration two to 15 times in terrestrial systems. These tunnels serve as “preferential flow paths,” even in rainy periods. Other burrowing insects, crustaceans and arachnids are suspected of having the same effect.
But what about rain gardens? Is soil life equally beneficial?
Removing nutrients is vitally important for rain gardens treating stormwater. Unfortunately, the effect on how nitrogen and phosphorus are retained and removed via soil fauna in rain garden settings hasn’t been looked at. And, based on what is known so far about how soil fauna affects N and P in forest settings and the little we know about worms through vermicomposting, the effects of soil fauna in rain gardens could be both positive and negative. Certainly soil fauna are good for plant growth since they transform nitrogen into more plant usable forms. Soil fauna also appear to boost the beneficial effects of soil mycorrhizae on plant growth.
But the net result of soil fauna in processing organic matter and in removing pathogens isn’t understood, with some research showing positive benefits and others negative benefits.
Some soil fauna may bioaccumulate meaningful amounts of soil-borne pollutants, like heavy metals such as zinc, cadmium, copper and lead. Other fauna may synergize plant uptake of pollutants or bioaccumulate other types of contaminants, such as toxic chemical byproducts or pharmaceuticals. Provided that the pollutants remain stationary or are safely removed, this is a good thing, but if the soil fauna moves out of the rain garden, pollutants may move to adjacent soils with the invertebrates.
Rain gardens feature highly engineered media and soils. Could it be that accommodating native beneficial soil fauna in the design will enhance rain garden performance? Conversely, perhaps some soil fauna are bad for rain garden performance and the design should purposely exclude them or reduce their impact. Stay tuned – there’s much more to learn.
Potential roles of soil fauna in improving the efficiency of rain gardens used as natural stormwater treatment systems (http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12525/abstract) Andrew S. Mehring and Lisa A. Levin, Center for Marine Biodiversity and Conservation, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA. Journal of Applied Ecology2015, 52, 1445-145. doi: 10.1111/1365-2664.12525
January 15, 2016 NewTerrain