Stabilization of carbon in composts and biochars in relation to carbon sequestration and soil fertility

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    Received 18 January 2012Received in revised form 20 February 2012Accepted 26 February 2012Available online 22 March 2012

    Keywords:Organic amendments

    1. Introduction Applying organic residues, such as those resulting from compost-

    Science of the Total Environment 424 (2012) 264270

    Contents lists available at SciVerse ScienceDirect

    Science of the Tot

    j ourna l homepage: www.e lseCarbon (C) sequestration in soil has been recognized by the Inter-governmental Panel on Climate Change (IPCC) as one of the possiblemeasures through which greenhouse gas emissions can be mitigated(Follett, 2001). Strategies for increasing its sequestration in soils havebeen investigated including conservation tillage, soil amendmentswith biosolids and organic wastes, and improved crop rotation (Lal,2004; Sissoko and Kpomblekou-A, 2010). However, capitalizing onthis potential climate-change mitigation measure is not a simpletask and the issue is complicated by the fact that intensive farmingtechniques generally lead to the depletion of C from soil, thus reduc-

    ing, to agricultural land could increase the amount of C stored inthese soils and contribute signicantly to the reduction of greenhousegas emissions (Bustamante et al., 2010). Thus composting can con-tribute in a positive way to the twin objectives of restoring soil qualityand sequestering C in soils. Applications of organic residues can leadeither to a build-up of soil C over time, or a reduction in the rate atwhich organic matter is depleted from soils. Therefore the applicationof organic residues is likely to reverse the decline in soil C storage thathas occurred in recent decades, thereby contributing to the build-upin the stable C fraction in soils (De Neve et al., 2003).

    One of the problems with the use of organic residues such as com-

    ing its capacity to act as a C sink (Lal, 2009;2010).

    Corresponding author at: Centre for Environmentalation (CERAR), University of South Australia, SA 50956218; fax: +61 8 8302 3124.

    E-mail address: Nanthi.Bolan@unisa.edu.au (N.S. Bo

    0048-9697/$ see front matter 2012 Elsevier B.V. Alldoi:10.1016/j.scitotenv.2012.02.061Heavy metalsImmobilizationDecompositionMineralizable nitrogenMicrobial biomass carbonThere have been increasing interests in the conversion of organic residues into biochars in order to reduce therate of decomposition, thereby enhancing carbon (C) sequestration in soils. However energy is required toinitiate the pyrolysis process during biochar production which can also lead to the release of greenhouse gas-ses. Alternative methods can be used to stabilize C in composts and other organic residues without impactingtheir quality. The objectives of this study include: (i) to compare the rate of decomposition among variousorganic amendments and (ii) to examine the effect of clay materials on the stabilization of C in organicamendments. The decomposition of a number of organic amendments (composts and biochars) was exam-ined by monitoring the release of carbon-dioxide using respiration experiments. The results indicated thatthe rate of decomposition as measured by half life (t1/2) varied between the organic amendments and washigher in sandy soil than in clay soil. The half life value ranged from 139 days in the sandy soil and187 days in the clay soil for poultry manure compost to 9989 days for green waste biochar. Addition ofclay materials to compost decreased the rate of decomposition, thereby increasing the stabilization of C.The half life value for poultry manure compost increased from 139 days to 620, 806 and 474 days with theaddition of goethite, gibbsite and allophane, respectively. The increase in the stabilization of C with the addi-tion of clay materials may be attributed to the immobilization of C, thereby preventing it from microbial de-composition. Stabilization of C in compost using clay materials did not impact negatively the value ofcomposts in improving soil quality as measured by potentially mineralizable nitrogen and microbial biomasscarbon in soil.

