6.3 Primary Succession
6.4 Secondary Succession
6.5 Soil Biodiversity
6_Soil-Organic-Matter
Soil Organic Matter
Overview
Title image "Organic Matter" by Wpsopo is license CC BY SA 3.0.
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Introduction
Lesson Objectives
Discuss the importance of soil organic matter (SOM) and biological community of soil.
Explain the process of primary and secondary ecological succession.
Explain the significance of soil biodiversity and organic matter.
Key Terms
biological community - two or more different plant or animal species that occupy the same geographical area at the same time
climax community - a biological community that has reached a stable ecosystem through the process of ecological succession
humus - dark organic matter formed by decomposed plant and animal matter.
mineral soil - type of soil that is formed from the weathering of rocks and inorganic material; composed primarily of sand, silt, and clay
organic soil - type of soil that is formed from sedimentation; composed primarily of organic material
primary succession - newly exposed or newly formed rock is colonized by living things for the first time
secondary succession - an area previously occupied by living things is disturbed—disrupted—then recolonized following the disturbance
soil biodiversity - the variability among living organisms from plants, bacteria, fungi, and animals
soil organic matter - the living component of soil consisting of plant or animal tissues in various stages of decomposition
succession - a series of progressive changes in the composition of an ecological community over time
tilth - condition of prepared soil
Introduction
Looking at a landscape with a complex, diverse community of plants and animals—such as a forest—can prompt thoughts about how it came to be. Once upon a time, that land must have been empty rock, yet today, it supports a rich ecological community consisting populations of different species that live together and interact with one another. Odds are, that didn't happen overnight!
Ecologists have a strong interest in understanding how communities form and change over time. In fact, they have spent a lot of time observing how complex communities, like forests, arise from empty land or bare rock. They study, for example, sites where volcanic eruptions, glacier retreats, or wildfires have taken place—where these events have cleared land or exposed rock.
Succession
In studying these sites over time, ecologists have seen gradual processes of change in ecological communities. In many cases, a community arising in a disturbed area will go through a series of shifts in composition, often over the course of many years. Over periods of years or decades, the plants that grow in any given place change. New species take the place of those that came before. This process is called plant succession or, more broadly, ecological succession—because as the plants change so do the microorganisms and animals. Ecological succession is a series of progressive changes in the species that make up a biological community over time. Ecologists usually identify two types of succession, which differ in their starting points:
- In primary succession, newly exposed or newly formed rock is colonized by living things for the first time.
- In secondary succession, an area that was previously occupied by living things is disturbed, then re-colonized following the disturbance.
Primary Succession
Primary succession occurs when new land is formed or bare rock is exposed, providing a habitat that can be colonized for the first time.
For example, primary succession may take place following the eruption of volcanoes, such as those on the Big Island of Hawaii. As lava flows into the ocean, new rock is formed. On the Big Island, approximately 32 acres of land are added each year. What happens to this land during primary succession?
First, weathering and other natural forces break down the substrate, which is rock, enough for the establishment of certain hearty plants and lichens with few soil requirements; these are known as pioneer species (see image below). These species help to further break down the mineral-rich lava into soil where other, less hardy species can grow and eventually replace the pioneer species. In addition, as these early species grow and die, they add to an ever-growing layer of decomposing organic material and contribute to soil formation.
This process repeats multiple times during succession. At each stage, new species move into an area, often due to changes in the environment made by the preceding species, and these new species may replace their predecessors. At some point, the community may reach a relatively stable state and stop changing in composition. However, it's unclear if there is always—or even usually—a stable endpoint to succession, as we'll discuss later.
Access for free at https://openstax.org/books/biology-2e/pages/45-6-community-ecology
Secondary Succession
In secondary succession, a previously occupied area is re-colonized following a disturbance that kills much or all of its community.
A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire. Wildfires will burn most vegetation and kill animals unable to flee the area. Their nutrients, however, are returned to the ground in the form of ash. Since a disturbed area already has nutrient-rich soil, it can be recolonized much more quickly than the bare rock of primary succession.
Before a fire, the vegetation of an oak and hickory forest would have been dominated by tall trees. Their height would have helped them acquire solar energy, while also shading the ground and other low-lying species. After the fire, however, these trees do not spring right back up. Instead, the first plants to grow back are usually annual plants—plants that live a single year—followed within a few years by quickly growing and spreading grasses. The early colonizers can be classified as pioneer species, as they are in primary succession.
Over many years, due at least in part to changes in the environment caused by the growth of grasses and other species, shrubs will emerge; these are usually followed by small pine, oak, and hickory trees. Eventually, barring further disturbances, the oak and hickory trees will become dominant and form a dense canopy, returning the community to its original state—its pre-fire composition. This process of succession takes about 150 years. This final stage is known as a climax community, which is a biological community that has reached a stable ecosystem through the process of ecological succession.
