REGULAR ARTICLE

Biochar from Miscanthus: a potential silicon fertilizer

David Houben & Philippe Sonnet &Jean-Thomas Cornelis

Received: 26 March 2013 /Accepted: 22 August 2013 /Published online: 9 October 2013# Springer Science+Business Media Dordrecht 2013

AbstractBackground and aims Silicon (Si) is largely recognizedto improve plant growth subjected to various biotic andabiotic stresses. As plants accumulate Si in the form ofreadily-soluble phytolith, we examine the possibility ofusing phytolith-rich biochar as a bio-available Si sourcefor increasing the agronomical productivity of Si high-accumulator plants while augmenting soil fertility and Csequestration.Methods By adding three different biochars (Miscanthusx giganteus straws, coffee husks and woody material) attwo different concentrations (1 % and 3 %; w/w) to soilsamples, we investigated the effects on the soil respira-tion, the chemical characteristics and the kinetic releaseof bio-available Si (CaCl2-extractable Si).Results Here we show that the biochar fromMiscanthusstraws was the most attractive amendment. Its incorpora-tion at a 3 % rate improved the soil fertility param-eters (pH and available cations) and combined thehighest mean residence time of carbon (C) in soil(MRT=50 years) with the highest rate of release of

bio-available Si. We attribute this result to thepresence of phytoliths in this biochar, as revealedby SEM-EDS analysis.Conclusions Not only did the biochar fromMiscanthusenhance both soil C sequestration and fertility, but theresults of this study suggest that it can also be consideredas a potential source of bio-available Si. Although ourconclusions should be substantiated in the field, wesuggest that Miscanthus biochar could be used as apotential source of bio-available silicon for the cultureof such crop as Si-accumulator plants growing, forinstance, in highly weathered tropical soils with lowcontent in carbon, nutrients and bio-available Si.

Keywords Biochar . Silicon . Phytoliths . Fertilizer . -

Carbon sequestration

Introduction

The ubiquity of charcoal in soil and terrestrial sedimentand the subsequent coupling with the oceanic environ-ment play a key role in a wide range of biogeochemicalprocesses (Schmidt and Noack 2000). Through itschemical composition and surface chemistry (Baldockand Smernik 2002; Cheng et al. 2008a), charcoal im-pacts the global carbon (C) cycle (Kuhlbusch 1998;Crutzen and Andreae 1990), the residence time of soilorganic carbon (SOC) (Skjemstad et al. 1996; Schmidtet al. 1999) and the mobility of both organic and

Plant Soil (2014) 374:871882DOI 10.1007/s11104-013-1885-8

Responsible Editor: Yong Chao Liang.

D. Houben : P. Sonnet : J.

inorganic pollutants in the Earths Critical Zone (Brndliet al. 2008; Beesley et al. 2011; Houben et al. 2013, inpress). Moreover, charcoal may greatly ameliorate thephysical and chemical properties of highly weatheredtropical soils (Glaser et al. 2002) characterized by theirvery low content in primary minerals and a high rate ofsoil organic matter mineralization (Tiessen et al. 1994;Zech et al. 1997). The pyrolysed biomass used for soilapplication is commonly named biochar (Lehmann andJoseph 2009). Biochar is usually considered a win-winsolution in terms of sustainable agriculture and the glob-al C cycle (Laird 2008). In comparison to burning(Crutzen and Andreae 1990), the conversion of largequantities of organic matter into stable C pools bypyrolysis is assumed to be an efficient process to act asa sink for atmospheric CO2 in the short-term atmo-, bio-,pedospheric C cycle (Lehmann 2007b; Powlson et al.2011). On the other hand, biochar is increasingly recog-nized as a soil conditioner because its incorporation hasbeen reported to enhance nutrient balance in the soil-plant system by supplying and, more importantly,retaining nutrients (Glaser et al. 2002; Lehmann et al.2003). However, the residence time of C and the soilconditioner role of biochar are dependent on pyrolysisconditions and the feedstock used for biochar produc-tion. Moreover, the evaluation of biochar for in situapplication is limited by the scarcity of long-term,large-scale field studies. As a result, potentiallypromising biochars need to be carefully identified priorto being implemented for field trials (Brewer et al.2011).