    2012 Elsevier B.V. All rights reserved.Article history:a b s t r a c ta r t i c l e i n f oStabilization of carbon in composts and band soil fertility

    N.S. Bolan a,b,, A. Kunhikrishnan c, G.K. Choppala a,b,a Centre for Environmental Risk Assessment and Remediation (CERAR), University of Southb Cooperative Research Centre for Contaminants Assessment and Remediation of the Enviroc Chemical Safety Division, Department of Agro-Food Safety, National Academy of Agricultud Department of Environmental Engineering, Gyeongnam National University of Science anSanderman and Baldock,

    Risk Assessment and Remedi-, Australia. Tel.: +61 8 8302

    lan).

    rights reserved.chars in relation to carbon sequestration

    . Thangarajan a,b, J.W. Chung d

    tralia, SA 5095, Australiaent (CRC CARE), University of South Australia, SA 5095, AustraliaScience, Suwon-si, Gyeonggi-do 441-707, Republic of Koreachnology, Dongjin-ro 33, Jinju, Gyeongnam, 660-758, Republic of Korea

    al Environment

    v ie r .com/ locate /sc i totenvposts and manures as a means of terrestrial C sequestration is theirrelatively fast rate of degradation, leading to the release of carbon-dioxide, thereby becoming a source (rather than a sink) for green-house gas emission (Godbout et al., 2010). Therefore there havebeen increasing interests in the conversion of organic residues intobiochars in order to reduce the rate of decomposition, thereby en-hancing C sequestration in soils (Kookana et al., 2011). However en-ergy is required to initiate the pyrolysis process during biochar

  • production which can also lead to the release of greenhouse gassessuch as carbon-monoxide and methane (Gaunt and Lehmann,2008). Alternative methods can be used to stabilize C in compostsand other organic residues without impacting their quality in relationto improving the physical, chemical and biological fertility of soils.The objectives of this study include: (i) to compare the rate of decom-position among various organic amendments and (ii) to examine theeffect of clay materials on the stabilization of C in organic amend-ments. The fertilization value of C-stabilized composts was examinedby monitoring potentially mineralizable N (PMN) and microbial bio-mass C in soil.

    2. Materials and methods

    using poultry manure compost and poultry manure biochar. The ef-

    2.3. Decomposition experiments

    The decomposition of organic amendments was measured by mon-itoring the release of carbon-dioxide (CO2) using respiration asks. Thesoil samples were mixed with unamended or amended composts andbiochars at a rate of 50 g C kg1 soil, corresponding to approximately25 Mg organic carbon per ha to a depth of 5 cm. The soil sampleswere placed in 3-dm3 jars containing a CO2 trap (30 mL of 1 MNaOH), and a beaker containingwater tomaintain awater-saturated at-mosphere. Additional jars containing only the CO2 traps and waterserved as blanks. The jars were then incubated in the dark at 24 C.Three jars per treatment were removed at various intervals over a peri-od of 360 days for the analysis of CO2 in theNaOH trap and total C in soilsamples. After precipitation of the trapped CO2 as BaCO3 using BaCl2, theamount of evolved CO2 was determined by back-titration of unreacted

    Table 1Characteristics of soils and organic amendments used in this study.

    265N.S. Bolan et al. / Science of the Total Environment 424 (2012) 264270Soil/amendments Sand(g kg1)

    Silt(g kg1)

    Claya

    (g kg1)

    Arable soil 218 384 398Vegetable soil 246 257 497Vineyard soil 228 138 634Turf soil 324 197 479Copper mine soil 387 235 378Shooting range soil 487 123 390Poultry manure compost Cow manure compost Poultry manure biochar Green waste biochar

    afect of stabilizing agents on the decomposition of poultry manureand poultry manure compost was examined using the vegetablesoil. The effect of unamended and amended poultry manure on mi-crobial biomass carbon and potential mineralizable nitrogen was ex-amined using the arable soil.

    2.2. Stabilization of carbon

    The stabilization of C in both composts and biochars was exam-ined by co-composting these samples in the presence of various claymaterials that include goethite, gibbsite and allophane. Around 1 kgof poultry manure compost or poultry manure biochar were mixedwith these amendments at the rate of 50 g kg1 and incubated inan aerobic digester supplied with air at 1.44 L min1 at a moisturecontent of 30% (w/w) in a temperature controlled room (37 C) for3 months. A control sample was also incubated in the absence ofthese amendments. We used chemical fractionation of C in compostsand biochars to monitor C stabilization (Bhupinderpal-Singh et al.,2004).2.1. Soil and organic amendments

    Four surface (015 cm depth) soil samples one each from differ-ent land-use practices (vineyard, vegetable cultivation, arable crop-ping and sports turf) were used in this study. To examine the effectof metal contamination on decomposition of organic amendments, acopper mine soil and a shooting range soil were also used. The or-ganic amendments used in this study include two composts (poultrymanure and cow manure composts) and two biochars (poultry ma-nure biochar and green waste biochar) (Table 1). Synthetic claysamples of goethite [(FeO(OH))], gibbsite [Al(OH)3] and allophane[(Al2O3)(SiO2)1.32.5(H2O)] were used to examine their effect onthe stabilization of C in composts and biochars.

