16.2: Soils, Organic Matter, and Greenhouse Gas Dynamics

Why should we care about soils?

Figure 16.2.1 Soil is the basis for life on Earth. Reproduced from © Okea/ Fotolia.

Soils are literally and figuratively the basis for life on Earth (Figure 16.2.1). Soils provide a substrate for plants to sink their roots into. Soils also provide the nutrients that plants need to survive, and by extension they provide the nutrients for most other living things on Earth. Soils store water. They also filter water by removing minerals and contaminants.

Figure 16.2.2 Map of soil degradation globally. The majority of managed lands experience moderate to very severe degradation including soil loss from wind and water erosion, toxicity from pollutants, low nutrient content from over harvesting, and compaction from overgrazing and the use of heavy machinery. Reproduced from the FAO.

In this way, healthy watersheds with abundant and intact soil resources help provide clean water to humans and other organisms. Some groups of soil microbes can consume and break down toxic chemicals, rendering them less dangerous to human and ecosystem health. The sustainability and productivity of agriculture depends on soils. However, much of the world’s soils have suffered damage and degradation from poor management (Figure 16.2.2). Erosion, overuse, compaction, and contamination from chemicals threaten the health of soils.

What is soil?

Soils are made up of a complex mixture of minerals and organic matter. The minerals come from rocks that are broken down through the process of weathering. The organic matter in soils comes from plants, microorganisms, and animals. Soils are teeming with life. There are more microorganisms in a teaspoon of soil than there are people on Earth. In addition to live organisms, soils are the repository of microbial by-products and dead microbial, plant, and animal tissues. The organic matter derived from live and dead tissues thus makes up an important part of the soil. As all these tissues contain carbon, storing organic matter in soils is the vehicle for storing carbon in soils.

Soils and greenhouse gas emissions

Soils exchange gases with the atmosphere. These gases include CO₂, methane, and nitrous oxide, the big three greenhouse gases. Microorganisms, roots, insects, and other soil animals that breathe oxygen (that is, aerobic organisms) release CO₂ through the process of respiration. Some soil microorganisms release methane to the atmosphere through anaerobic respiration (that is, respiration in the absence of oxygen), while still other soil microorganisms consume methane from the atmosphere and respire CO₂. Soils are the largest natural source of nitrous oxide, which is produced predominantly by yet another group of soil microorganisms. Both methane and nitrous oxide are very potent greenhouse gases with more warming power than CO₂. Thus, even relatively low methane and nitrous oxide emissions from soils can have a big impact on climate.

Microbes are not the only source of greenhouse gases in soils. In addition to biological sources of greenhouse gases, soils can foster the conditions needed for nonbiological (for example, geochemical) reactions that produce greenhouse gases. Geochemical greenhouse gas production in soils is thought to play a less important role than microbiological processes at a global scale, thus in this chapter we will focus on the microbial greenhouse gas emissions and ways to reduce these emissions.

All ecosystems exhibit some greenhouse gas emissions. The production of greenhouse gases is a by-product of natural microbial processes and an indicator of life. However, some agricultural and forestry activities can increase greenhouse gas emissions from soils. For example, the use of nitrogen fertilizers on agricultural soils can stimulate the production and emission of nitrous oxide. Irrigation, especially flood irrigation, can create the anaerobic conditions needed for both methane and nitrous oxide production and emissions. Plowing and tillage can release stored carbon and nitrogen, making it accessible to microbes that release CO₂, methane, and nitrous oxide to the atmosphere. Agriculture accounts for more than half of the nitrous oxide and methane emissions globally and approximately 25% of the total greenhouse gas emissions worldwide. Forestry and other land uses result in similar levels of emissions.

