Suburban vs. Urban Soil Nutrient Levels in Milwaukee, Wisconsin
Megan Dreger, Joy Preis
11/ 7/ 10
Our study looked into nutrient levels in soils from naturally-occurring forested areas present in urban and suburban parks of Milwaukee, Wisconsin. Soil samples were collected from the parks and tested for pH, phosphorous, nitrogen, and potassium levels with a soil analysis kit. Phosphorus was the only nutrient we found a significant difference with between the urban and suburban park soil samples (P = 0.0001656). Soil nitrogen, potassium, and pH levels were not significantly different between the two park categories.
Urban ecology is an expanding field of research as urban populations around the globe continue to grow and, consequently, surrounding rural areas are converted to urban landscape to provide needed space. It is predicted that over 50% of people living in Africa and approximately 87% of those living in North America will be in urban spaces by the year 2030 (Lorenz & Lal, 2009). Many urban materials and processes create a distinct environment and conditions that influence local and regional atmospheric conditions as well as local soil characteristics (Lorenz & Lal, 2009). In an urban area, natural soils may be sealed over and artificial soils may be put in place leading to changes in soil structure and distribution of soil nutrients (Lorenz & Lal, 2009).
Urban soil is directly and indirectly subjected to anthropogenic disturbances such as combustion processes of cars and industry, application of fertilizer, construction of impervious surfaces, pollution from storm water runoff, and soil mixing, to name a few (Lorenz & Lal, 2009). These disturbances affect the organic matter as well as the carbon, phosphorous, and nitrogen retention in urban soil (Lorenz & Lal, 2009; Zhu et al, 2004). A new soil classification, Technosol, has even been introduced to account for the distinct properties that characterize soil frequently exposed to the urban atmosphere and technical disturbances (Lorenz & Lal, 2009). Technosols tend to contain waste, pavement, processed oil products, and other substances from urban infrastructure and human activity (Lorenz & Lal, 2009). The specific local, regional, and global ecosystem changes that follow these disturbances, especially when on-going, are largely unknown due to the rapid rate of urbanization and lack of research into how the process of urbanization influences an ecosystem on any scale (Carreiro & Tripler, 2005).
Similar disturbances are present in suburban areas, though presumably not as frequent or condensed in space, since the infrastructure and human population are usually more spread out. In rural areas, the disturbances common in the urban setting are least prevalent, and the landscape is commonly less densely populated than both urban and suburban areas. There is a great need for comparative research on the ecosystems in urban, suburban, and rural areas to determine the impact of urbanization on various ecosystem components, like soil, and to predict future conditions for the environment on smaller and larger scales based on predicted growth of urban areas worldwide (Carreiro & Tripler, 2005; MacDonald, 1997). The aim of this paper is to contribute some comparative researchon urban and suburban soil nutrient levels to the field of urban ecology, where it is greatly needed.
Our research focuses on the comparison of urban and suburban soil nutrients with multiple soil samples from naturally-occurring forested areas in urban and suburban parks of Milwaukee, Wisconsin. The samples were taken from naturally-occurring forested areas with the purpose of gathering soil that was most natural in respect to the area rather than collecting soil from areas that could have been treated and monitored by park grounds-keepers. We hypothesized that there would be a difference in soil nutrient levels between urban and suburban soil samples and predicted the suburban soil would have higher levels of the nutrients due to suburban soil less frequently experiencing the aforementioned common city disturbances: soil mixing, construction of impervious surfaces, etc.
Materials and Methods
On October 14-17, 2010, we went out and collected samples from urban and suburban parks of Milwaukee, Wisconsin. To be considered an urban park, the park location had to be specifically in the zip code of Milwaukee, while the suburban parks were those parks located in cities of Milwaukee County but not the zip code of Milwaukee itself. Eight urban and eight suburban parks were selected for collection of soil samples. Two soil samples were collected from each park to give a better representation of the park than one soil sample would provide yet remain from being too time-consuming in relation to collection, testing, and analysis. Therefore, a total of 32 soil samples were collected from 16 separate parks. The samples were from 0.15 m below the soil surface in naturally-occurring forest areas of the selected parks.