    The effect of soil type on carbon decomposition was examinedMajor clay minerals include kaolinite, chlorite, illite and feldspar.NaOH using 0.3 M HCl. The amount of C released was calculated usingthe following equation (Bloem et al., 2006):

    R MWC VbVs M 0:5=DW 1

    where R is the amount of C released (g C kg1 soil), MWC is themolec-ularweight of C (12 g mol1), Vb is the volume ofHCl for blank titration(L), Vs is the volume of HCl for sample titration (L), M is the concentra-tion of HCl (0.3 M), DW is the dry weight of the soil (kg), and 0.5 is thefactor to account for the fact that 2 mol of OH is consumed by 1 mol ofCO2.

    2.4. Fractionation of soil carbon

    Fractionation of C in both unamended and amended poultry ma-nure compost and biochar was measured to examine the effect of Cstabilization using various clay materials on the redistribution of C.The procedure used to fractionate soil C sequentially is similar tothat of Bhupinderpal-Singh et al. (2004) for soil sulfur and C. This se-quential fractionation technique was found suitable for fractionatingsulfur and phosphorus into various pools in soils (Hedley et al.,1982). The initial design of the fractionation was to determine rstthe exchangeable ionic forms of C in the organic amendmentsextracted on to ion exchange resins (resin-C). Solid-phase non-exchangeable forms of C were extracted in alkali extractions of twostrengths. Hence, conceptually labile organic C fraction would beextracted by 0.1 M NaOH and less labile fractions extracted by 1 MNaOH (Hedley et al., 1982). The C fractions not extracted with eitherresin or the 0.1 M NaOH extractant were termed as the recalcitrant orchemically stable fraction. Stabilization of C in organic amendmentsusing this fractionation technique was also compared with the rela-tionship between pyrophosphate extractable C and Fe and Al(Matus et al., 2008).

    pH Total C(g kg1)

    Total N(g kg1)

    Total Cu(mg kg1)

    Total Pb(mg kg1)

    6.14 7.21 0.812 21.53 4.356.78 10.5 1.12 17.82 2.127.28 6.12 0.721 175.2 3.237.34 11.2 0.634 10.31 1.675.67 3.21 0.123 450.2 586.66.12 4.56 0.134 157.4 346.78.12 301 23.5 10.42 3.017.87 367 17.8 7.32 1.568.17 456 2.25 3.45 2.348.67 576 1.56 2.27 0.57

  • forms of C in organic amendments and soils (Marschner et al.,2008). Both physical and chemical fractionation studies involving or-ganic residues have demonstrated that labile C fractions decomposefaster than non-labile C fractions (Bernal et al., 1998). The physicaland chemical structures including surface area, condensation grade

    0 100 200 300 400Incubation period (days)

    0

    20

    40

    60

    Carb

    on re

    mai

    ning

    (g/kg

    soil)

    y=60e-7e-05xR2=0.864

    y=60e-2E-04xR2=0.922

    y=60e-0.005xR2=0.741

    y=60e-0.009xR2=0.795

    Perc

    enta

    ge o

    f tot

    al c

    arbo

    n

    0%

    20%

    40%

    60%

    80%

    100%

    120%

    Resin0.1 M NaOH1 M NaOHResidual

    PM CM BPM BGW

    aa

    b b

    a a

    b

    ab

    ba a

    Fig. 2. Carbon fractions in poultry manure compost (PM), cow manure compost (CM),poultry manure biochar (BPM) and green waste biochar (BGW). Within various frac-

