What I Have Learned About Urban Soils: Part 1

By Richard V. Pouyat, Ph.D. Emeritus Scientist, U.S. Forest Service

I have been involved in conducting research in urban ecosystems for over 30 years. And while I was pulling together a presentation for a recent virtual symposium on urban soils (Los Angeles Urban Soil Symposium) hosted by TreePeople, I asked myself, what in a nutshell have I learned about urban soils during that time? Well, I tried to keep it within 5 topics, but in the end I came up with 6. In part 1, I cover the first three. Here they are:

1. Cities are the answer to continued human existence on the planet Earth

To address this topic, I have to introduce two important points:

First, ecosystems are connected

It can be argued that densely populated areas, such as our cities, suburban areas, and peri-urban towns are the answer to the world’s human population growth, which continues to grow exponentially. In fact, it is estimated that the world’s human population is expected to exceed 11 billion people before the end of the 21st century (United Nations, 2015). Where these billions of people end up living will have a profound effect on the Earth’s ecosystems. If we can pack ourselves into densely populated areas and do so in a sustainable way (more on this later), we will have a much better chance of saving highly productive agricultural and forested lands and vital native ecosystems and habitats.

To illustrate, imagine a city as an ecosystem with inputs and outputs of resources and waste (above graphic). In both situations the importation of resources and subsequent release of waste cause an “ecological footprint” on those ecosystems connected with a human populated area. For example, the importation of food and export of sewage sludge to and from the city or town will have impacts on the ecosystems connected to the flow of food and waste, with the impacted ecosystems representing the “ecological footprint” of the urban area. Thus, an important objective in the development of sustainable cities is to minimize that footprint. Good examples include the recycling of waste resulting from food consumption, or to produce food locally thereby minimizing trace gas emissions from both transporting and packaging of food. For this and other examples, urban soil plays an important role in the development of sustainable and resilient cities.

Second, urban soil is the brown infrastructure of cities and towns

Soil and the ecosystem services they provide are crucial for the development of a broad array of “nature-based solutions” to help cities avoid and adapt to climate change and enhance the quality of life for the people who live there. Within an urban context, how may this occur? To answer that question, let’s look at the provision of ecosystem services in urban landscapes (see above graphic):

1. Working Ecosystems, which reduce the need for the importation of food (thus emissions), helps “fill” nutritional food deserts, and based on prior experience, e.g., Victory Gardens during WWII , there’s a high potential for this to happen.
2. Eco-Engineered Ecosystems provide natural solutions to stormwater runoff, air pollution, and the recycling of waste.
3. Amenity Ecosystems, which are extremely important for human quality of life and health and outdoor educational opportunities.

These ecosystem services help reduce the “footprint” of cities and if designed appropriately can greatly enhance a city’s resilience to climate change — not to mention enhancing the quality of life of urban inhabitants!

Like with all ecosystems, all of the above is made possible by “supporting” ecosystem services such as nutrient cycling, water storage, and carbon storage, much of which is made possible by soils. Thus, in an urban context, I like to think of soils as the “brown infrastructure” of our cities and towns.

2. We have a lot to learn about urban soil!

Pedology is a branch of soil science focused on the formation of soil, its characteristics, and classification. H. Jenny in 1941 (later refined in 1961) proposed the five factors of soil formation, which enabled pedologists to develop concepts for soil formation in the field, for mapping, and classification (see above graphic). Note, at the time, Jenny considered humans with all other organisms. It was not until the 1990s that a sixth factor, the “a” for anthropogenic, was suggested due to the scale of human modifications to soil formation — big effects in a relatively short period of time (Amundson and Jenny 1991; Pouyat 1991). Thus, in pedology the fact that humans play an important role in soil formation is a relatively new idea! The same can be said for the field of ecology, which only recently has embraced urban ecology as a sub-discipline.

With the ushering in of a “new paradigm” in pedology, soil scientists began to investigate soils in highly impacted ecosystems. In fact, the term “urban soil” was used for the first time in the scientific literature in 1964 by a Russian soil scientist L.T. Zemlyanitskiy, while the discipline of soil science was recognized in Europe since the mid to late nineteenth century. Even with the acknowledgment of urban soils as early as 1964, urban soil science did not catch on until the 1990s (when, by the way, I conducted my dissertation research!). At that time, only a handful of scientific papers were published on urban soils. However, by the end of that decade things began to exponentially pick up, so much so that by 2017, more than 1500 papers were published on urban soils! Before you think that number was impressive, however, compare that output to over 40,000 articles being published per year in soil science. And today, we continue to be on a steep learning curve, which means we have a lot more to learn! That’s okay, since in my view makes urban soil research even more exciting!

