Urban Soils
A book review by Tom Sizmur
University of Reading
t.sizmur@reading.ac.uk
ECG Bulletin July 2020
University of Reading
t.sizmur@reading.ac.uk
ECG Bulletin July 2020
Estimates of the global land area currently being covered by urban conurbations range from 0.5% to 3%, but our understanding of how soils function largely stems from research undertaken in semi-natural or farmed ecosystems. Urban soils are highly disturbed because they are used as waste dumps, exposed to air pollution, subjected to urban heat island effects, or sealed entirely by concrete or asphalt.
Urban Soils is part of the ‘Advances in Soil Science’ series. This boasts an impressive collection of volumes, mostly edited, as this book is, by the Rattan Lal and B. A. Stewart. This book contains 18 chapters that span topics such as soil organic matter, food security, environmental contaminants, and ecosystem services in urban soils. I dipped into a few chapters that particularly appealed to me.
Urban Soils is part of the ‘Advances in Soil Science’ series. This boasts an impressive collection of volumes, mostly edited, as this book is, by the Rattan Lal and B. A. Stewart. This book contains 18 chapters that span topics such as soil organic matter, food security, environmental contaminants, and ecosystem services in urban soils. I dipped into a few chapters that particularly appealed to me.
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Chapters 3 to 6 deal with the impact of urbanisation on the soil carbon cycle. There is a lack of consensus on whether urban soils are a net sink or a net source of carbon. Globally, urban areas are more likely to be situated on soil types with lower than average soil organic carbon content (you don’t find many cities built on peat bogs!), and so the potential to mineralise and emit large quantities of carbon to the atmosphere is limited. Chapter 3 provides case studies based in Moscow to illustrate how land management through history can increase the carbon stocks of urban soils.
A new paradigm, describing how soil organic carbon becomes stabilised in soils is introduced. This points to the roles of microorganisms, environmental conditions, and the soil physical structure as key mediators. Chapter 4 applies the new paradigm to urban soils. Urban environments are generally warmer, wetter, and enriched in aerially deposited nitrogen, which may accelerate microbial mineralisation of soil organic matter and the release of carbon from soils as carbon dioxide. Urban soils also contain elevated concentrations of heavy metals, which negatively impact plant productivity, thus decreasing the potential for urban soils to sequester carbon. However, a considerable portion of the carbon in urban soils is fossil fuel-derived black carbon (i.e. soot), which has a slower turnover in soils than plant litter. Chapter 4 also highlights several direct anthropogenic drivers of the soil carbon cycle in urban soils related to changes in land use or land cover, and including the introduction of land management practices, such as the use of organic and inorganic fertilisers; the removal of above-ground biomass (i.e. mowing the lawn); compaction; topsoil removal; and/or sealing. |
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Chapter 5 discusses how the carbon and nitrogen cycles in urban soils underpin the delivery of ecosystem functions and services, such as the mitigation of heat stress, regulating the storage and flow of water, and providing space for recreation and a habitat for urban wildlife. The delivery of these ecosystem services (particularly the latter) is hampered by poor interconnectivity of urban soils that are isolated in patches across the urban landscape.
The relationships between urban soil organic carbon and soil forming factors (climate, parent material, time, vegetation and anthropogenic influence) are explored in Chapter 6. The chapter also compiles international datasets to estimate a global urban soil organic carbon stock of 6.57 Gt of carbon. However, the authors acknowledge that true stock may be much higher because most datasets only report carbon stocks in the uppermost 30 cm, highlighting a key knowledge gap.
One of the most well-studied aspects of urban soils is the elevated concentrations of contaminants they contain. While this is mentioned in several chapters of the book, it is dealt with directly in Chapter 8, where the bioavailability and fate of four common urban contaminants (lead, arsenic, cadmium and polyaromatic hydrocarbons (PAHs)) are discussed comprehensively. Lead is not readily taken up by plants, but poses the greatest risk to human health when lead contaminated soil is directly ingested or inhaled. The fate of arsenic in soils is often dictated by its valence, with arsenate, As(V), generally less mobile and bioavailable than arsenite, As(III). The extent to which cadmium leaches from urban soil is largely influenced by soil pH, with its mobility greatly increased in acidic conditions. Unlike lead, arsenic, and cadmium some PAHs may be remediated by microbial degradation. Their environmental fate depends on molecular weight, with low molecular weight PAHs more likely to be taken up by plants than high molecular weight compounds. Molecular structure, along with mass, determines their rate of microbial degradation.
Chapters 14 and 15 tackle the issue of food production in urban environments. While much attention and scientific research has been directed towards how we might produce food to feed a global population of 9-10 billion by 2050, much less attention has been focussed on how this food might be supplied to the approximately 6.5 billion people that are likely to be living in urban areas by this time. Chapter 14 explains the role urban agriculture might play in satisfying the future demand for food. However, urban soils are generally less fertile than rural soils, primarily due to chemical and physical degradation. This degradation can include the presence of contaminants (e.g. heavy metals), compaction, and low levels of soil organic matter. As a result, many authors report lower yields of vegetable crops on urban farms compared to rural farms, although this is sometimes reversed if urban soils are irrigated or receive higher chemical inputs (e.g. fertilisers and pesticides). Urban soils are particularly well placed to benefit from nutrients from liquid and solid waste materials produced by the residents of urban areas through the transformation of urban wastes into valuable products for soils (e.g. composts), as explained in Chapter 10.
