Biochar

Printed on recycled paper U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2010–3117

January 2011

Biochar for Soil Fertility and Natural Carbon Sequestration

Biochar is charcoal (similar to chars generated by forest fires) that is made for incorporation into soils to 

increase soil fertility while providing natural carbon sequestration. The incorporation of biochar into soils can 

preserve and enrich soils and also slow the rate at which climate change is affecting our planet. Studies on 

biochar, such as those cited in this Fact Sheet, are applicable to both fire science and soil science.

The application of biochar to soil has been proposed as a way to store carbon in the 

soil for longer periods than if biomass was left to decay. Besides increasing the storage 

of carbon in soils, the application of biochar to soil can improve soil fertility and increase 

crop production. Since biochar is different from better-known, activated charcoal, information is needed on the effects of its formation conditions on both its physical and chemical properties before farmers begin to use it for large-scale incorporation into soils. 

One of a nation’s most important assets is the fertility of its soil. The cultivation 

of soil causes a steady decrease in the soil’s organic content and fertility. This steady 

decrease in fertility is compensated for by addition of fertilizers, but the decrease in 

organic content is a more permanent problem. Incorporation of biochar can slow or 

reverse this trend. 

When plant materials—such as 

wood, bark, corn stalks, switchgrass, 

and the like, which scientists generically 

term biomass—are heated in a low- or 

no-oxygen atmosphere (this process is 

termed pyrolysis), the resulting product 

is called biochar. For example, pyrolysis of wood yields a familiar biochar: 

charcoal, once the staple fuel for 

backyard cookouts.

Carbon in the form of carbon 

dioxide (CO2), which is released into the 

atmosphere when carbon-based fuels 

are burned and when biomass decays, 

is a major contributor to global climate 

change. For this reason, carbon sequestration (the process of capturing CO2

before it escapes into the atmosphere) 

has the potential to slow global warming. Increasingly, scientists are looking at 

biochars as additives that can simultaneously enrich soil and sequester carbon 

that would otherwise be released into the 

atmosphere if biomass were simply left 

aboveground to decay.

Plant biomass is primarily a mixture of cellulose and lignin with small 

amounts of inorganic material called ash. 

Waste biomass such as beetle-killed trees 

or agricultural residues can be put to beneficial use by conversion to biochar. 

The pyrolysis of plant biomass to 

generate biochar converts much of the 

aliphatic carbon (which contains no carbon ring structures) to a very stable form 

of aromatic carbon (which contains rings 

made of carbon, like graphite) which is 

stable in soils for hundreds of years. The 

term biochar has been adopted to mean 

char produced from waste biomass for 

the purpose of land application. 

Douglas-fir mortality caused by Douglas-fir beetle in the Uncompahgre National Forest

near Telluride, Colorado. Photograph taken by Justin Backsen of the U.S. Forest Service.
As a means of predicting biochar 

characteristics, the U.S. Geological 

Survey (USGS) made biochar by charring 

a variety of feedstocks including purified 

plant components (such as cellulose and 

lignin) and plant materials (such as wood, 

bark, and switchgrass) under a nitrogen 

atmosphere at temperatures ranging from 

200°C to 500°C (about 400°F to 900°F) 

for times ranging from 1 hr (hour) to 

168 hr. Changes in composition were 

examined by mass loss, elemental composition (carbon, hydrogen, and oxygen), 

acid-functional-group content, and infrared and 13C Nuclear Magnetic Resonance 

(NMR) spectroscopy (Rutherford and 

others, 2008). Changes in structure were 

examined by porosity and surface area 

(Rutherford and others, 2004). 

The USGS has obtained bulk quantities of biochar reference material from 

two gasification processes which produce 

biochar as a byproduct. The reference 

materials were homogenized and split 

to ensure uniformity. The uniformity of 

these materials was verified by subsampling and subsequent chemical and 

physical analyses. These two reference 

chars provide a consistent material to the 

scientific research community for comparison of analytical techniques between 

laboratories or as a basis for comparison 

of characteristics of one type of char 

to another. 

The two main families of carbon 

compounds are aliphatic and aromatic. 

Comparison of mass loss, combined with 

13C NMR spectroscopy, showed that 

the initial rapid loss of material during 

pyrolysis is due to the loss of aliphatic 

components, which are either vaporized 

or converted to aromatic carbon early in 

the charring process. Aromatic carbon is 

a more stable form than aliphatic carbon. 

The biochars were also analyzed 

for acid functional group content. Acid 

functional groups are related to cationexchange capacity, which is important to 

soil fertility, as it assists in the retention 

of nutrients (nitrogen and phosphorus) 

and cations (like potassium) in soil. 

In the USGS experiments, the highest 

concentrations of acid functional groups 

developed at low charring temperatures 

(250°C) and long charring times (greater 

than 72 hr). Cellulose appears to be the 

primary source of the acid functional 

groups in the biochar. 

Surface area and porosity of the biochar plays a significant role in soil fertility. In contrast to the optimum conditions 

for the formation of the acid functional 

groups, more intense charring conditions 

(higher temperatures and longer charring 

times) were required for the formation of 

porosity and surface area in the biochar. 

The effect of formation conditions 

and feedstock on physical and chemical 

characteristics of the resultant char will 

be important if biochar is to be successfully incorporated into agricultural 

soils. Using this information provided 

by USGS, formation conditions and 

feedstocks may be tailored to optimize 

desirable characteristics (such as acid 

functional group content) and to minimize less-desirable characteristics (such 

as degradability or mass loss). The results 

—Colleen E. Rostad (cerostad@usgs.gov)

David W. Rutherford (dwruther@usgs.gov)

generated by USGS provide insight into 

optimum formation conditions for chars 

with the best properties to simultaneously 

enhance soil fertility and provide longterm carbon sequestration.

References Cited

Rutherford, D.W., Wershaw, R.L., and 

Reeves, J.B., III, 2008, Development 

of acid functional groups and lactones 

during the thermal degradation of 

wood and wood components: U.S. 

Geological Survey Scientific Investigations Report 2007–5013, 43 p. 

(Also available at http://pubs.usgs.gov/

sir/2007/5013)

Rutherford, D.W., Wershaw, R.L., and 

Cox, L.G., 2004, Changes in composition and porosity occurring during 

the thermal degradation of wood and 

wood components: U.S. Geological 

Survey Scientific Investigations Report 

2004–5292, 78 p. (Also online at 

http://pubs.usgs.gov/sir/2004/5292/.)

Pine shavings, before and after charring.

For more information, contact:

Chief Scientist, Hydrology

Branch of Hydrologic Research

U.S. Geological Survey

409 National Center

12201 Sunrise Valley Drive

Reston, VA 20192