Access for free at https://openstax.org/books/biology-2e/pages/45-6-community-ecology
Soil Biodiversity
The unsung hero of forests is the soil from which the trees and plants grow. Soil biodiversity reflects the mix of living organisms in the soil. These organisms interact with one another and with plants and small animals forming a web of biological activity. Soil is by far the most biologically diverse part of Earth. The soil food web includes beetles, springtails, mites, worms, spiders, ants, nematodes, fungi, bacteria, and other organisms. These organisms improve the entry and storage of water, resistance to erosion, plant nutrition, and break down of organic matter. A wide variety of organisms provides checks and balances to the soil food web through population control, mobility, and survival from season to season. On the basis of organic matter content, soils are characterized as mineral soil or organic soil. Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process. At any given time, it consists of a range of materials from the intact original tissues of plants and animals to the substantially decomposed mixture of materials known as humus.
Organic matter within the soil serves several functions. Organic matter is critical for soil health and for soil productivity by providing energy for soil microbes, supporting and stabilizing soil structure, increasing water storage, storing and supplying nutrients, building soil biodiversity, storing carbon, and buffering chemical behavior such as pH. From a practical agricultural standpoint, it is important for two main reasons: (i) as a “revolving nutrient fund”; and (ii) as an agent to improve soil structure, maintain tilth and minimize erosion.
As a revolving nutrient fund, organic matter serves two main functions:
- As soil organic matter is derived mainly from plant residues, it contains all of the essential plant nutrients. Therefore, accumulated organic matter is a storehouse of plant nutrients.
- The stable organic fraction (humus) adsorbs and holds nutrients in a plant-available form.
Organic matter releases nutrients in a plant-available form upon decomposition. In order to maintain this nutrient cycling system, the rate of organic matter addition from crop residues, manure and any other sources must equal the rate of decomposition. In addition, the rate of organic matter addition must also consider the rate of uptake by plants and losses by leaching and erosion.
Where the rate of addition is less than the rate of decomposition, soil organic matter declines. Conversely, where the rate of addition is higher than the rate of decomposition, soil organic matter increases. The term steady state describes a condition where the rate of addition is equal to the rate of decomposition.
An important part of soils’ biological communities are bacteria. Bacteria are tiny, one-celled organisms—generally 4/100,000 of an inch wide (1 µm) and somewhat longer in length. What bacteria lack in size, they make up for in numbers. A teaspoon of productive soil generally contains between 100 million and 1 billion bacteria. In terms of mass, that is as much as two cows per acre.
Most bacteria are decomposers that consume simple carbon compounds, such as root secretions and fresh plant litter. By this process, bacteria convert energy from soil organic matter into forms useful to the rest of the organisms in the soil food web. A number of decomposers can break down pesticides and pollutants in soil. Decomposers are especially important in immobilizing, or retaining, nutrients in their cells; thus, they prevent the loss of nutrients, such as nitrogen, from the rooting zone.
Bacteria alter the soil environment to the extent that the soil environment will favor certain plant communities over others, playing a large role in succession. Before plants can become established on fresh sediments, the bacterial community must establish, starting with photosynthetic bacteria. These fix atmospheric nitrogen and carbon, produce organic matter, and immobilize enough nitrogen and other nutrients to initiate nitrogen cycling processes in the young soil. Then, early successional plant species can grow. As the plant community is established, different types of organic matter enter the soil and change the type of food available to bacteria. In turn, the altered bacterial community changes soil structure and the environment for plants. Some researchers think it may be possible to control the plant species in a place by managing the soil bacteria community.
Certain strains of the soil bacteria Pseudomonas fluorescens have anti-fungal activity that inhibits some plant pathogens. P. fluorescens and other Pseudomonas and Xanthomonas species can increase plant growth in several ways. They may produce a compound that inhibits the growth of pathogens or reduces invasion of the plant by a pathogen. They may also produce compounds (growth factors) that directly increase plant growth.
These plant growth-enhancing bacteria occur naturally in soils, but not always in high enough numbers to have a dramatic effect. In the future, farmers may be able to inoculate seeds with anti-fungal bacteria, such as P. fluorescens, to ensure that the bacteria reduce pathogens around the seed and root of the crop.
Dig Deeper
Attributions
"Community Ecology" by Bear, et. al. is licensed under CC BY-NC-SA 4.0.
"Ecological Succession" by Khan Academy is licensed under CC BY-NC-SA 4.0.
"FAO Soils Portal: Soil Biodiversity" by the Food and Agriculture Organization of the United Nations is copyrighted and used with permission.
Organic Matter by Victorian Resources Online is licensed under CC BY 4.0.
"Plant Succession" by the United States National Park Service is in the Public Domain.
"Soil Bacteria" by Elaine R. Ingham, United States Department of Agriculture Natural Resources Conservation Service, is in the Public Domain.
"Soil Biodiversity" by the United States Department of Agriculture Natural Resources Conservation Service, is in the Public Domain.
"The Importance of Soil Organic Matter: Chapter 1" by the Food and Agriculture Organization of the United Nations is copyrighted and used with permission.