    266 N.S. Bolan et al. / Science of the Total Environment 424 (2012) 2642702.5. Microbial biomass carbon

    Microbial biomass C and PMN in the arable soil treated withamended and unamended poultry manure compost were measuredto examine the effect of C stabilization using various amendmentson the fertilization value of composts. Microbial biomass was mea-sured using the chloroform-fumigation extraction method (Vance etal., 1987). The arable cropping soil was incubated with unamendedor amended poultry manure composts at eld capacity for 14 days.To measure microbial biomass C, 10 g of incubated soil sampleswere fumigated with ethanol-free CHCl3 in a desiccator. The fumigat-ed and non-fumigated samples were extracted using 0.5 M K2SO4 andthe extract was ltered and analyzed for dissolved organic carbon(DOC) using a TOC analyzer. Microbial biomass C was calculatedusing the following equation:

    Microbial biomass C mg=kg EC=KC 2

    where EC=(organic C extracted from fumigated soils (mg/kg)organic C extracted from non-fumigated soils (mg/kg)) andKC=0.45, value which represents the efciency of organic C extrac-tion (Jenkinson and Ladd, 1981).

    2.6. Potentially mineralizable nitrogen

    Potentially mineralizable N as determined by anaerobic incuba-tion is often considered to reect the organic matter pools being min-eralized (Keeney and Bremner, 1996). Anaerobic mineralizable N isdetermined as the difference between the ammonium (NH4+) ionsmeasured in two sets of 2 M KCl extracts. One is extracted immediate-ly (1:10 soil: extractant, 1 h shaking) and the other in the same man-ner after a seven-day incubation at 40 C with the soil covered bywater. The concentration of NH4+ ions in the 2 M KCl extracts wasmeasured following the method proposed by Mulvaney (1996).

    2.7. Analysis of data

    The rst-order decay rate equation was used to calculate the de-composition rate of C:

    N N0exp kt 3

    where N0 is the initial amount of C recovered at day 0 (g C/kg soil), Nis the concentration of residual C in the soil at that instant in time(g C/kg soil), t is time (day) and k is the rst-order decay rate con-stant. The C half-life (t1/2=time (day) taken to reduce C concentra-tion to half of the initial value) was calculated from the rateconstant (k).

    All experiments were conducted with three replicates. The datacollected were analyzed statistically using SPSS 17 software. Duncan'smultiple range test was used to compare the means of the treatments,variability in the data was expressed as the standard deviation, and aPb0.05 was considered to be statistically signicant.

    3. Results and discussion

    3.1. Decomposition of organic amendments

    The results indicated that the rate of decomposition as measuredby half life (t1/2) varied between the organic amendments (Fig. 1).The rate of decomposition of composts was much higher than bio-chars, with t1/2 values ranging from 139 days (poultry manure com-post) to 9989 days (green waste biochar). It has often been noticedthat the rate of decomposition is much higher for manures and com-posts than biochars (Steinbeiss et al., 2009; Wardle et al., 2008)

    which has been attributed to the difference in the nature of C in theorganic amendments. Flavel and Murphy (2006) noticed that C min-eralization varied between various organic amendments that includepoultry manure, green waste compost, straw compost and vermicom-post which they attributed to the differences in C quality parametersas measured by 13C NMR spectroscopy. Steinbeiss et al. (2009) no-ticed that the mean residence times for biochars varied between 4and 29 years depending on soil type and quality of biochar.Zimmerman (2010) noticed that the rate of degradation of biocharswas controlled more strongly by combustion temperature and dura-tion than the source material.

    The sequential C fractionation analysis of the unamended com-posts and biochars indicated that while most of the C in the compostsremained in the labile fractions (resin and 0.1 M NaOH), the majorityof C in biochar samples remained as non-labile residual fraction(Fig. 2). It is important to recognize that these extraction proceduresrepresent only arbitrary partitioning among different forms (i.e., op-erational in nature), since reagents are not completely selective forone particular form. Hence a suite of physical and chemical fraction-ation, and spectroscopic techniques are required to identify various

    Fig. 1. Rate of decomposition of poultry manure compost (PM: ), cow manure com-post (CM: ), poultry manure biochar (BPM: ) and green waste biochar (BGW: ) invegetable soil. Each value represents the mean of three replicates with standard devi-ation shown by error bars.tions, means indicated by a common letter are not signicantly different at the 5% level.

  • and particle size of biochars control their stability in soils (Wardle etal., 2008).

