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Soil Biology & Biochemistry 43 (2011) 1563e1568

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Soil Biology & Biochemistry

journal homepage: www.elsevier .com/locate/soi lb io

Quantitative analysis of biochar in field soil

Roger T. Koide*, Krittika Petprakob, Matthew PeoplesDepartment of Horticulture and Graduate Program in Ecology, The Pennsylvania State University, University Park, PA 16802, USA

a r t i c l e i n f o

Article history:Received 21 October 2010Received in revised form4 April 2011Accepted 7 April 2011Available online 22 April 2011

Keywords:AnalysisBiocharCarbon sequestrationLoss on ignitionSoil organic matterQuantification

* Corresponding author. Tel.: þ1 814 863 0710; faxE-mail addresses: rxk13@psu.edu (R.T. Koide), kup

peoplma@gmail.com (M. Peoples).

0038-0717/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.soilbio.2011.04.006

a b s t r a c t

Biochar is used with increasing frequency as a soil amendment because of its potentially beneficialeffects on soil carbon sequestration, crop yield, nutrient leaching and greenhouse gas emissions. Simplemethods for the analysis of biochar in soil, however, are currently unavailable. Therefore, we haveadapted the “loss on ignition” method for this purpose. The technique requires knowledge of theproportions of both biochar and biochar-free soil that are lost on ignition. One can use values determinedprior to the amendment of the soil with biochar, assuming that the values do not change after biochar isincorporated in the soil. We tested these assumptions. Over the course of 15 months, the assumptionsproved to be valid under our test conditions. The technique accurately determined a wide range ofbiochar concentrations in field soil.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Thermochemical conversion (pyrolysis) is one route for theproduction of liquid fuel from biomass (Boateng et al., 2006). One ofthe byproducts from pyrolysis is biochar, which may be useful asa soil amendment due to its unusual chemical and physical char-acteristics. Biochar contains high concentrations of carbon that canbe rather recalcitrant to decomposition, so it may stably sequestercarbon (Glaser et al., 2002). It can increase soil aeration (Laird,2008) and reduce soil emissions of N2O, a greenhouse gas(Spokas et al., 2009; Singh et al., 2010). It has increased crop yieldthrough various mechanisms including stimulation of beneficialsoil microbes such as mycorrhizal fungi (Warnock et al., 2007),increase of soil base saturation (Glaser et al., 2002; Major et al.,2010a,b), increase in water holding capacity (Glaser et al., 2002;Steiner et al., 2007), and retention of nutrients in the portion ofthe soil column containing roots, thus improving nutrient useefficiency (Chan et al., 2007; Steiner et al., 2008).

There are multiple situations in which it might be important tomeasure the amount of biochar in soil. Effective and fair carbonsequestration compensation schemes may require documentingboth the temporal and spatial stability of biochar. Biochar is capable

: þ1 814 863.6139.146@psu.edu (K. Petprakob),

All rights reserved.

of migrating in soils (Major et al., 2010a,b) and, while it is generallyassumed to be recalcitrant, a portion of most biochars, particularlyfresh biochars, is “easily decomposable” (Lehmann et al., 2009).Therefore, repeated analyses of biochar in time and space becomenecessary. Moreover, because biochar has the potential to improvecrop growth and reduce leaching and greenhouse gas emissions(see above), its cost-effective use will require knowledge of therelationship between biochar quantity and the magnitude of thesebenefits.

Despite the clear need for a simple and routine way to quantifybiochar in soils, the complex chemistry of biochar and the difficultyin distinguishing biochar from other forms of organic matter hasresulted in methods that are either extremely labor intensive orrequire specialized instrumentation, both of which are impracticalfor routine analysis by most scientists (Manning and Lopez-Capel,2009). For example, biochar in soils has been quantified by hand-sorting (Zackrisson et al., 1996), in situ analysis via scanning calo-rimetry, NMR spectroscopy or infrared spectroscopy (Schmidt andNoack, 2000; Manning and Lopez-Capel, 2009; Nguyen et al.,2009), analysis of molecular markers (Manning and Lopez-Capel,2009), or by preferential removal of inorganic and non-biocharorganic C by selective oxidation or acid treatment followed byanalysis of residual organic material (presumably equivalent tobiochar) by NMR, optical, or mass spectroscopy, or thermalconductivity (Schmidt and Noack, 2000; Elmquist et al., 2004;Manning and Lopez-Capel, 2009). None of these methods issuited to routine analysis of biochar in soils bymost agronomists or

