8/9/2019 Carbon Sequestration With Biochar - stability and effect on decomposition of soil organic matter

1/3

Carbon sequestration with biochar stability and effect on decomposition of soil organic

matter

This article has been downloaded from IOPscience. Please scroll down to see the full text article.

2009 IOP Conf. Ser.: Earth Environ. Sci. 6 242010

(http://iopscience.iop.org/1755-1315/6/24/242010)

Download details:

IP Address: 98.203.174.133

The article was downloaded on 04/05/2010 at 18:42

Please note that terms and conditions apply.

View the table of contents for this issue, or go to thejournal homepage for more

ome Search Collections Journals About Contact us My IOPscience

http://iopscience.iop.org/page/termshttp://iopscience.iop.org/1755-1315/6/24http://iopscience.iop.org/1755-1315http://iopscience.iop.org/http://iopscience.iop.org/searchhttp://iopscience.iop.org/collectionshttp://iopscience.iop.org/journalshttp://iopscience.iop.org/page/aboutioppublishinghttp://iopscience.iop.org/contacthttp://iopscience.iop.org/myiopsciencehttp://iopscience.iop.org/myiopsciencehttp://iopscience.iop.org/contacthttp://iopscience.iop.org/page/aboutioppublishinghttp://iopscience.iop.org/journalshttp://iopscience.iop.org/collectionshttp://iopscience.iop.org/searchhttp://iopscience.iop.org/http://iopscience.iop.org/1755-1315http://iopscience.iop.org/1755-1315/6/24http://iopscience.iop.org/page/terms

8/9/2019 Carbon Sequestration With Biochar - stability and effect on decomposition of soil organic matter

2/3

P24.02

Carbon sequestration with biochar - stability and effect on decomposition of soil organic matter

Sander Bruun(1), T El-Zahery(2), L Jensen(1)

(1) Plant and Soil Science Laboratory, Department of Agriculture and Ecology, Faculty of Life Sciences,

University of Copenhagen, Frederiksberg, Denmark

(2) Department of Soil Science, Faculty of Agriculture, University of Mansura, Mansura, Egypt

Thermo-chemical conversion covers several technologies which can be used to extract energy from biomass

including pyrolysis and gasification. Under high temperature and limited oxygen conditions, solid biomassis converted into syngas, bio-oil and biochar. The syngas and bio-oil can be combusted for energy. The

biochar is also an excellent fuel, but recently the use of biochar for carbon sequestration and improvement ofsoil properties has gained increasing attention. Removing additional biomass from agricultural fields for

bioenergy production may mean that the soils become depleted of nutrients and organic matter. As thebiochar contains most of the nutrients from the biomass, returning it to the land where the biomass wasproduced means that the nutrients are recycled and utilised sustainably. However, biochar also has a range of

other positive effects on soil quality such as increased water holding capacity and cation exchange capacity.

In addition, biochar is highly resistant against microbial degradation, which means that it is stabilized in thesoil. Because the carbon in the biochar was recently fixed during plant growth, the process can beconsidered as a means for carbon sequestration. The high stability of char in soil is incontrovertible, but

most available information comes from the natural occurrence of char in sediments and in soils from areaswith a history of vegetation burning. Laboratory incubations allow investigation of differences in stability of

chars of different origin and thermal alteration. By using isotope techniques very sensitive measurementscan be obtained. However, the high stability of char necessitates long incubations to get a better impressionof the development of the degradation rate with time. A recently published ten year litter bag experiment in

a Swedish forest (Wardle, Nielsson and Zackrisson, Science, 2008, 230, 629) indicated that the presence ofchar was leading to losses of soil organic matter. This could mean that any carbon sequestered in biochar

could be offset by increased mineralization of soil organic matter. However, the study was criticized becauseof several artifacts pertaining to the litter bag method (Lehman and Sohi, Science, 2008, 321, 1295). Most