Whereas the impact of biochar incorporation on soilproperties is the object of numerous publications, itspotential ability to enrich the soil in bio-available silicon(Si) has still not been the subject of characterization.This is particularly surprising given increasing evidenceof the beneficial effects of Si for higher plants (rice,wheat, barley, banana, sugarcane) growing under vari-ous biotic and abiotic stresses (Korndrfer and Lepsch2001; Ma 2004; Guntzer et al. 2012). Acting as resistantstructural components, Si deposits provide a better up-right position which promotes photosynthesis and createa hard outer layer that serves as a defense against fungaland insect attacks (Epstein 1994; Marschner 1995;Blanger et al. 2003; Vermeire et al. 2011). It is alsorecognized that Si can alleviate the phytotoxicity of Aland other metal ions such as Mn and Cd (Hodson andEvans 1995; Liang et al. 2007; Vatehov et al. 2012).Although limited under optimum growth conditions,

such beneficial Si effects are obvious under stress con-ditions (Epstein 1994). Therefore, in order to reduce theseverity of several economically important diseases inbarley, corn, cucumber, grape, rice, strawberry andwheat, Si application as soil amendment has beenrecommended for maximum production in highlyweathered soils (Datnoff et al. 2001, 2007). Todate, traditional Si amendments consist mainly ofsilicate slag and natural silicate minerals. However,though they are cost-effective because they arederived from by-products of furnaces, silicate slagsoften contain only a small proportion of easilysoluble Si (Gascho 2001) and may present hazard-ous levels of heavy metals (Berthelsen et al.2001). On the other hand, silicate minerals suchas wollastonite (CaSiO3) are characterized by highprice and limited mineral reserves and dont havethe ability to combine beneficial effects other thanSi amendments such as the increase of soil fertilityand storage of soil organic carbon. More sustain-able alternatives are thus needed.

Silicon is taken up by plants and mainly deposited intranspiration sites where polymerization of hydratedamorphous silica occurs to form phytoliths (SiO2.nH2O)(Epstein 1999; Marschner 1995). Because disappearanceof the amorphous structure of phytoliths occurs above500800 C (Krull et al. 2003), pyrolyzed biomass belowthis temperature should still contain unmodifiedphytoliths. Since the solubility of amorphous silica suchas phytoliths is higher (1.82 mM Si) than its crystallinecounterpart quartz (0.10.25 mM Si) (Drees et al. 1989;Fraysse et al. 2006), their presence in plant-derivedbiochar could render biochar a potential bio-available silicon source. Therefore, biochar couldturn out to be a promising cost-effective and anenvironmentally friendly alternative to convention-al Si amendments. The importance of the questionas to whether biochar can successfully provide bio-available Si is especially meaningful for soil intensivelyused for cropping Si high-accumulator plants, such aswheat and rice, as these species represent the premiercrops for the nutrition of mankind but are susceptible toa variety of diseases and insect pests if the Si supply islow (Epstein 2001).

In this study, we investigated the effects of threeplant-derived biochar amendments on soil fertility, re-spiratory C losses from soil and release of bio-availableSi in soil solution. The final goal was to determinewhether biochar could be used as an amendment for

872 Plant Soil (2014) 374:871882

Si high-accumulator plants while increasing the soilfertility and C sequestration.

Materials and methods

Soil

The forest soil used as the substrate in this study (argicBt horizon) originates from the loessic silt belt in centralBelgium. This well-drained and acidic (pH 3.84.3) soilis classified as a Luvisol with an argic horizon occurringat 25 cm depth. The carbon content of the argic horizonis 0.43 %. The cation exchange capacity of the argic Bthorizon is below 10 cmolc/kg and mainly influenced byAl. The soil was dried at ambient temperature, sievedthrough a 2-mm plastic sieve and then stored in the darkat 20 C prior to use.

Biochar

Commercial grade biochars were obtained by PyregGmbH (Drth, Germany) using industrial pyrolysisreactors. The feedstocks for the biochars were coffeehusks (BC), Miscanthus (Miscanthus x giganteus)straws (BM) and woody material from a woodchipproduction (BW). Operating conditions of productionwere similar for the three biochars and consisted of aresidence time in the reactor of 30 min and an endtemperature of pyrolysis of 600 C.