R.T. Koide et al. / Soil Biology & Biochemistry 43 (2011) 1563e15681564

ecologists engaged in soil amendment studies. Our goal, therefore,was to adapt the familiar “loss on ignition” method for use in theroutine analysis of biochar in soil.

2. Material and methods

2.1. Development of the method

Our method for analysis of biochar in soils is based on the “losson ignition” method for the analysis of soil organic matter (Heiriet al., 2001). Loss on ignition is considered to be superior to wetcombustion for the estimation of soil organic matter because itaccounts for a greater fraction of the organic matter present in thesample, but it can overestimate soil organic matter if there issubstantial carbonate present in the sample (Byers et al., 1978). Atemperature of 550 �C is generally assumed to oxidize theconventional (non-thermally altered) organic matter in most soils(Ball, 1964; Heiri et al., 2001). In preliminary tests we determinedthat 550 �C also thermally oxidized the organic portion of biocharproduced from hardwood via conventional charcoaling (slowpyrolysis) methods. We therefore subsequently used 550 �C in allexperiments of this study.

Because both conventional soil organic matter and biochar areoxidized in the same process, the weight that is lost on ignitionfrom a soil sample containing biochar can be given in the followingequation:

LI ¼ OMþ CHLI (1)

where:LI is the total weight of the sample that is lost on ignition,OM is the weight of the native, non-biochar organic matter

resident in the soil (frequently referred to as thermally unalteredorganicmatter, Schmidt andNoack, 2000) that is lost on ignition, and

CHLI is the weight of the biochar in the soil sample that is lost onignition.

The portion of the biochar in the soil sample exposed to 550 �Cthat is not lost on ignition is the mineral component of biochar orash. Our soil, described below, did not contain measurableconcentrations of carbonate.

In the absence of biochar, OM is operationally defined as thatportion of the soil that is lost on ignition (Heiri et al., 2001). It isassumed to be the non-biochar organic matter resident in the soil.Therefore, in a soil sample containing biochar:

OM ¼ qðW � CHÞ (2)

where:q is the proportion (by weight) of pure soil (without biochar)

that is lost on ignition,W is the weight of the soil sample containing biochar, andCH is the weight of the biochar in the sample (including that

which is lost on ignition plus the ash component of the biochar thatremains after combustion).

CHLI, the weight of biochar lost on ignition, can be given by thefollowing equation:

CHLI ¼ y CH (3)

where:y is the proportion (by weight) of biochar that is lost on ignition.

We assume this is the organic fraction of the biochar.Substituting Eqs. (2) and (3) into Eq. (1), we obtain:

LI ¼ qðW � CHÞ þ y CH (4)

Rearranging the equation to solve for CH, we obtain:

CH ¼ ðLI� q WÞ=ðy� qÞ (5)

Thus, the weight of biochar in a soil sample can be calculatedfrom the weight that is lost on ignition from the soil sample con-taining biochar, the weight of the soil sample containing biochar,the proportion of pure soil (not including the biochar) that is loston ignition (q), and the proportion of the biochar that is lost onignition (y).

2.2. Testing the assumptions of the method

The proposed method for biochar analysis assumes knowledgeof q, the proportion of the pure soil (without biochar) that is lost onignition, and y, the proportion of the pure biochar (without soil)that is lost on ignition. We imagine that in most cases it will bedifficult to separate biochar from a soil sample, especially if thebiochar is fine, in order to routinely determine either q or y.However, in cases in which biochar is experimentally added to soil,samples of pure biochar can be analyzed to determine y andsamples of pure soil can be analyzed to determine q. In order forthat approach to be useful, however, one must assume that y and qdo not change as a consequence of residence of the biochar in thesoil. We tested these assumptions by adding biochar to soil in thefield and, 15 months later, collecting field samples and separatingthe biochar from the remaining soil (see below) in order to deter-mine whether q and y remained unaltered.