importantly, the litter bags does not allow for contact with mineral soil. The purpose of the current study isto i) compare the degradation rate of char thermally altered to different degrees and ii) test if biocharincreases the degradation of soil organic matter. Two incubation experiments were conducted; one exploring

the effect of thermal alteration on the stability of biochar and the other testing interaction between litter, charand soil organic matter. In the first experiment, char was produced from homogeneously 14C labelled wheat

straw at 225C, 300C and 375C under limited oxygen supply. The resulting char and straw were dried at70C and used in a two-year soil incubation where the evolved 14C was trapped in a NaOH and counted on

a scintillation counter. In the second experiment, different combinations of 14C labelled wheat straw andchar was incubated together with unlabelled straw and char. In addition, 14C soil organic matter in a soilwhich had been labeled 40 years ago was incubated with unlabelled char and wheat straw. The results of the

first experiment showed that there was an initial flush of carbon evolution from all the chars included in thestudy with no apparent lag phase. In contrast, there was a distinct lag phase for the straw. The small amount

of carbonates in the chars produced at 225C and 300C excluded carbonates as the source of the initial

flush. The initial flush is therefore more likely to be associated with oxidation of the char particle surfaces,which has been observed in other studies. After the initial flush, the carbon evolution rate quickly decreasedto very low values for all the chars, whereas it remained high for the litter before the evolution for this

material also faded eventually. After two years of incubation, 56% of the carbon originally added as strawhad been evolved as CO2. For char produced at 225C, 9.3% was lost while for 300C, 3.1% had been lost.The rate of carbon evolution after two years of incubation was 0.082 d-1 for straw, 0.060 d-1, 0.011

d-1, and 0.008 d-1 for char produced at 225C, 300C and 375C. Therefore it was concluded thatchar is very resistant against microbial degradation and that the resistance increases with the degree of

thermal alteration, also after the initial flush of carbon evolution. This confirms that the potential for carbonsequestration using biochar is large. However, although biochar produced at high temperatures is more

stable, it may not necessarily lead to higher carbon sequestration because the efficiency of formation ishigher at lower temperatures since less carbon is released during biochar production. Thus after two years,

44% of the carbon was sequestered in the soil when straw litter was added, whereas as much as 81% or only

20% was sequestered when char produced at 225C or 300C, respectively, was added to the soil. For char

1

8/9/2019 Carbon Sequestration With Biochar - stability and effect on decomposition of soil organic matter

3/3

produced at 375C almost all carbon was lost already during production. This means that the treatment

actually resulted in the highest carbon sequestration after two years from biochar production at 225C.

However after longer period of time the char produced at higher temperatures is likely to become more

favorable. The results of the second experiment did not indicate any increased decomposition of either strawor soil organic matter due to char addition. In fact, there was a small tendency for biochar to decrease

decomposition of soil organic matter. However, addition of straw resulted in higher reductions soil organic

matter decomposition rates. There are several possible explanations for the reduction in the decomposition

of soil organic matter. First of all, the added straw and char will suck up water from the soil and reduce the

water potential in the soil and thereby perhaps also the decomposition of the organic matter. A reason for the

reduction after addition of litter could be the immobilization of nutrients, which decreases the availability of

nutrients for microorganisms decomposing the soil organic matter. Whether this could also explain the

reduction of the rates of soil organic matter decomposition in the treatments where char is added is

questionable as the char has a considerably smaller potential for nutrient immobilization. Thus, based on the

high stability of char there seems to be a great potential for carbon sequestration based on biochar

production. This potential is dependant on the temperature at which the biochar is produced and upon the

time frame at which the sequestration is assessed. Also the energy recovery in the thermo-chemical

conversion will have to be considered. Finally we did not find any evidence that the biochar increases thedecomposition of soil organic matter. There is thus no indication the carbon sequestered in the biochar will

be offset by an increased release of carbon dioxide because of increased decomposition of soil organic or

recenty added plant litters. All of this support the assertion that biochar presents a potentially very effective

method for soil carbon sequestration.

2