3. Is there such a thing as a typical urban soil?

The photographs above represent what most people think of when they think of urban soil (which, by the way was the primary focus of early urban soil research). Urban soils are thought of as being highly disturbed, sealed by impervious surfaces, or highly maintained as in a typical turf grass lawn. But, let us consider some data to see if these views of urban soil are true.

If we look at soil data collected in Baltimore, MD, as part of the Baltimore Ecosystem Study (BES), one of two Long Term Ecological Research (LTER) sites in the United States (the other is the Central Arizona Phoenix (CAP) LTER, which has done outstanding research on urban soils as well), we find that there may not be a typical soil type in urban areas (see Pouyat et al. 2007). We sampled almost 200 plots, which were randomly selected based on land use and cover, such as lawns, vacant lots, and woodland areas. Soils were sampled to a depth of 0–10 cms and measured for 20 soil characteristics and we were surprised by what we found! First, there was a very wide range in measurements for all characteristics measured. As an example, pH ranged from 3.3 (very acidic) to 7.6 (slightly basic). Second, the mean (or average) for each characteristic came out to be well within the range of horticultural recommendations for soil tests. In other words, the average soil sampled in the City of Baltimore had the potential to support very productive plants. For example, mean phosphorus (P) concentrations were 527 mg kg-1 (or parts per million, ppm), which is very high when compared to the suggested range of 30–60 ppm.

In a nutshell, these results suggest that there were a very wide range of measurements found in Baltimore and that range is much higher than one would expect if there were a “typical urban soil”. Additionally, the average measured characteristic fell well within, or even exceeded, the range required by most horticultural plants.

Another example can be found in an article by Ramirez et al. (2014), but this time using soil biodiversity as a measure, but in a much smaller area than a major city like Baltimore. In their study, DNA profiles of microbes (bacteria and archaea) inhabiting the soils of Central Park, N.Y. (840 acres) were compared to global profiles. As you can see in the above figure, where phylotype richness (a measure of biodiversity) increases as the number of sites are added, the difference between the Central Park and Global curves is minimal.

This is truly an amazing result. How can this be? A possible explanation can be found in soil pH data that was collected for Central Park in the late 1980s and early 1990s. Soil microbes are very responsive to differences in soil pH. The range of the data collected over a 15 year period shows that soil pH in the park ranged from being very acidic (pH 3.0 or less) to very alkaline (pH of 8 or more) — a range that would not be expected to be found in an 850 acre park, let alone a forest or grassland.

So, there you have it, Part 1 of what I have learned about urban soils in over 30 years of observation and research. Look for Part 2 coming soon!

References:

Amundson, R., and H. Jenny. 1991. The place of humans in the state factor theory of ecosystems and their soils. Soil Sci. 151:99–109.

Jenny, H. 1941. Factors of soil formation. McGraw-Hill, New York.

Jenny, H. 1961. Derivation of state factor equations of soils and ecosystems. Soil Sci. Soc. Am. Proc. 25:385–388.

Newman, P.W.G. 1999. Sustainability and cities: extending the metabolism model. Landscape and Urban Planning 44: 219–226.

Pouyat, R.V. 1991. The urban–rural gradient: An opportunity to better understand human impacts on forest soils. Proc. of the Society of American Foresters, 1990 Annual Conv., Washington, DC. Soc. of Am. Foresters, Washington, DC.

Pouyat, R.V., I.D. Yesilonis, J. Russell-Anelli, and N.K. Neerchal. 2007a. Soil chemical and physical properties that differentiate urban land-use and cover types. Soil Sci. Soc. Am. J. 71:1010–1019.

Ramirez, K. S. et al. Biogeographic patterns in below-ground diversity in New York City’s Central Park are similar to those observed globally. Proc. R. Soc. B 281, 20141988 (2014).

United Nations (2015) World population prospects: the 2015 revision, key findings and advance tables. Working Paper ESA/P/WP.241. United Nations, Department of Economic and Social Affairs, Population Division, New York

Zemlyanitskiy, L.T. 1963. Characteristics of the soils in the cities. Sov. Soil Sci. 5:468–475.