Topsoil Development in Bioretention Cells

TOPSOIL DEVELOPMENT IN BIORETENTION CELLS

What are the Implications?

Emily Mitchell Ayers, Ph.D.

The Low Impact Development Center, Inc. Pedogenesis: soil development 2 Pedogenesis in bioretention cells 3 Importance for engineers 4 The development of a “topsoil” in bioretention can be expected to affect:  Hydraulic performance  Increased porosity  Prevention of clogging  Pollutant removal performance  Increased cation exchange capacity  Increased bacterial activity Biological actors 5 Actors Activities

 Earthworms  Create pores  Create aggregates  Other invertebrates  Break down and  Plant roots incorporate organic  Fungal hyphae matter into the soil  Soil organic matter  Stabilize pores and aggregates Soil Organic Matter

 Binds soil particles together into stable aggregates

 Maintains soil structure and macroporosity

 Provides food source for bacteria

 Adsorbs pollutants Source: Sinauer Associates Plant roots (and mycorrhizal fungi)

 Break up compacted soils  Create macropores  Stabilize macropores  Increase soil organic matter  Provide habitat and food source for bacteria Source: www.hedging.co.uk Earthworms

 Create macropores through burrowing

 Incorporate organic matter into soil matrix

 Form stable soil aggregates

Source: www.ecocharlie.co.uk Field study

9 Objective: Catalog the biotic communities and physical properties of rain garden soils

Methodology: Conduct soil surveys at a series of rain gardens forming a chronosequence Soil animals were found at all sites 10 Characteristic depth profile 11

Leisman, 1957 Conclusions from field study 12

 Biological activity is ubiquitous in bioretention soils

 Earthworms and other soil invertebrates rapidly colonize bioretention cells in spite of their physical isolation

 Bioretention cells develop a characteristic soil profile with exponentially decreasing macro-biological activity with depth

 No visible evidence of clogging due to normal operation was found at any of the field sites Soil surface in a 10-year old bioretention cell 13 Implications

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 Biological activity changes bioretention soils

 These changes may be beneficial to the bioretention cell’s function and longevity Suggestions for bioretention design and maintenance

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 Design to promote topsoil development  Seed bioretention with organisms  Seed bioretention with mycorrhizal fungi  Limit sand content of Bioretention Soil Medium to encourage earthworms  Plant densely  Maintain so as to protect life and topsoil integrity  Avoid disturbance of topsoil and mulch  Mow to encourage high productivity  Limit pesticide use in watershed  Avoid use of sodium chloride deicers in watershed Thank You

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Please feel free to contact me at: [email protected]

Special thanks to my dissertation advisor, Dr. Patrick Kangas, at the University of Maryland Department of Environmental Science and Technology