The soil samples were prepared by allowing soil to dry overnight, and then each sample was sifted to remove other organic matter (small rocks, root pieces, etc.) and have a uniform sample for testing. Next, all samples were tested using LaMotte Soil Test Kit Garden Guide Model EL/EM code 5679/5934. The first test we ran on each soil sample was for the pH level of the soil. The initial step was to add pH Indicator (5701) to the fourth line on a test tube (0755). Next, three 0.5 spoons (0698) of soil sample were added to the test tube and the test tube was then capped. The contents within the capped test tube were then gently mixed for one minute before being allowed to settle out for ten minutes in the test tube. The color of the solution after the ten minutes was matched with a reaction color found on the pH color Chart (1353) (see appendix A) and results were recorded (see appendix B) (LaMotte, n.d.).
The second test we ran on each sample was for phosphorus. The first step was adding phosphorus extracting solution (5704) to the sixth line of a new test tube (0755). The next step was adding three 0.5 spoons (0698) of the soil sample to the phosphorous extracting solution in the test tube. After capping the test tube, the contents were gently mixed for one minute before the cap was removed and the soil left to settle to the test tube bottom. The clear liquid extract above the settled soil was then transferred to a clean test tube (0755) up to the third line using a pipet (0364). Next, we added six drops of Phosphorus Indicator Reagent (5705) to the clear extract, capped the test tube, and mixed the contents for one minute. Then, the cap was removed from the test tube, and one Phosphorus test tablet (5706) was added and, subsequently, dissolved completely into the solution (LaMotte, n.d.). The resulting color of the extract solution was then matched to a bar on the Phosphorus color Chart (1372) (see appendix C) and results were recorded (see appendix B) (LaMotte, n.d.).
The next test we ran on each sample was for nitrogen. The first step was to add nitrogen extracting solution (5702) to the sixth line of a new test tube (0755), and then add two 0.5 spoons (0698) of soil sample to the test tube. The test tube was then capped and the contents were gently mixed for one minute. Following this, the test tube cap was removed and the soil left to settle out. Then, the clear extract resting atop the soil layer was transferred to a clean test tube (0755) up to the third line using a clean pipet (0364). Two 0.25 spoons (0695) of nitrogen indicator reagent (5703) were added to the extract before the tube was capped and the contents mixed again for one minute. The contents were left to develop for five minutes, and then the resulting color of the extract mixture was matched to a color category on the Nitrogen color Chart (1371) (see appendix D) and results were recorded (see appendix B) (LaMotte, n.d.).
The final test we ran on each sample was for potassium. The initial step was adding potassium extracting solution (5707) to the seventh line of a new test tube (0755). The second step was adding four 0.5 spoons (0698) of soil sample to the test tube with potassium extracting solution and capping the top. Next, the contents were mixed vigorously for one minute before uncapping the test tube and allowing the soil and solution to settle. The liquid extract above the soil at the bottom of the tube was transferred to a clean test tube (0755) up to the fifth line using a pipet (0364). The next step involved adding one potassium indicator tablet (5708) to the extracted liquid (LaMotte, n.d.). Then the test tube was capped and the contents within mixed until the tablet dissolved and a purplish color appeared (LaMotte, n.d.). Once the purplish color appeared, two drops at a time of potassium test solution (5709) were added to the mixture in the tube, swirling the test tube after each additional drop to mix the contents of the test tube. When the color changed from purplish to blue, using the potassium end point color chart (1352) as a guide, we stopped adding drops (see appendix E). Then, we recorded results (see appendix B) using the number of drops added to quantify the potassium level as higher or lower (LaMotte, n.d.).
Then we input our data into Microsoft Excel for Windows, 2007 version, where we assigned a given number to represent trace, low, medium, and high. Trace was given the value one. Low was given the value two. Medium was given the value three. High was given the value four. With the data we input into Microsoft Excel, we performed one tail, type two t-tests on the data sets for each nutrient tested and soil pH.
There was a not significant difference between the pH in soil from urban parks as compared to soil from suburban parks (Fig. 1, P= 1). After averaging the pH level results, the soil from urban parks had a neutral pH (Mean 7.38) and the soil from suburban parks had the same neutral pH (Mean 7.38).
