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Storing Carbon in Soil: Why and How?
Charles W. Rice

Carbon is a primary element of all organic life forms on Earth. Carbon also is distributed in geologic material, oceans and the atmosphere. Concern has been mounting about the rapid buildup of carbon dioxide in the atmosphere — which is increasing by more than 3 billion tons per year. Industrialization and the burning of fossil fuels (coal, oil and natural gas) have accelerated this buildup. Carbon dioxide is a gas that absorbs heat, and thus contributes to the greenhouse effect.
 
The potential ramification of elevated atmospheric carbon dioxide on climate change makes it necessary to reduce carbon dioxide emissions — through increased energy efficiency and greater use of non-carbon energy — or to sequester carbon dioxide by injecting it into geologic formations and oceans or enhancing its uptake by terrestrial and aquatic ecosystems.
 
Terrestrial ecosystems, both plants and soils, provide an attractive mechanism for carbon sequestration because we can manage them. We can manage plant growth to increase plants’ capacity to uptake carbon dioxide. And we can manage plant growth so that soils in turn store carbon for long periods of time. Agricultural lands are a good example.

Why soils?

The estimated amount of carbon stored in world soils is about 1,100 to 1,600 petagrams (one petagram is one billion metric tons), more than twice the carbon in living vegetation (560 petagrams) or in the atmosphere (750 petagrams). Hence, even relatively small changes in soil carbon storage per unit area could have a significant impact on the global carbon balance.
 
Carbon sequestration in soils occurs through plant production. Plants convert carbon dioxide into tissue through photosynthesis. After the plants die, plant material is decomposed, primarily by soil microorganisms, and much of the carbon in the plant material is eventually released through respiration back to theatmosphere as carbon dioxide.

[Chuck Rice of Kansas State University displays a carbon-rich soil sample from the Konza Prairie Biological Station near Manhatten, Kan. The soil's dark color shows the presence of organic material. Courtesy of Dan Donnert, Kansas State University.]

But some of it remains when organic materials decay and leave behind organic residues, often called humus. These residues can persist in soils for hundreds or even thousands of years. At the same time, many factors can slow the decay of organic materials and, as a result, affect a soil’s capacity for storing carbon. Inherent factors include climate variables (temperature and rainfall), clay content and mineralogy.
 
It is possible to manage agricultural lands to maximize the amount of carbon those soils can store. The work my colleagues and I have undertaken on the agricultural lands of Kansas attempts to map the benefits of such soil management.

Staying in the soil

Climate affects soil carbon sequestration in two ways. First is the production of organic material entering the soil. Warm, moist climates generally have greater plant productivity. Cooler climates limit plant production. Hot climates may limit production because of reduced water availability, making water the limiting factor. Climate also affects the rate of microbial decomposition of plant material and soil organic matter. As temperature increases, microbial activity generally increases.
 
Soil water content also is important. Optimal microbial activity occurs at or near field capacity — the maximum amount of water that soil can hold against gravity. As soil becomes waterlogged, decomposition slows and becomes less complete. Peat soils are a common result. Decomposition also slows as soils dry.
 
Clay content stabilizes organic carbon by two processes. First, organic carbon chemically bonds to clay surfaces, which slows degradation. Clays with high adsorption capacities, such as montmorillinitic clays, retain the organic molecules. Secondly, soils with greater clay contents have a higher potential to form aggregates, which trap organic carbon and physically protect it from microbial degradation.
 
Generally, ecosystems that provide high quantities of plant material have the greatest potential to store carbon. Tropical ecosystems often provide some of the highest amounts of plant biomass, although these amounts are balanced by high rates of decomposition. Soils formed under tallgrass prairie, such as those my colleagues and I are studying on the Konza Prairie Biological Station near Manhattan, Kan., have high amounts of soil carbon. These amounts partly result from a high rate of plant productivity, with approximately 60 to 80 percent occurring below ground. The amount of carbon stored in these soils is equivalent to soils of tropical forests.
 
Even in one handful of soil, not all carbon is the same and differs by its degradability. Soil organic carbon often is divided into three pools: active, intermediate or slow, and recalcitrant. These three pools have different rates of turnover with the active pool on the order of months to years, the slow pool decades, and the recalcitrant pool hundreds to thousands of years. The active pool includes microbial biomass and labile organic compounds that make up less than 5 percent of the soil organic carbon. The slow pool usually makes up 20 to 40 percent, the recalcitrant 60 to 70 percent.
 
The goal of sequestering carbon in soils is to promote carbon transformations into the intermediate and recalcitrant pools. If more of the carbon ends up in the slow or recalcitrant pool, then it is less subject to loss and can remain in the soil for hundreds or thousands of years.
 
Carbon in soils under natural ecosystems often is at high levels and is considered at equilibrium, thus unable to sequester additional carbon. However, we have shown that in the soil beneath a native tallgrass prairie, soil carbon increased by 6 percent as deep as 50 centimeters when the tallgrass prairie was exposed to elevated carbon dioxide. The extra carbon dioxide increased plant production, which in turn increased how much carbon was incorporated into the soil. Most of the carbon was sequestered into a relatively slow pool, but some of the carbon was integrated into recalcitrant fractions, indicating longer-term storage.
 
