Geotimes
Web
Feature
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