In June 2009, we rototilled to a depth of 25 cm a portion of a fieldformerly planted to corn at the Russell Larson Research andEducation Center. Its location is 40�42045.6500N, 77�57026.2100W.This field was not the source of the soil described in Section 2.3, butit possesses the very same soil type. The soil is Hagerstown soil:fine, mixed, semiactive, mesic Typic Hapludalfs (USDAeNRCS,2010). The surface texture is a silt loam with subsurface texturesof a silty clay loam and silty clay. After rototilling, we laid out 4-1m2

plots in each of three blocks. Two plots in each block were amendedwith biochar while two plots were not and served as controls. Plotswere separated from each other by 2 m. The biochar plots receivedapproximately 5.7 kg (12.5 lbs) biochar, equivalent to approxi-mately 56.8 tonne ha�1. The biochar was produced from domestichardwood via conventional, slow pyrolysis and obtained fromHumphrey Charcoal (Brookville, PA, USA). In blocks 2 and 3we usedmesh size #6 biochar; the pieces of biochar were smaller than3.4 mm in the smallest dimension. In block 1 we used mesh size#10 biochar; the pieces of biochar were smaller than 1.7 mm in thesmallest dimension. Other than size, the biochars were identical.After spreading the biochar uniformly over the plots, we mixed itinto the soil with spades to a depth of approximately 25 cm, turningthe soil 3e4 times for thorough mixing. The area was then plantedto sweet corn (Delectable, Rupp Seeds Wauseon, OH, USA), and thecorn was harvested later that season. In May 2010, the same areawas planted to soybean (FS H535A90, Growmark, Bloomington, IL,USA). In September 2010, when the biochar had been in the soil for15 months, we collected soil samples to a depth of 15 cm from eachof the 12 plots using a standard 2.5 cm diameter soil sampler(Oakfield Apparatus, Inc., Oakfield, WI, USA). Two cores were takenfrom each control (no biochar) plot and four cores were taken fromeach biochar plot.

We then separated out approximately 15 ml of biochar piecesfrom each biochar plot soil sample. The separation was facilitatedby the fact that the biochar pieces were relatively large (see above).We employed a 5 mm soil sieve to perform the initial separation ofbiochar from soil. Most of the biochar in the sample remained onthe sieve. The biochar pieces and soil aggregates remaining on thesieve were easily separated. The soil aggregates were retained aspart of the pure soil fraction. The smaller aggregates of soil passing

Fig. 1. Plot of actual biochar weight in sample vs. calculated biochar weight in samplebased on use of Eq. (5), assuming q ¼ 0.0429 and y ¼ 0.861. The data points representduplicate samples of biochar mixed in field soil at 0, 1, 2, 5 and 10% biochar by weight.

R.T. Koide et al. / Soil Biology & Biochemistry 43 (2011) 1563e1568 1565

through the sievewere picked free of biochar using forceps. The soilaggregates that passed through the sieve were added to the largeraggregates remaining on the sieve and collectively comprised thepure soil fraction. The smaller pieces of biochar were added to thelarger pieces retained by the sieve to comprise the biochar fraction.

The biochar samples separated from soil were cleaned ofadhering soil by placing them overnight in 50 ml polyethylenecentrifuge tubes filled with a 0.5% (by weight) aqueous solution ofsodium hexametaphosphate, (NaPO3)6. The tubes were thenshaken vigorously to dislodge the soil particles, and the biochar wasrinsed four times with distilled water. They were then allowed tosoak in distilled water for a few hours before being drained anddried in an oven at 70 �C for temporary storage.

