16.1: Natural and Working Lands in the Terrestrial Carbon Cycle

Natural ecosystems and working lands (those used for agriculture and forestry) play an important role in the terrestrial carbon cycle. Forests, grasslands, crop fields, wetlands—in fact, all ecosystems with plants and soils—exchange carbon dioxide (CO₂) with the atmosphere. Approximately 86% of the Earth’s land surface is rural natural or working land, housing approximately 45% of the world’s population. Although most of the natural and working lands can be found in rural areas, they are not limited to rural areas. And urban, suburban, and peri-urban areas—even residential lawns and gardens—cycle CO₂. Plants take up CO₂ from the atmosphere during the process of photosynthesis. About half of the CO₂ that plants take up gets released back to the atmosphere via plant respiration, together with oxygen and water vapor. The remaining CO₂ absorbed by plants is converted into plant tissues such as stems, roots, leaves, fruits, and flowers. From CO₂, plants get the carbon they use as a major building block to make and sustain their tissues. Plants are at the base of the food chain and thus provide energy, in the form of carbon, together with nutrients from the soil to most of the rest of the organisms on Earth. Plants feed the world.

How do plants feed the world? Some organisms (for example, herbivores) harvest and eat live plant parts, but most of the carbon and nutrients in plants go to feed microorganisms. Plants regularly slough tissues, akin to animals shedding hair or skin. When plants shed their tissues, or when whole plants die, the tissues get deposited on or in the soil as plant litter. Some of this plant litter subsequently becomes food for microorganisms living in the soil (for example, bacteria and fungi). Like plants, microorganisms play a critical role in the local, regional, and global cycles of carbon. Soil microorganisms use enzymes to help break down plant litter during the process of decomposition. The carbon captured by microorganisms during decomposition is used for energy and to build microbial bodies. Microorganisms also respire CO₂, thus completing the carbon cycle by returning some of the carbon initially captured by plants via photosynthesis back to the atmosphere (Figure 16.1.1).

Figure 16.1.1 The global carbon cycle. Carbon cycles on the scale of seconds to centuries among the atmosphere, plants, animals, soils, and soil microbes. Geologic reserves cycle carbon on the scale of centuries to millennia. The exception is when humans mine carbon from geologic reserves in the form of coal, gas, oil, and other fossil fuels. The burning of fossil fuels dramatically accelerates the release of geologic carbon to the atmosphere. Adapted from UCAR Center for Science Education.

Plants and microorganisms live in every biome on the Earth’s surface, and their activity can be clearly seen at a global scale in the graph of atmospheric CO₂ data from Mauna Loa volcano on the island of Hawaii (Figure 16.1.2). There are two prominent features of this graph. The first feature is the steep rise in atmospheric CO₂ concentrations from the start of the record in 1958 to present. The long-term rise in atmospheric CO₂ concentrations is due to increasing emissions from human activity. The second prominent feature in the graph is the annual “wiggle” in the atmospheric CO₂ data. The wiggle occurs as a result of the breathing of the biosphere. Every time the line goes down, CO₂ uptake by plants has exceeded the release of CO₂ by microbial respiration. This in turn results in the net removal of carbon from the atmosphere and its storage in the biosphere. This annual downturn in CO₂ is a natural process driven by plant uptake of CO₂ during photosynthesis at a global scale. The global minimum of atmospheric CO₂ concentration generally occurs in September or October, which in the Northern Hemisphere occur in summer and early fall. The Northern Hemisphere has more land mass than the Southern Hemisphere and thus more land plants and associated carbon uptake. For this reason, the Northern Hemisphere growing season is the main driver of the annual low point in atmospheric CO₂ concentration.

Figure 16.1.2 Atmospheric carbon dioxide (CO₂) as measured from the top of Mauna Loa Volcano, on the Island of Hawaii. The upward trend in the data results from human activities that release CO₂ to the atmosphere—the use of fossil fuels, biomass burning, and deforestation, among others. The annual troughs and peaks result from photosynthesis by plants and respiration, primarily by microorganisms. Data from Pieter Tans, NOAA/ESRL; and Ralph Keeling, Scripps Institution of Oceanography.

The annual peak in atmospheric CO₂ concentration occurs when the respiration of microorganisms that live predominantly in the soil exceeds plant uptake of CO₂ from the atmosphere. Soil microorganisms in many parts of the world are active in decomposition all year long and respond to periods of plant litter deposition during the plant growing season and at the end of the growing season when plants drop their leaves or completely senesce. During the growing season, plant uptake of CO₂ exceeds microbial respiration. When plant activity slows or comes to a halt during the Northern Hemisphere winter and early spring, microbial respiration of CO₂ exceeds plant uptake. This causes the global atmospheric concentration of CO₂ to increase to a maximum, generally in May.

Figure 16.1.3 Global carbon stocks. One petagram (Pg) equals 1015 grams. Deep rock reserves are the largest pool of carbon on Earth, with oceans storing the second largest amount. Soils are the next largest pool, followed by the atmosphere and plants. There is considerable uncertainty in the exact size of the Earth’s carbon stocks, as it is impossible to accurately measure the full pool directly. Estimates are derived from strategic direct measurements, remote sensing (satellite, airplane), and computer models. Data from University of New Hampshire 2008.

Plants and soils don’t just exchange carbon with the atmosphere; they are also a reservoir of stored carbon, together with the atmosphere, oceans, freshwaters, and geologic reserves (Figure 16.1.3). The reservoirs where carbon is stored are also referred to as pools or stocks. On the land surface, soils are the largest reservoir of carbon. The total amount of carbon in soils is not well understood, because it is difficult to estimate the amount of carbon below the top meter, even though scientists know that deep soils contain carbon. Plants represent a smaller carbon stock but play a key role as conduits of carbon capture from the atmosphere. The interaction among the atmosphere, plants, and soil organic carbon pools can significantly affect the rate of climate change. The more carbon that is stored in soils and plants, the less that is stored in the atmosphere where it can absorb heat and warm the climate. Below, we will explore how carbon is cycled and is stored in soils and plants and how ecosystems can be managed to increase carbon storage to help slow climate change.