Figure 1: Urban park soil pH in comparison to suburban park soil pH
There was a significant difference between the phosphorus level in soil from urban parks as compared to soil sampled from suburban parks (Fig. 2, P-Value = 1.6 X ). The soil from urban parks had a higher amount of phosphorus (Mean 2.8) whereas soil from suburban parks had a lower amount of phosphorus (Mean 1.6).
Figure 2: Urban park soil phosphorus level in comparison to suburban park soil phosphorus level
There was no significant difference between the nitrogen level in soil from urban parks as compared to the nitrogen level in soil from suburban parks (Fig. 2, P-Value = 0.14). The soil from suburban parks had a higher level of nitrogen (Mean 1.4) while soil from urban parks had a lower level of nitrogen (Mean 1.2).
Figure 3: Urban park soil nitrogen level in comparison to suburban park soil nitrogen level
Additionally, there was no significant difference between the potassium level in soil from urban parks as compared to soil from suburban parks (Fig. 2, P-Value = 0.68). The soil from suburban parks had a higher amount of potassium (Mean 3.3) whereas soil from urban parks had a lower amount of potassium (Mean 3.2).
Figure 4: Urban park soil potassium level in comparison to suburban park soil potassium level
Our hypothesis of a difference in nutrient level and specific prediction that the soil nutrient levels of phosphorus, nitrogen, and potassium would be higher in suburban park soil as compared to urban park soil was not supported by our results. There was significant difference between the phosphorus level for suburban and urban park soil, but there was not a significant difference between the nitrogen and potassium found in urban and suburban park soil. The pH level of the soils taken from the suburban and urban park areas was not significantly different either. Since phosphorus was the only soil nutrient out of the three tested that differed significantly in level between the urban and suburban parks, we concluded that our hypothesis was not supported. One nutrient with a significant difference measured would not provide enough support when the other two nutrients levels had not been significantly different. Additionally, the urban park soil had a greater level of phosphorus (Mean = 2.8) than the suburban park soil (Mean = 1.6), which goes against our prediction that suburban park soil would have higher levels of all the nutrients tested: phosphorus, nitrogen, and potassium.
The results of this study might have been different depending if we would have got samples deeper in the ground by obtaining a soil core sample from .381 m down, a standard depth for soil core samples. This greater depth would allow analysis of conditions farther away from the soil surface, which could provide nutrient level measurements that are more distinct between urban and suburban soil possibly due to differences in leaching, compaction, and amount of soil mixing taking place. Sampling soil from other locations that are nearer to urban and suburban infrastructure might also yield greater differences in soil nutrient levels because the soil from these sites would experience greater exposure to human and industrial activity.
This experiment could be improved by performing and analyzing the testing of soil samples for nutrient levels together to come to a consensus on what intensity of color constitutes what kind of level rating. Since we preformed our testing separately due to time constraints and for convenience, the ratings may not have been as consistent in nature due to individual and subjective judgments on color intensity. Also, the pH level was estimated by one researcher to be between whole number values (i.e. 7.5) with some samples while the other researcher recorded pH results of samples only with the whole number values present on the pH chart. The experiment would be more scientifically sound if the testing and analysis of the soil samples was performed together instead of separately to ensure consistency in analysis of test results.
Though our investigation did not find significant differences in soil nutrients between urban and suburban soil samples from naturally-forested areas of parks, many more studies will need to be conducted with urban and suburban soil to come closer to any overall conclusions on how soil nutrients are affected in these areas and why. More research also needs to be conducted on how biogeochemical cycles work in urban soil and if the growth in urban land use will affect global nutrient cycles (Lorenz & Lal, 2009). Comparative studies of soil also need to be done between urban areas and rural areas to get a better idea of the changes in the environment we can expect to see in coming years. Areas with urban-rural gradients may be especially useful for studying ecosystem responses to city disturbances and influencing future urban and other land use planning (Carreiro & Tripler, 2005). With urbanization on the rise around the world, studies of urban ecosystems and their differences in comparison to more natural ecosystems and landscape are critical to minimize harm to the environment and its biota.
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