The concentrations of major elements in the biocharswere determined after calcinations at 950 C followedby borate fusion (Chao and Sanzolone 1992) (Table 1).Briefly, a crushed sample of 150 mg of the ignitionresidue was melted at 1,000 C in a graphite cruciblein the presence of 0.4 g Li-tetraborate and 1.6 g Li-metaborate. After dissolution of fusion beads in 10 %HNO3, elemental contents were quantified by inductivelycoupled plasma atomic emission spectrometry (ICP-AES; Thermo Jarrell Ash, IRIS Advantage). Total C

and N contents were measured on ground biochar sam-ples by dry combustion with a CNS analyser (Flash EA1112 Series). The pH of each biochar was measuredpotentiometrically in a 5:25 (W/V) suspension of deion-ized H2O (Table 1).

Biochar particles were observed and analyzed semi-quantitatively using a field emission gun scanningelectron microscope (FEG-SEM; Zeiss Ultra55) andan in-lens secondary electron detector. The FEG-SEM was equipped with an energy-dispersive X-rayspectrometer (EDS) for element detection and elementmapping (silicon drift detector, Quantax, Bruker).Voltage was set at 20 kV. Image and element rastermaps were imaged using Esprit software (Bruker).

Substrate preparation

The mixtures were prepared and homogenized by thor-oughly mixing the soil with 1 % and 3 % (w/w) of eachbiochar in plastic containers. The water holding capac-ity of soil-biochar mixtures was placed at the fieldcapacity prior to the soil respiration experiment.

Soil respiration measurements

The soil respiration was measured by incubating anamount of moist soil-biochar mixture equivalent to 20 gof dry mixture in hermetic flasks kept in a dark room at20 C. The experiment was conducted in three replicatesand lasted 76 days. Flasks with untreated soil andwithout soil samples served as controls and blanks,respectively. A 30-ml vial was filled with 25 ml of0.497 M NaOH solution and inserted inside soil-incubation chambers for capturing the released CO2 fromrespiration. Soil-derived CO2-C was then calculated ev-ery 47 days using the alkali absorption/conductivitymethod (Rodella and Saboya 1999). The electrical con-ductivity of the NaOH solution inside incubation cham-ber (cx), of the standard 0.497 MNaOH solution (c1) andof the standard 0.248 M Na2CO3 solution (c2) were

Table 1 Chemical composition of biochar from coffee husk (BC), woody material (BW) and Miscanthus straw (BM)

pH Al Ca Fe K Mg Mn Na P Si C N C/Ng kg1 % %

BC 10.1 7.0 18.3 5.3 6.2 6.2 0.4 1.6 3.9 29.8 76.6 0.7 103BW 10.7 11.0 59.0 14.6 18.0 5.1 1.8 2.8 4.0 125.0 66.4 2.5 27BM 10.1 7.1 8.3 2.1 10.6 3.5 0.5 1.9 2.5 38.3 52.8 0.3 189

Plant Soil (2014) 374:871882 873

used to estimate the mass of absorbed CO2 by theexpression:

mg CO2 22 c1cxc1c2

V C

where V is the volume (ml) of the standard NaOH solu-tion and C is its concentration expressed in mol l1 (M).

Similar to Steinbeiss et al. (2009) and Bolan et al.(2012), the decay rate of C in soils (k) was calculatedusing the first-order decay rate equation:

N N0exp kt where N0 is the initial amount of C (mg g

1 soil), N isthe concentration of remaining C in the soil at time t(i.e. N0 minus cumulative CO2-C amount emitted attime t; mg g1 soil), t is time (year) and k is the first-order decay rate constant. Mean residence time of C insoil (MRT) is defined as the inverse of the decay rate(1/k) (Olson 1963). Goodness of fit was assessed usingcoefficients of determination (R2) obtained by the leastsquare regression of measured versus predicted Nvalues.

