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Urban Soils
As urbanization continues, more and more lands such as forestland and rangeland are turned into urban areas, and cities can generate more than 80% of urban and industrial wastes that might be added to the soils. As soils have been extensively impacted by human activities, some properties of soils in urban areas are different from natural soils. It is important to understand the properties and management of urban soils as they are critical resources.

There are three types of urban soils：

a)    Soils present a man-made surface layer at least 50 cm containing a mixture of human artifacts and are highly transformed by human activities through mixing and filling and by contamination.

b)    Soils that are used in parks and gardens are closer to agricultural soils but have different compositions.

c)    Soils impacted by construction activities are often sealed.

Use of Soils in Urban Areas
Soils in urban areas provide a wide range of uses. They can support infrastructure such as buildings, roads, and parking lots and shelter for cables and pipes underground. They are used to support plants on roadsides, parks, and public and private gardens in both urban and suburban areas. They are also used for industrial production activities such as waste disposal and for recreation such as playgrounds.

Coarse Fragments
Due to the additions of human artificial materials such as concrete and bricks, the coarse fragment contents are usually very high in urban soils. Coarse fragments are particles with sizes ranging from 2 mm to 25 cm in diameter. Soils with higher amounts of coarse particles can influence soil bulk density, increase hydraulic conductivity and occurrences of macropores, and decrease water holding capacity. Although there is a greater coarse fragment content in urban soils, the soils often have a low infiltration rate due to heavy compaction.

Soil Compaction and Sealing
Soils are often compacted in urban areas due to deliberate compaction and unintentional compaction. Deliberate compaction results from heavy equipment and construction activities in order to increase strength for construction foundations. Unintentional compaction is caused by vehicle and foot traffic. Soil compaction can negatively impact other soil physical properties, including soil structure, porosity, infiltration rate, aggregate stability, and penetration resistance. Compacted soils reduce water infiltration rate and restrict root growth and penetration into the soil. In addition, in urban areas, soils are commonly sealed by grey infrastructure. For example, soils are sealed by pavements and buildings with asphalt and concrete. Soil sealling inhibits material and energy exchanges between the soil and other compartments and increases water runoff in cities.

Soil Structure
Due to the lack of fine particles to bridge between coarse particles, the structure of urban soils is commonly poor. Soil compaction can crush the large fraction of soil aggregates, reducing the pore space. This leads to poor aeration and could influence the growth of vegetation in urban areas. Soils with a poor structure can also lead to soil erosion, poor infiltration and water logging.

pH
Commonly, the pH of urban soils is slightly to strongly alkaline, which is higher than 7 and can be up to 8.6. This can be the result of the contaminants of building wastes and rubble which release calcareous solutions. Soil pH can impact soil chemical processes greatly. A high soil pH can induce nutrient imbalance and immobilize heavy metal particles. Nutrient imbalance can influence plant growth. Moreover, a study showed that 40% Ni was found in soils if the soil pH exceeded 8.1.

Contamination
Urban soils may contain more chemical contaminants than natural soils due to human activities. Common chemical additives are heavy metals (e.g., Cd, Pb, and Zn ) and organic pollutants (e.g., hydrocarbons, polychlorobiphenyls, and herbicides). Urban soils play an important role in absorbing pollutants; however, urban soils will shift to a pollution source from a pollution sink if the capacity is exceeded. Soil contamination has become a public concern as it poses a risk to the environment and human health.

Biological Properties
The contamination of urban soil can influence living organisms and biodiversity negatively. The disturbances of natural lands damage natural habitats of native species. Relatively few species are able to adapt to the urban environment. A study in Italy indicates that the uptake of heavy metals (Cd, Cu, Pb, and Zn) by earthworms is impacted by soil comtamination. Soil compaction, contamination, and sealing can also result in a decrease in soil microbial activity. Study found that microbial biomass could be reduced by 50% in urban soils compared to their natural counterparts.

Management of Urban Soils
Restoration of soil health is essential to improving environment and human health in urban areas. It is important to provide more green and open spaces in cities such as recreation areas and parks to promote human well-being. To improve biodiversity, exisiting habitats and vegetation cover in urban areas should be maintained and restored. Other managment practices may include promoting urban agriculture and green roofs.