    3.2. Effect of soil type

    The rate of decomposition of poultry manure compost was slightlyless in the vineyard soil than soils from other land use practices(Fig. 3). There was no signicant effect of soil type on the rate of de-composition of poultry manure biochar. The vineyard soil contains aslightly higher level of Cu than other soils (Table 1) resulting fromregular application of Cu fungicides, which may be one of the reasonsfor the decrease in the decomposition of composts. The vineyard soilis also heavy textured containing more clay particles than other soils.Depending on the microbial activity of soils, organic amendmentstend to decompose slower in clay soils than sandy soils which hasbeen attributed to greater immobilization or occlusion of C andlower aeration in the former soils (Khalil et al., 2005). Sissoko andKpomblekou-A (2010) noticed that the t1/2 values for broiler litterranged from 18 to 693 days for a range of soils which they attributedto the difference in texture and microbial activity between the soils.With the same input of organic material, clay soils usually accumulatemore organic matter than sandy soils (Laganire et al., 2010). The sta-

    MANURES BIOCHAR

    Carb

    on re

    leas

    ed (g

    /kg so

    il)

    0

    20

    40

    60

    PM-C PM-SR PM-CM BPM-C BPM-SR BPM-CM

    a

    b

    c

    a

    139

    170

    383

    a a3465 3889 4912

    Fig. 4. Decomposition of poultry manure compost (PM) and poultry manure biochar(BPM) in uncontaminated vegetable soil (C), shooting range soil (SR) and Cu minesoil (CM). The number above the bar indicates the half life (t1/2) in days. Within ma-nures and biochar, means indicated by a common letter are not signicantly differentat the 5% level.

    MANURES BIOCHAR

    Carb

    on re

    leas

    ed (g

    /kg so

    il)

    0

    20

    40

    60

    PM-C BPM-C BPM-Cl BPM-Fe BPM-AlPM-AlPM-FePM-Cl

    139

    474620

    806

    3465 3856 4012 5120

    a

    bb

    c

    a a a a

    Fig. 5. Decomposition of poultry manure compost (PM) and poultry manure biochar(BPM) in the absence (C) and presence of goethite (Fe), gibbsite (Al) and allophaneclay (Cl) in vegetable soil. The number above the bar indicates the half life (t1/2) indays. Within manures and biochar, means indicated by a common letter are not signif-

    267N.S. Bolan et al. / Science of the Total Environment 424 (2012) 264270bilizing effect of organic matter in soils has been ascribed to adsorp-tion of organics onto surfaces such as clays, encapsulation betweenclay particles or entrapment in small pores in aggregates inaccessibleto microbes (Oades, 1989). In sandy soils, microorganisms have moreaccess to organic matter than in heavy textured soils where microbialaccess is limited because of sorption of organic C onto soil minerals(Sissoko and Kpomblekou-A, 2010).

    3.3. Effect of soil contamination

    The rate of decomposition was less in the Cu mine and shootingrange soils than the uncontaminated soil and the effect was morepronounced in the case of poultry manure composts than biochar(Fig. 4). Metal contamination inhibits microbial activity, thereby af-fecting the decomposition of organic matter (Giller et al., 1998). Forexample, Sauve (2006) and Zeng et al. (2006) noticed a signicantdecrease in soil microbial activity when the level of Cu and Zn in-creased above 285 and 500 mg/kg soil, respectively. Similarly,Clemente et al. (2006) have shown that C mineralization of freshand composted cow manure was less in Zn and Pb contaminatedsoils than uncontaminated soils which they attributed to the

    MANURES

    Carb

    on re

    leas

    ed (g

    /kg so

    il)

    0

    20

    40

    60

    BIOCHARPM-V PM-W PM-A PM-T BPM-V BPM-W BPM-A BPM-T

    139

    187

    152155a

    b

    aa

    3465 3670 3851 3520a a aa

    Fig. 3. Decomposition of poultry manure compost (PM) and poultry manure biochar(BPM) in vegetable soil (V), vineyard soil (W), arable soil (A) and turf soil (T). Thenumber above the bar indicates the half life (t1/2) in days. Within manures and biochar,

    means indicated by a common letter are not signicantly different at the 5% level.adsorption of these metals to manures thereby inhibiting the releaseof C. It has also been shown that a number of metals including Cu andPb form both soluble and insoluble complexes with organic matter,thereby inhibiting the supply of free C for decomposition (Bolan etal., 2011; Kunhikrishnan et al., 2011).