Physico-chemical characterization

At the end of the incubation period, soils were air-driedat ambient temperature. The pH was then measuredpotentiometrically in a 5:25 (W/V) suspension of de-ionized H2O. The cation exchange capacity (CEC))and the base saturation (%BS) of the exchange com-plex (((Ca, Mg, K, Na)*100)/CEC) were assessed byleaching the soil with 1 M ammonium acetate at pH 7(for Ca2+, Mg2+, K+ and Na+) and then 1 M KCl (forCEC) (Chapman 1965).

Bio-available Si evaluation

The pool of so-called bio-available Si, i.e. silicic acidin the aqueous phase, was assessed by 5-hour CaCl2extraction (0.01 M, 1:10 W/V) (Haysom and Chapman1975) on air-dried soil samples. The amount of Sireleased in the extract represents the immediatelyavailable Si fraction of the readily soluble Si pool(Berthelsen et al. 2001; Sauer et al. 2006; Corneliset al. 2011; Haynes et al. 2013). At the end of theextraction, the supernatant was separated from thesolid residue by centrifugation (3,000g; 10 min) foranalysis. The remaining extract was decanted and the

residue was re-extracted. A total of six successiveextractions were conducted at 202 C and the con-centration of Si in each extract was analyzed by ICP-AES. This procedure was designed to investigate theability of soils (i.e., untreated soil and soil-biocharmixture) to replenish the bio-available Si pool. Therelease of Si with time was fitted by the linear relation-ship of the kinetic Elovich equation (Chien andClayton 1980; Jalali 2006):

qt a b*ln t where qt is the cumulative amount of Si released at timet, t is the time of release, a and b are constants and denotethe intercept and the slope of the curve respectively. Theconstant b is an index of the rate of Si release from soils.Goodness of fit was assessed using coefficients of de-termination (R2) obtained by the least square regressionof measured versus predicted qt values.

Statistical analysis

Average results for the different treatments were com-pared using one-way analysis of variance (ANOVA)followed by Fishers test (p

(Table 2). The exchangeable complex of the biochar-amended soils was more saturated in Ca2+ and Mg2+,implying a base saturation (%BS) up to five fold higherafter addition of biochar (Table 2).

At the completion of the incubation experiment(76 days), the amounts of CO2 released by the untreatedsoil and soil-biochar mixtures were not significantlydifferent (p

determine not only the current bio-available Si pool butalso the soils ability to replenish it. The contrastingimpacts of biochar addition on Si released by succes-sive extractions are depicted in Fig. 3. Whereas thecumulative Si amount released in BM-3 % treatmentwas higher relative to the untreated soil, the cumulativeSi amounts released by BW-3 % and, to a lesser extent,by BC-1 % were lower. According to the total amountof Si released after six successive extractions and re-sults of Fishers test for multiple comparisons (meanfollowed by same letter did not differ significantly atthe 5 % level; p BW-1 % (60.3b mg kg1) BC-3 % (58.6bc mg kg1) = BM-1 % (57.9bc mg kg1) Soil (54.4 cd mg kg1) BC-1 % (49.7de mg kg1) BW-3 % (47.9e mg kg1).

Discussion

Impact of biochar on soil fertility and respiration

Alkalinizing effects of biochar amendments (BW >BC = BM) were consistent with their respective initialpH (Table 1). The applications of biochar containing

Fig. 2 Cumulative carbon(C) mineralized per g of Cin soil (mg CO2-C.gC

1)during 76 days in theuntreated soil and in the soilamended with 1 % or 3 %of biochar produced fromcoffee grounds (BC), woodymaterials (BW) andMiscanthus straws (BM).Each point represents theaverage of three replicates

Fig. 3 Cumulative amountof Si released over time bysuccessive extractions with0.01 M CaCl2. Each pointrepresents the average ofthree replicates. The stan-dard deviation is taken intoaccount in the symbol sizes

876 Plant Soil (2014) 374:871882

ash add free bases to the soil solution and thereforeincrease the soil pH (Glaser et al. 2002). According toYuan et al. (2011), biochar is more effective in amelio-rating soil acidity than its plant feedstock because theN from biochar is less accessible for microorganismsand subsequent NH4

