The Heart of Green Infrastructure – The Soil
Image source: The Green Infrastructure Research Group
SESL is assisting the ACT Government with their Green Infrastructure Strategy. Green Infrastructure is the network of waterways and green spaces in an urban environment. Spaces include parks, gardens, sports fields, waterways, reserves, roof gardens and green walls. These areas provide visual amenity, social benefits, cleaner air, flood protection, habitat for biodiversity and cleaner storm water.
A key benefit of green infrastructure is mitigating the urban heat island effect. This is a phenomenon where an urban area is hotter than surrounding rural areas, and where the city remains hotter in the evening. Cities can be, on average, between 1 and 5 degrees celcius warmer than their rural surroundings. The cause stems from differences in how the sun’s energy is dissipated. An urban setting has more hard surfaces such as roads and buildings to reflect heat back into the atmosphere, whilst rural settings have more vegetative cover which provides a cooling effect.
Image source: Heat Island Group
Additionally, damp soil has a significantly greater thermal conductivity and heat capacity than dry soil and concrete. A wet soil will absorb heat faster than a dry soil, and a wet soil will experience less of a temperature increase, which is ideal for plant roots and soil microbes. The evaporating component of heat loss (QE) will also help cool the surface.
What, therefore, is at the heart of urban heat island mitigation? On the surface (pun intended) the answer appears to be plants – but the answer is soil. Plants are the actual method by which the atmosphere is cooled, they provide oxygen, clean the air, offer mental health benefits, and provide food and habitat for wildlife. Yet soil is the foundation for plants, providing the water and nutrients plants need to survive. For flourishing green spaces you need to start in the soil.
The relationship between plants and soil is not mutually exclusive. Plants need healthy soil, and soil needs plant cover. Bare soil is quickly compacted and eroded, leading to dust problems, run-off and siltation. Soils also act as filters, scrubbing nutrients from storm water and minimizing polluted run-off. Unfortunately soil function is usually negatively impacted by urbanization. Nutrient cycling ceases, surfaces are compacted or paved and soil oxygen and water content declines. Nutrients and effective rooting volume decline, runoff increases and the status and longevity of vegetation declines.
Poor soil management and planning often leads to plant death or poor performance, and does nothing to improve runoff and soil storage. Consideration of soil requirements and choice of the soil approach should be detailed at the beginning of every project in the early design phase (Leake and Haege 2014).
Soil, therefore, is an essential component of of Green Infrastructure.