The amount sequestered over the eight-year experimental period was equivalent to 6 megagrams per hectare. If one million acres absorbed this much carbon, it would store the same amount released by burning 4.3 million tons of coal. Furthermore, much of the carbon that was added from plant material was stored in macroaggregates larger than 250 micrometers, supporting the theory that physical protection of soil carbon is an integral part of carbon sequestration.

The potential of a tallgrass prairie

Agriculture in the 1800s and early 1900s relied on plowing the soil with low crop yields and on removing crop residues. This combination of agricultural practices resulted in reduced replenishment of organic material (carbon) to the soil. As a result, approximately 50 percent of the soil organic carbon (soil organic matter) has been lost over a period of 50 to 100 years of cultivation. However, this loss of soil carbon leaves space for new carbon. In recent decades, higher yields, retention of crop residues and development of conservation tillage practices have begun to increase soil carbon. Advances in crop and soil management practices can potentially allow soils to store more carbon.

[Late afternoon/early evening on the tallgrass prairie of the Konza Prairie Biological Station in Kansas. Courtesy of Alan K. Knapp, Kansas State University.]

No-tillage is one management practice that often preserves or increases soil carbon. My colleagues and I performed a study in western Kansas in which native sod was planted to a winter wheat-grain, sorghum-fallow rotation using either no-tillage or tillage to prepare the seedbed and plant the seed. After 10 years, the mass of aggregates larger than 2,000 micrometers in the top 5 centimeters was reduced and redistributed into aggregates of less than 250 micrometers when native sod was converted to cropping. The amount of carbon in macroaggregates of greater than 250 micrometers in native sod was double that observed in conventional tillage. No-tillage conserved the same amount of macroaggregates that naturally occur in native prairie soil. The organic carbon associated with the macroaggregates was preferentially lost with cultivation. No-tillage soils have a higher potential for storing injected carbon for a long time.
 
In addition to preserving soil carbon from native conditions, no-tillage can increase soil carbon in soils that were previously cultivated and contained reduced levels. In my study of maize that ran continuously for 10 years at Kansas State University, no-tillage increased soil carbon by 9 percent when compared with tilled soil. Water-stable aggregates increased in no-tillage compared with tillage, especially in aggregates larger than 2,000 micrometers. The number of macroaggregates greater than 250 micrometers increased, as did the carbon associated with the aggregates, preferential to the smaller size aggregates. When manure was added as a nitrogen source, no-tillage also held more of the manure’s carbon: the no-tillage soil held 32 percent more carbon from the manure than the tilled soil. Thus, manure added both nitrogen and carbon.
 More frequent planting of crops (almost year-round) infuses the soil with extra plant material and increases the amount of carbon stored. In western Kansas, intensifying cropping systems by conversion from wheat-fallow rotation to wheat-grain, sorghum-fallow rotation has increased soil carbon levels.
 
Another factor that determines storage capacity is the quality of plant carbon entering the soil. Our research shows that carbon from roots may contribute more to soil organic matter formation than does carbon from straw. The reason for this difference between roots and above-ground material is not clear, but roots have a higher ratio of carbon to nitrogen, which would slow decomposition and encourage formation of humus. This conversion of carbon into humus is important because humus is part of the recalcitrant pool and the carbon in humus lasts longer in the soil. This quality factor suggests that plant breeding may provide avenues for increased carbon sequestration, either by changing plant composition of carbon compounds so that more carbon will be converted to soil organic matter, or by altering ratios of roots to shoots.
 
Microorganisms convert plant carbon into soil organic carbon. Differences in the soil microbial community can affect the ratio of carbon converted to carbon dioxide vs. to soil organic carbon. In research on the Konza Prairie that changed water relations in a tallgrass prairie, the soil microbial community was changed to favor fungi. Because bacteria tend to respire more plant carbon to carbon dioxide, while fungi tend to retain more carbon in the soil, the result was a greater retention of carbon into microbial products in the soil. Further research needs to be conducted on potential manipulation of the soil microbial community to find biogeochemical transformations of carbon that remain in soil.

Other pluses to soil carbon

Managing agricultural soils for sequestering carbon will yield additional benefits. When carbon is part of the soil organic matter fraction, the soil’s capacity to hold basic cations increases, which in turn improves soil fertility. Soil organic matter also improves water holding capacity, thus increasing plants’ ability to withstand short droughts. Soil carbon improves the structure of the soil, which results in improved drainage and aeration and better root growth. For the microbial community, carbon provides an energy source resulting in greater nutrient cycling and biodiversity. In addition, management practices that increase soil carbon also tend to reduce soil erosion, reduce energy inputs and improve soil resources. Increasing a soil’s capacity to store carbon means increasing how much carbon it contains, which in turn increases crop productivity and enhances soil, water and air quality.



Rice is a professor of soil microbiology at Kansas State University. Email: cwrice@ksu.edu. Learn more about the Konza Prairie Biological Station at www.konza.ksu.edu


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