Organic matter versus organic carbon versus inorganic carbon

The organic matter in soils contains carbon that was originally stored in the atmosphere and subsequently captured by plants via photosynthesis. There is a wide range of soil organic carbon (SOC) contents in soils globally. This is because both inputs and outputs of SOC are sensitive to a suite of environmental factors:

• Climate and weather: temperature, precipitation, storms, drought
• Geology: rock type and weathering rate
• Soil age: landscape and landform stability
• Biology: vegetation, microorganisms, and animals

Carbon capture via photosynthesis differs among ecosystems. The plants of temperate and tropical wetlands and tropical rain forests are among the most productive globally, meaning that they capture the most carbon annually. Wetlands are productive where water and dissolved and suspended nutrients are constantly flowing and providing a regular renewal of resources. Tropical rain forests are productive because their location near the equator promises abundant sunlight and near-constant warm, moist conditions that favor continuous plant growth throughout the year.

The pool of SOC also differs among ecosystems. The amount of organic carbon storage in soils is a function of the difference between carbon inputs and carbon losses. Where organic carbon losses via decomposition and leaching (or other physical removal) equal organic carbon inputs to the soil, the size of the SOC pool remains the same. Where decomposition rates are lower than carbon inputs, SOC can accumulate. Northern peatlands store the most SOC among the world’s terrestrial biomes. Although plant growth and associated carbon uptake is low compared with ecosystems like wetlands or tropical forests, the rate of SOC loss is even slower as a result of cold temperatures and unfavorable conditions for microbial decomposition (for example, anoxia). This facilitates the gradual buildup of large quantities of SOC.

So far, we have focused on organic carbon pools. Soil also contains inorganic carbon. Inorganic carbon is primarily made up of calcium and magnesium carbonates and enters soils through the weathering of carbonate rocks. Soils contain about 1,000 petagrams (Pg; 1 Pg = 1015 g) of inorganic carbon (equivalent to 1,000 gigatons) in the top meter, globally, mostly concentrated in deserts and semiarid regions. Increasing atmospheric CO₂ associated with climate change and the increase in soil acidity from certain land uses can result in the loss of inorganic carbon in soils.

Soil organic carbon sequestration

To review, when the rate of inputs of SOC exceeds the rate of losses, SOC accumulates. Another term for this is SOC sequestration. Soils have tremendous capacity to sequester SOC. It has been estimated that soils store between 1,500 and 3,500 petagrams of organic carbon. The low estimate of SOC is double the amount of carbon stored in the atmosphere (750 petagrams) and almost three times the carbon stored in vegetation globally (560 petagrams) (Figure 16.1.3). We do not have precise estimates of SOC pools, because soils vary greatly from place to place and are heterogeneous, deep, and largely hidden from view. This makes measuring the total amount of SOC challenging at a global scale.

With so many microorganisms in soils, it is a wonder that any SOC can escape microbial decomposition and become sequestered. However, there are several ways that organic matter and its associated carbon can get stored in soils:

• Organic carbon can chemically react with soil minerals or other organic compounds to form strong bonds. These bonds can be difficult for microbes to break, leading to the persistence of organic carbon in soils.
• Organic carbon can accumulate if it is deposited deep down in the soil. Microbial activity is greatest near the surface of soils (top 10 to 30 centimeters) and declines with depth. This is because most of the carbon and nutrients that microbes need to survive is deposited on or near the surface by plants. However, roots can penetrate to deep soil depths and inject organic matter into soils as they slough tissues. In some seasonally dry environments, like parts of the Amazon basin, roots extend down almost 20 meters. Water can also transport organic carbon into deep soils as it percolates down. Burrowing animals and insects are good agents of organic carbon transport and deposition into deep soils.
• Organic carbon can persist in soils if conditions are more favorable for plant growth than for microbial decomposition or physical losses. The organic-rich northern peatlands discussed above are an example of where SOC accumulates because microbial decomposition is inhibited by the lack of oxygen in soils and cold temperatures.
• Organic carbon can persist in soils if the organic matter it is derived from is chemically or physically difficult for microbes to break down. This happens with materials like compost, wood, and waxy tissues or in cases where the microbial community lacks the enzymes to break down specific types of chemical compounds.