We performed “loss on ignition” analyses of samples of controlplot soils, which never contained biochar (10 g), samples of purebiochar separated from the biochar-amended soil plots (2.5 g),samples of pure soil separated from the biochar-amended soil plots(10 g), as well as three samples of biochar (2.5 g) that had neverbeen added to soil. Loss on ignition was determined as the differ-ence in weight between the sample after drying at 105 �C and afterheating in a muffle furnace at 550 �C for 4 h. The values of q and ywere determined from these measurements according to theexplanations given beneath Eqs. (2) and (3), respectively. Bycomparing biochar that had not been added to soil with biocharthat had been in the soil for 15 months, we tested the assumptionthat y did not change. By comparing soil from the control plots withsoil separated from biochar-amended soil plots, we tested theassumption that q did not change.

2.3. Testing the method

We tested our method by comparing actual and calculatedbiochar contents of approximately 5 g samples of field soil to whichwe added varying amounts of biochar. There were two replicateseach of 1%, 2%, 5% and 10% biochar by weight. The biochar, suppliedby Humphrey Charcoal (Brookville, PA, USA), was the same as usedabove (Section 2.2) but, in this case, the biochar was ground toa fine powder with mortar and pestle before mixing with field soilin order to increase the homogeneity of the samples. The soil wasobtained from an agricultural field of the Russell Larson Agricul-tural Research and Education Center of the College of AgriculturalSciences, Penn State University, located in Rock Springs, PA, USA,40�42044.4300N, 77�57019.2900W. This field was not the source of thesoil described in Section 2.2, but it possesses the same Hagerstownsoil (Fine, mixed, semiactive, mesic Typic Hapludalfs) (USDAeNRCS,2010). The soil was collected from the top 15 cm and air dried andsieved to pass a 2 mm sieve prior to mixing with the biochar. Inaddition to the four concentrations of biochar in soil (above),duplicate samples of 100% soil (no biochar) and 100% biochar (nosoil) were also prepared for loss on ignition analysis in order todetermine the values of q and y. In all cases, weight loss on ignitionwas calculated as the difference between the weight of samplesafter drying at 105 �C and their weight following 4 h at 550 �C ina muffle furnace (Barnstead Thermolyne 48000, Dubuque, IA, USA).In the 5 cm (diameter) ceramic crucibles we employed for the losson ignition procedure, the 4 h duration of heating was sufficient tocompletely oxidize 5 g sample of pure biochar to ash, and tocompletely oxidize the biochar in the 5 g soil samples.

3. Results

3.1. Testing the assumptions of the method

Despite using different size biochar in block 1 compared toblocks 2 and 3, there were no significant effects of block on either q,

the proportion of the pure soil lost on ignition (p ¼ 0.53) or y, theproportion of biochar lost on ignition (p ¼ 0.88). There was nosignificant effect of the 15 month residence time of biochar in thefield soil on q; according to the analysis of variance, the q of the soilin the control plots (mean 0.045, se 0.002) was not significantlydifferent (p¼ 0.79) from that of the soil separated from the biochar-amended soil plots (mean 0.046, se 0.002). Moreover, there was nosignificant effect of the 15 month residence time of biochar in thefield soil on y; the y of the biochar taken from the biochar-amendedsoil plots (mean 0.878, se 0.010) was not significantly different(p¼ 0.42) from that of the biochar that had never been added to soil(mean 0.861, se 0.009).

3.2. Testing the method

The average q of the duplicate pure soil samples was 0.0429 (se0.0003). The average y of duplicate pure biochar samples was 0.861(se 0.009). Using these values and Eq. (5), we found a linear rela-tionship between actual and calculated biochar weights in theprepared soilebiochar mixtures described above (Fig. 1). Themixtures ranged from 0% biochar up to 10% biochar (by weight).The slope was very close to one (0.960) and there was little scatterin the data (r2 ¼ 0.996). Therefore, the method proved to accuratelydetermine the actual weight of biochar in each sample over a widerange of biochar concentrations in soil.