    3.4. Effect of stabilizing agents

    The addition of stabilizing agents, goethite, gibbsite and allophaneclay decreased the rate of decomposition of poultry manure compost,and the effect was less pronounced in the case of poultry manure bio-char (Fig. 5). The decrease in the rate of decomposition in compostsmay be attributed to the difference in the redistribution of C fractionsbetween the unamended and amended samples (Fig. 6). In the ab-sence of these stabilizing agents, most of the C remained in the labilefractions (resin and 0.1 M NaOH). However, in the presence of stabi-lizing agents, there was an increase in the non-labile and residualfractions which are less available for decomposition. There was a pos-itive correlation between pyrophosphate extractable C and Fe and Alicantly different at the 5% level.

  • microbial growth (Ros et al., 2006). The results indicate that stabiliza-tion of C using goethite, gibbsite or allophane clay is unlikely to affectthe microbial growth in soils.

    BIOCHAR

    of 60%

    Resin0.1 M NaOH1 M NaOHResidual

    BPM-C BPM-Fe BPM-Al BPM-Cl

    a a a a

    a a a a

    ar (BPM) in the absence (C) and presence of goethite (Fe), gibbsite (Al) and allophane claydifferent at the 5% level.

    mas

    s ca

    rbon

    (mg/k

    g)

    400

    600

    800

    1000a)b b b b

    268 N.S. Bolan et al. / Science of the Total Environment 424 (2012) 264270in the clay amended compost samples (data not shown) indicatingthe formation of Fe and Alorganic matter complexes (Matus et al.,2008). It has been shown that Fe and Al oxides and allophanic claysimmobilize soil C, thereby preventing it from microbial decomposi-tion (Day et al., 1994; Wada, 1985). For example, Sikora (2004) no-ticed that while the addition of water treatment residue containinghigh Al content did not affect the decomposition of soil organic mat-ter, iron-rich residue addition decreased the rate of decompositionwhich they attributed to a decrease in soil microbial activity. Scheelet al. (2008) and Schwesig et al. (2003) have shown that Al decreasesthe biodegradability of DOC thereby increasing C stabilization whichthey attributed mainly to the reduced bioavailability of DOC afterAl-induced precipitation.

    Allophane clays have been shown to form stable organicmineralcomplexes through inner-sphere ligand-exchange reactions which,together with physical protection mechanisms, would play a role inincreasing the mean residence time of different C forms in soils(Dahlgren et al., 2004). Both the equilibrium organic matter contentsand its mean residence time in Andosols are often found to be much

    MANURES

    Perc

    enta

    ge

    0%

    20%

    40%

    PM-C PM-Fe PM-Al PM-Cl

    a

    a

    a

    b

    b

    b

    b

    b

    b

    b

    b

    b

    Fig. 6. Distribution of carbon in poultry manure compost (PM) and poultry manure bioch(Cl). Within various fractions, means indicated by a common letter are not signicantly tota

    l car

    bon

    80%

    100%

    120%

    a b b bhigher than for any other soil type (Partt, 2009). A large organicmatter content is common in volcanic ash soils (Broquen et al.,2005) and preservation of organic matter in these soils is inuencedby a number of processes including: (i) protection of organic matterby complexation with iron, aluminum and allophane; (ii) reducedbacterial activity that results from the presence of free iron and alu-minum; (iii) low pH and poor availability of nutrients, especiallyphosphorus to soil microorganisms involved in organic matter degra-dation (Partt, 2009). Soil microaggregates play an important role inthe long-term sequestration of C because they can protect soil organicmatter against decomposition (Lal, 2009). This suggests that allopha-nic clays in volcanic ash soils play an important role in C stabilization.By forming very stable allophaneorganic matter complexes, thesesoils also have a large capacity for sequestering C.

