Investigations in Urban Soils - Baltimore Ecosystem Study

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(bacteria, fungi, algae), and invertebrates (protozoa, worms, insects). The dark color we associate with healthy topsoil comes from decaying organic material, often called humus. There is space between the mineral and organic particles. Scientists call these spaces pores, and they are filled with varying amounts of air and water (Figure 3), in which nutrients are dissolved. The ideal soil for optimal plant growth is composed of 45% mineral, 5% organic matter, 25% air and 25% water. In compacted soil, such as urban soil, the pore space is greatly reduced (Figure 4). Soil Functions Soil is incredible. The trees and other plants we study and cherish could not survive without soil. Without trees and other plants we would not have shade, food, timber, energy, paper, rubber, and many other commodities, and many animals would not have a natural habitat. Soil supports our buildings and roads, filters and stores water, and absorbs our many pollutants including exhaust and fertilizers. Soils even help absorb noise! The humble soil is home to more species of living things than the famous rain forests – and these nearly invisible creatures recycle organic waste so that plants and other organisms can survive. Without soil organisms, we would be up to our ears in dead plants, animals and microbes. With “help” from humans, soil can also become contaminated with pollutants, compacted, and eroded (Figure 5). Then, soil will be less able to support soil organisms and plant life and may endanger human health. Soil itself becomes a pollutant when it gets blown or washed away into streams and estuaries (erosion). Linking Structure and Function in Soil Proper functioning of the soil depends on its healthy structure. Soil texture (mixture of sand/silt/clay) affects the amount of water soil can hold, the movement of air and nutrients through the soil, and resistance to erosion. Humans can take precautions to avoid compacting soil, which destroys the pore space and limits the oxygen supply to roots and soil organisms. Frequent soil disturbance also affects the distribution of plants. While many native plants and animals may not be used to recovering from these changes, exotic species that are more adapted to urban environments often flourish in human altered soils, soon out competing natives. Soil as a Habitat The soil is a great place to live! The temperature and moisture under the soil does not fluctuate as much as it does above-ground over the course of a day or a year. The vast number of pore spaces in soil provides great places for tiny organisms to live and find food (Figure 6). The “community” of organisms that flourishes there is connected to each other in various ways. Microscopic bacteria, fungi, and protozoa digest leaf litter, softening it for other decomposers. Earthworms, mites, millipedes, and ants are a few of the larger decomposers that shred and mix the organic material. They Draft 2 - (9.28.04) Baltimore Ecosystem Study – Investigations in Urban Soils page 3
page 4 Baltimore Ecosystem Study Draft 2 - (9.28.04) Soils Urban in Investigations are, in turn, eaten by predacious insects (spiders, centipedes, beetles), which are food for shrews, moles, birds and others. Many of these organisms spend their entire lives in the soil; others spend a phase of metamorphosis there before becoming surfacedwelling adults; and others simply create nests or burrows in the soil. Soil Organisms Soil organisms provide important services to the ecosystem: (1) Decomposition of organic matter – 90% is carried out by microbes. (2) Nutrient cycling – microbes do the job, but larger organisms determine the rate. (3) Bioturbation – mixing of soil and creation of pores. Microbes Microbes are microscopic organisms such as fungi and bacteria that digest organic material outside their bodies and absorb the nutrients. This process softens the material and releases nutrients, such as calcium, nitrogen, and phosphorus, into the soil where plants can access them. Microbe activity is central to recycling nutrients between the air, water, and organisms in an ecosystem. Fungi are slow decomposers that dominate the leaf litter in rural forests. Active fungal communities can take up to five years to decompose a year’s worth of leaf litter. Although many fungi produce visible mushrooms in the late summer and fall, chemical decomposition is carried out by their network of microscopic filaments, called hyphae. The by-product of fungal decomposition is ammonium, a great source of nitrogen for plants. Fungi proliferate best in thick layers of moist, undisturbed leaf litter. These conditions are becoming rare to non-existent in urban Baltimore. Nitrogen Cycle in Soils All living things need nitrogen. It is used to make the basic units of all life: DNA, amino acids and proteins. However, most nitrogen exists as a gas in the atmosphere, which very few living things can use. There are a small number of bacteria that are able to transform nitrogen gas into ammonia, through a process called nitrogen fixation. These bacteria use the ammonia for their own growth, or they excrete it into the surrounding soil or water. Once in the soil, ammonia can either be absorbed by plants or it can be used by highly specialized bacteria which transform it to nitrate. Plants can absorb nitrate as well as ammonia as their source of nitrogen for growth. Animals obtain their nitrogen by eating plants, or other animals. Once an organism dies, the nitrogen in its proteins is broken down to ammonia again by decomposing bacteria, transformed into nitrate by nitrifying bacteria, and absorbed again by other plants. In this way, nitrogen in the soil and water is recycled over and over. At every turn of the cycle, nitrogen fixing bacteria add a little nitrogen, while some nitrogen is released back into the atmosphere by another group of bacteria in a process called denitrification (Figure 7). These different processes occur in different places in the landscape. Nitrogen gas is transformed into ammonia at the surface of soils and near the roots of plants. While ammonia stays bound to soil particles, nitrate in excess of plant demand can be leached out of the soil because it is water-soluble. Once nitrate is leached from the soil it can flow into groundwater and accumulate in lakes, wetlands, rivers and estuaries. There, in the waterlogged soils, denitrifying bacteria transform some of the nitrate back into nitrogen gas and release it into the atmosphere (Figure 8). The nitrogen cycle of a landscape is self-adjusting. The activity of all of these microorganisms in the soil and water maintains a balance between the amount of ammonia and the amount of nitrate in the system, and between the amount of nitrogen fixed in upland areas, and the amount released back into the atmosphere in wetlands. If there is an excess of one form of nitrogen, there are microorganisms that will quickly transform it into another form, thereby retaining a balance. However, this balance is disturbed if the amount of nitrogen input (say, from fertilizer) overloads the system. This can result in the loss of nitrogen from the overloaded ecosystem and its overabundance in receiving waters. – Investigations in Urban Soils tudy
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Investigations in Urban Soils - Baltimore Ecosystem Study

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