4. Discussion

There are currently no standard methods for the analysis ofbiochar in soils. In most cases, biochar content of soils has beendetermined by laborious or instrument-intensive techniques(Zackrisson et al., 1996; Schmidt and Noack, 2000; Elmquist et al.,2004; Manning and Lopez-Capel, 2009; Nguyen et al., 2009).None of these is suitable for routine analysis in typical ecological oragronomic studies due to the requirement for specialized apparatusor the sheer labor involved in hand-sorting. Because of thesimplicity of the instrumentation involved (muffle furnace andbalance), our method of analyzing biochar in soil samples is ideally

Fig. 2. Plot of actual biochar weight in sample vs. three calculated values of biocharweight in sample based on use of Eq. (5), assuming y ¼ 0.861 and three values of q(0.0429 and 90% and 110% of 0.0429). The data points represent duplicate samples ofbiochar mixed in field soil at 0, 1, 2, 5 and 10% biochar by weight.

R.T. Koide et al. / Soil Biology & Biochemistry 43 (2011) 1563e15681566

suited to routine determinations. It is far simpler than othermethods that require more sophisticated instrumentation, theresults of which are difficult to interpret and do not yield biocharweights directly (Schmidt and Noack, 2000; Manning and Lopez-Capel, 2009). Moreover, our method does not assume thatbiochar and non-thermally altered soil organic matter are distin-guishable either analytically or in terms of their recalcitrance tooxidation. Instead, our adaptation of the “loss on ignition” meth-odology involves the thermal oxidation of all forms of organicmatter in the soil, including biochar.

The method does require knowledge of q, the proportion ofweight of pure soil that is lost on ignition and y, the proportion ofweight of biochar that is lost on ignition. Heterogeneity among soilsamples in organic matter content such as caused by random bits ofplant litter will result in variation in q from sample to sample, butsuch variation cannot by itself alter the mean of the calculatedbiochar content as long as a sufficient number of samples areanalyzed. While we have tested our method only on biocharmanufactured from hardwood via conventional, slow pyrolyticcharcoaling methods, it should also be applicable to other forms ofbiochar that can be thermally oxidized using the conventional “losson ignition” conditions as long as y is determinable.

When biochar cannot be separated from soil samples in order todetermine q and y such as was done in this study, it may beacceptable to use values for q and y determined prior to the addi-tion of biochar to the soil. Evenwhen biochar was incorporated intoa field soil for a period of 15 months during which a typical crop-ping rotation occurred (corn and soybeans), q and y did not changesignificantly in the test soil using the test biochar. In other soils orwith other types of biochar, q and y could behave differently. Theassumptions of temporal stability of q and y would have to bevalidated for each study.

Indeed, we recognize the potential for y to change followingthe addition of biochar to soil. For example, y may be reduced if,over time, ash comprises a greater proportion of the remainingweight as the “easily decomposable fraction” of biochar (Lehmannet al., 2009) decomposes or as the volatile fraction is lost (Breweret al., 2009). Moreover, as biochar ages in the soil, it may becomeenriched in oxygen and depleted in carbon (Cheng et al., 2008;Hammes and Schmidt, 2009; Nguyen et al., 2009), and thiscould also possibly alter the value of y. All of these phenomenacould be influenced by the quality of the biochar, which is influ-enced by choice of feedstock and pyrolysis conditions (Breweret al., 2009).

The value of q could be altered by biochar addition to soilbecause root growth can be stimulated by biochar in soil (Makotoet al., 2010). Moreover, biochar can influence microbial communi-ties, and soil microbial communities may influence the quality andquantity of soil OM. For example, biochar may stimulate mycor-rhizal fungi (Watrud et al., 1978; Saito,1989; Ishii et al., 1999; Ezawaet al., 2002; Yamato et al., 2006; Rondon et al., 2007;Warnock et al.,2007), and mycorrhizal fungi produce a soil-aggregating materialcalled glomalin, a decomposition-resistant glycoprotein thatapparently comprises a large fraction of stable organic matter insoils (Wright and Upadhyaya, 1996, 1998; Treseder and Allen, 2000;Rillig et al., 2001). Mycorrhizal fungi also significantly enhance soilstructure and erosion resistance due to the production of glomalinand of vast lengths of hyphae (see Miller and Jastrow, 1992;Gianinazzi and Schuepp, 1994; Boswell et al., 1998; Kabir andKoide, 1999). Soil aggregation stabilizes organic matter againstrapid decomposition by reducing access by microorganisms (Elliottand Coleman, 1988; Jastrow, 1996; Jastrow and Miller, 1998; Tisdalland Oades, 1979; Tisdall et al., 1997; Jastrow et al., 2005). Soilaggregation also reduces soil erosion, which further limitsdecomposition of organic matter in the soil (Peterson et al., 1998).

