10 2Natural systems and greenspaces
accumulations of many pollutants and toxins in the soil. Also residential septic
systems and cesspools add concentrated water, organic matter, and nutrients
in spots.
Digging down to see an urban soil profile often reveals layers of sand or rubble
(e.g., pieces of bricks, mortar, concrete) with sharp boundaries, sometimes with
aburied layer of mixed organic matter and mineral particles, which reflect the
history of building activities on the site. The sand and rubble layers are porous,
and the layers with mortar or concrete have a high pH.
Soil compactionand associated poor plant growth is widespread in urban
areas, from the 50--100 cm thick compacted material under ballfields to reg-
ularly walked trails. Construction equipment often significantly compacts the
subsoil, so when a site is later covered with topsoil, water cannot drain well
and accumulates in the topsoil inhibiting plant growth. Even soil compaction
due to vibrations from nearby vehicle traffic, trains, and diverse equipment is
significant and widespread in urban regions.
Overall, in metropolitan areas the history of human construction is a major
determinant of soil conditions. In the urban-region ring, agricultural and other
land-use history plus geographic/topographic location are progressively more
important determinants of soil conditions.Microclimate and air pollutants
Microclimate and air pollution, the two subjects introduced here, are
of major ecological importance in urban regions (Landsberg 1981, Oke 1987,
Ahrens 1991 ,Schmandt and Clarkson 1992, Moran and Morgan1994,Forman
1995,Smith1996,Santamouris 2001, Arnfield 2003).Microclimate,thehistory of
weather conditions in small spaces, differs on north vs. south slopes, upslope vs.
downslope, near vs. far from a coast or other water body, and on different sides
of buildings. Wind, solar angle, and source of water vapor are the major reasons.
Solar radiation composed of short wavelengths (the visible spectrum) and long
wavelengths (infrared radiation) is absorbed by soil, vegetation, and especially
dark impermeable surfaces, which then reradiate energy to the sky (especially
at night) in the form of infrared radiation. Infrared radiation is effectively heat,
so the air is heated. The abundance of dark impermeable surfaces around cities
helps produce aurban heat-island effect(Landsberg 1981, Moran and Morgan 1994,
Arnfield 2003, Hough 2004). Upward-moving city air at night carries pollutants
out and draws in air from the surroundings. However, if atemperatureinversion(a
warmair layer, e.g., over the metro area) forms, the air is blocked from moving
upward, so heat, particles, and gaseous pollutants in the air accumulate.