Chapter 15 makes a strong case for empowering cities to meet their demand for food by exploiting hydroponics and aeroponics to establish vertical farming as a step towards fully integrated, resilient and sustainable cities. By contrast, Chapter 14 highlights the need for more research before best management practices for urban soils and agriculture may be formulated and disseminated. Chapter 18 urges us to look to the past and learn from the lessons (both successes and failures) of ancient civilisations (e.g. Mayan, Byzantine, Harappan, and Mesopotamian), and build upon this historic knowledge to restore our urban soils. It recommends that we should blend well-understood approaches (e.g. use of vegetation and compost) with more innovative ones (e.g. bioremediation, green roofs, and synthetic soils) to secure an urban food supply for an uncertain future.
While the Preface, Chapter 1 (Introduction) and Chapter 18 (the final chapter) provide a global picture of urbanisation and the impact of future urbanisation on the functioning of urban ‘Anthrosol’ or ‘Technosol’ soils, the authors are largely drawn from the US, with contributors also from Russia, Poland, Mexico and India. As a result, the book lacks perspectives on how urbanisation impacts soils in South America, Africa, and South East Asia. This omission is particularly disappointing since almost all the future increases in urbanisation in the 21st century are expected to occur in developing nations (particularly Africa and Asia).
This book is a collection of papers by different authors and should be approached as such. Some of these chapters provide primary data, while others offer a review of the literature on a particular subtopic. The book is an ideal starting material for graduate students who wish to obtain a grounding in soil science within an urban setting. Some chapters will also be useful for city planners who need to consider urban soils, ecosystem services, natural flood management, green infrastructure and food security when planning the expansion of our cities. However, I do not recommend reading this book from cover to cover. Identify key chapters, as I did, that interest you the most.
Reference
Lal, R. and Stewart, B.A., Urban Soils, CRC Press, Boca Raton, Florida, 2018, eBook, ISBN: 9781315154251.
The relationships between urban soil organic carbon and soil forming factors (climate, parent material, time, vegetation and anthropogenic influence) are explored in Chapter 6. The chapter also compiles international datasets to estimate a global urban soil organic carbon stock of 6.57 Gt of carbon. However, the authors acknowledge that true stock may be much higher because most datasets only report carbon stocks in the uppermost 30 cm, highlighting a key knowledge gap.
One of the most well-studied aspects of urban soils is the elevated concentrations of contaminants they contain. While this is mentioned in several chapters of the book, it is dealt with directly in Chapter 8, where the bioavailability and fate of four common urban contaminants (lead, arsenic, cadmium and polyaromatic hydrocarbons (PAHs)) are discussed comprehensively. Lead is not readily taken up by plants, but poses the greatest risk to human health when lead contaminated soil is directly ingested or inhaled. The fate of arsenic in soils is often dictated by its valence, with arsenate, As(V), generally less mobile and bioavailable than arsenite, As(III). The extent to which cadmium leaches from urban soil is largely influenced by soil pH, with its mobility greatly increased in acidic conditions. Unlike lead, arsenic, and cadmium some PAHs may be remediated by microbial degradation. Their environmental fate depends on molecular weight, with low molecular weight PAHs more likely to be taken up by plants than high molecular weight compounds. Molecular structure, along with mass, determines their rate of microbial degradation.
Chapters 14 and 15 tackle the issue of food production in urban environments. While much attention and scientific research has been directed towards how we might produce food to feed a global population of 9-10 billion by 2050, much less attention has been focussed on how this food might be supplied to the approximately 6.5 billion people that are likely to be living in urban areas by this time. Chapter 14 explains the role urban agriculture might play in satisfying the future demand for food. However, urban soils are generally less fertile than rural soils, primarily due to chemical and physical degradation. This degradation can include the presence of contaminants (e.g. heavy metals), compaction, and low levels of soil organic matter. As a result, many authors report lower yields of vegetable crops on urban farms compared to rural farms, although this is sometimes reversed if urban soils are irrigated or receive higher chemical inputs (e.g. fertilisers and pesticides). Urban soils are particularly well placed to benefit from nutrients from liquid and solid waste materials produced by the residents of urban areas through the transformation of urban wastes into valuable products for soils (e.g. composts), as explained in Chapter 10.
Chapter 15 makes a strong case for empowering cities to meet their demand for food by exploiting hydroponics and aeroponics to establish vertical farming as a step towards fully integrated, resilient and sustainable cities. By contrast, Chapter 14 highlights the need for more research before best management practices for urban soils and agriculture may be formulated and disseminated. Chapter 18 urges us to look to the past and learn from the lessons (both successes and failures) of ancient civilisations (e.g. Mayan, Byzantine, Harappan, and Mesopotamian), and build upon this historic knowledge to restore our urban soils. It recommends that we should blend well-understood approaches (e.g. use of vegetation and compost) with more innovative ones (e.g. bioremediation, green roofs, and synthetic soils) to secure an urban food supply for an uncertain future.
While the Preface, Chapter 1 (Introduction) and Chapter 18 (the final chapter) provide a global picture of urbanisation and the impact of future urbanisation on the functioning of urban ‘Anthrosol’ or ‘Technosol’ soils, the authors are largely drawn from the US, with contributors also from Russia, Poland, Mexico and India. As a result, the book lacks perspectives on how urbanisation impacts soils in South America, Africa, and South East Asia. This omission is particularly disappointing since almost all the future increases in urbanisation in the 21st century are expected to occur in developing nations (particularly Africa and Asia).
This book is a collection of papers by different authors and should be approached as such. Some of these chapters provide primary data, while others offer a review of the literature on a particular subtopic. The book is an ideal starting material for graduate students who wish to obtain a grounding in soil science within an urban setting. Some chapters will also be useful for city planners who need to consider urban soils, ecosystem services, natural flood management, green infrastructure and food security when planning the expansion of our cities. However, I do not recommend reading this book from cover to cover. Identify key chapters, as I did, that interest you the most.
Reference
Lal, R. and Stewart, B.A., Urban Soils, CRC Press, Boca Raton, Florida, 2018, eBook, ISBN: 9781315154251.