    3.5. Microbial biomass carbon

    Microbial biomass C as measured by the fumigation techniqueranged from 98.6 to 887 mg kg1 (Fig. 7). There was an increase inmicrobial biomass C in the compost amended arable soil over the con-trol soil. But there was no signicant difference in microbial biomassC between any of the compost treatments. Compost addition in-creases the microbial biomass C by providing increased substrate foricro

    bial

    bio

    200M

    0

    b)Soil alone PM PM-Cl PM-Fe PM-Al

    Soil alone PM PM-Cl PM-Fe PM-Al

    a

    Pote

    ntia

    lly m

    iner

    aliza

    ble

    nitro

    gen

    (mg/k

    g)

    0

    50

    100

    150

    200

    250

    300

    a

    b bb

    c

    Fig. 7. Effect of poultry manure (PM) compost addition on (a) soil microbial biomassand (b) potentially mineralizable nitrogen (PMN) in arable soil. Means indicated by acommon letter are not signicantly different at the 5% level.

  • Cl

    po

    ch

    n rat

    A

    such

    269N.S. Bolan et al. / Science of the Total Environment 424 (2012) 2642703.6. Potentially mineralizable nitrogen (PMN)

    The PMN for the control soil was 33.5 mg kg1 which is about4.13% of total N in soil. The addition of both the unamended andamended composts increased the PMN value in soil (Fig. 7). ThePMN values for compost amended soils ranged from 19.9 to 24.6%of the total N input from both composts and soil. However, therewas a slight decrease in mineralizable N in soils treated with compostin the presence of gibbsite. Presence of free Al3+ ions in soil solutionhas been shown to inhibit various microbial functions including themineralization of organic N in soils (Giller et al., 1998). It has often

    +

    Com

    Bio

    Organic residues

    CarboSequest

    Fig. 8. Schematic diagram illustrating the role of clay mineralsbeen shown that the addition of organic amendments increases thePMN value in soils and the change in PMN depends on both the qual-ity and quantity of organic amendment addition (Chae and Tabatabai,1986; Jin et al., 2011). The results indicate that stabilization of C usinggoethite or allophane clay is unlikely to affect the fertilization value ofcomposts as a N source.

    4. Conclusions

    Biochars are found to be more stable than composts and can beused to enhance C sequestration in soils. There have been increasinginterests in examining the value of biochar in improving soil qualityin terms of physical, chemical and biological fertility. However, thelong-term C storage function of biochars contradicts their functionin relation to improving soil quality that requires a certain biodegrad-ability of the biochar materials. Co-composting of organic amend-ments with clay materials is effective in the stabilization of C in soil(Fig. 8). Similarly the application of compost to soils containing highclay content is likely to achieve a greater C stabilization. Stabilizationof C in composts using clay materials not only maintains their value inimproving soil quality as measured by potentially mineralizable Nand microbial biomass C but also would add to the long-term soil Cpool. However, the addition of clay materials to composts to enhanceC stabilization depends on the cost and availability of these clay ma-terials. Allophane is cheap and abundant in many countries includingNew Zealand, and can be obtained in large quantities from depositswhich are readily accessible. It is also safe to use allophane becauseit is a natural material and does not require extensive chemicalmodication before usage. Similarly, brown ochre can be used as anatural source of iron oxide to enhance C stabilization in organicwastes.

    Acknowledgments

    This research was supported by the Co-operative Research Centrefor Contamination Assessment and Remediation of the Environment(CRC CARE) and the University of South Australia. This study wasalso supported by the Ministry of Education, Science and Technology(MEST) and the Ministry of Knowledge Economy (MKE), Republic of

    ay

    st

    Soil

    ar

    CO2

    ion

    Biochar

    llophane

    as allophane in enhancing the stabilization of C in composts.Korea through Gyeongnam National University of Science and Tech-nology as a Hub University for Industrial Collaboration (HUNIC).

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    Stabilization of carbon in composts and biochars in relation to carbon sequestration and soil fertility1. Introduction2. Materials and methods2.1. Soil and organic amendments2.2. Stabilization of carbon2.3. Decomposition experiments2.4. Fractionation of soil carbon2.5. Microbial biomass carbon2.6. Potentially mineralizable nitrogen2.7. Analysis of data

    3. Results and discussion3.1. Decomposition of organic amendments3.2. Effect of soil type3.3. Effect of soil contamination3.4. Effect of stabilizing agents3.5. Microbial biomass carbon3.6. Potentially mineralizable nitrogen (PMN)

    4. ConclusionsAcknowledgmentsReferences

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