If, by altering microbial activity, biochar influences the quality ofthermally unaltered soil organic matter, its presence could, overtime, influence the value of q.

While we did not observe significant changes in q or y following15 months of biochar in soil, because of the potential for change wetestedwhether significant under- or overestimations of either q or ycould result in significant error in the calculation of biochar weight.We approached this problem by calculating biochar weights first byassuming y ¼ 0.861 while assuming qwas either 90% or 110% of theassumed correct value of 0.0429 (q ¼ 0.0472 and q ¼ 0.0386,respectively) and, secondly, by assuming q ¼ 0.0429 whileassuming y was either 90% or 110% of the assumed correct value of0.861 (y ¼ 0.947 and y ¼ 0.775, respectively). In the first instance,the results are shown in Fig. 2. In the second, the results are shownin Fig. 3.

Fig. 2 indicates that in our system a 10% error in the value of qresults in a nearly constant absolute error in biochar weight ofapproximately �0.025 g across the range of tested weights ofbiochar from 0 to nearly 0.5 g per sample (equivalent to concen-trations of biochar from 0 to 10% of soil by weight). A systematicabsolute error may be acceptable, particularly for studies in whichcomparisons among treatments are the major consideration.

Fig. 3 indicates that a 10% error in the value of y results ina variable absolute error in the calculation of biochar weight. At thelowest actual biochar weight (0.05 g, equivalent to 1% biocharconcentration) the absolute error is comparatively small (approxi-mately �0.005 g). At the highest actual biochar weight (close to0.5 g, amounting to 10% biochar concentration) the absolute error iscomparatively large (approximately�0.05 g). However, the percenterror was approximately the same across the entire range of testedweights of biochar; a 10% error in y resulted in an approximately10% error in the calculation of biochar weight, irrespective of bio-char weight. Given our results, it does not seem likely that in theexperimental use of biochar in field soils y would change as muchas 10%. Thus, the error in y for our test circumstances would appearto be acceptable given the fact that no other analytical method iscapable of accurately determining biochar weight. However, our

Fig. 3. Plot of actual biochar weight in sample vs. three calculated values of biocharweight in sample based on use of Eq. (5), assuming q ¼ 0.0429 and three values of y(0.861 and 90% and 110% of 0.861). The data points represent duplicate samples ofbiochar mixed in field soil at 0, 1, 2, 5 and 10% biochar by weight.

R.T. Koide et al. / Soil Biology & Biochemistry 43 (2011) 1563e1568 1567

assumptions obviously require validation for each soil and biochartype.

In some soils, 20e25 tonne biochar ha�1 have been applied withbeneficial effect (Major et al., 2010a,b; Jin et al., 2007). If we assumethat the biochar is placed in the top 15 cm of soil and that the soilhas a dry specific gravity of 1.2 g cm�3, the concentration of biocharapplied at the rate of 25 tonne ha�1 would be 1.4% of soil dryweight, well within the range of this study (0e10%). We concludethat this method, based on the “loss on ignition” methodology, iscapable of accurately determining biochar in field soils at realisticconcentrations assuming that both y and q are known.

Acknowledgments

We thank Elizabeth Reid for help in laying out the plots of thefield study and two anonymous reviewers for suggestions leadingto improvement of this manuscript. We acknowledge financialsupport from the Pennsylvania Soybean Promotion Board, theNortheast Sun Grant, the USDA NIFA competitive grants program,and the College of Agricultural Sciences and Department of Horti-culture of the Pennsylvania State University.

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