+ nitrification is therefore lower.As observed in our study, CEC measured by Novaket al. (2009) at the end of a 67-day incubation periodwas not significantly higher in the presence of 0.52 %pecan shell-based biochar compared to the untreatedsoil. The absence of any significant CEC increase waslikely because oxidation of these freshly made biocharswas not sufficiently advanced to create the negativesurface charge traditionally observed in aged biochar(Cheng et al. 2006). The enhancement of nutrientavailability after biochar application (increase of basesaturation, %BS) can be attributed to the presence ofthese nutrients (Ca, Mg and K) in the ash componentsof the biochar (Laird et al. 2010; Schulz and Glaser2012) which decreases the Al saturation of the soilcomplex exchange. Our data show the positive impactof biochar on soil fertility as suggested in Glaser et al.(2002). Ash in biochar rapidly releases free bases suchas Ca, Mg and K to the soil solution thereby not onlyincreasing soil pH but also providing readily availablenutrients for plant growth. However, it must be notedthat the exchangeable Mg content in the BW-3 % treat-ment was significantly lower than in the BW-1 %treatment (Table 1). This might be due to the high inputof Ca with BW-3 %, as indicated by the significantincrease of exchangeable Ca content (Table 1), whichdisplaced Mg from the exchange complex. Accordingto Grove et al. (1981), the strong increase of soil pHmight also render Mg less soluble because it favors Mgprecipitation, possibly as a mixture of Al, Mg doublehydroxide or as poorly ordered Mg silicate.

For the BW-treatment at 3 %, the significant increaseof CO2 per gram of soil already occurred at the verybeginning of the incubation experiment. Such an initialflush of CO2 can be attributed to microbial decomposi-tion of an easily degradable C fraction provided by thebiochar itself (Bruun et al. 2012; Zimmerman et al.2011). Because the three biochars used were producedunder similar conditions, differences in Cmineralizationper unit soil C can only be due to differences in terms ofbiochar composition. Krull et al. (2003) showed that C4grass-derived biochars presented higher contents of or-ganic C occluded within phytoliths than C3 grass- orwood-derived biochars. Relative to other soil organic

carbon constituents, this occluded C has been shown tobe resistant against decomposition (Parr and Sullivan2005). Besides the impact of chemical composition,molecular typology and biological properties, we thuspartly attribute the lower C mineralization in the pres-ence of BM to the higher content of organic C occludedwithin biochar phytoliths since Miscanthus belongs tothe C4 grass group. Moreover, the potential of black Cfor enhancing C sequestration in soil can be partly offsetby its capacity to stimulate loss of native soil organic C(Wardle et al. 2008). For similar production conditions,it has been reported that the priming potential of biocharfor the loss of native organic carbon was dependent onbiochar feedstock (Cross and Sohi 2011). According toSmith et al. (2010), the source of this labile C does notoriginate from the stable carbonized components of thebiochar but rather from condensates from the bio-oilformed during pyrolysis and adsorbed during cooling.According to the coefficient of determination (R2)values, which expresses the degree of conformity be-tween experimental data and the equation-predictedvalues, the C mineralization was well described usingfirst-order equations (Table 3). In the presence ofbiochar the MRTof C, calculated based on the decay rateconstant (k), ranged from 14 to 50 years. Irrespective ofthe rate of biochar application, the MRTwas the highestin soils amended with BM, reflecting the highest poten-tial of this biochar for slower C turnover rate. Moreover,for BC and BM, the MRTof C increased with the rate ofbiochar application. According to Thies and Rillig(2009), this might result from a higher metabolic effi-ciency of themicrobial community, a change inmicrobialpopulation abundance or a higher chemisorption of re-spired CO2 to the biochar surface. If sorbed, CO2 wouldnot be recovered in the assay, reducing artificially the

Table 3 Decay rate of carbon (k) obtained using a first-orderdecay equation that models the C remaining at time t in soils(Bolan et al. 2012). R2 is the coefficient of determination be-tween measured and predicted values. Mean residence time(MRT) corresponds to 1/k

k (year1) R2 MRT (year)

Soil 0.21 0.96 5BC-1 % 0.07 0.98 14BC-3 % 0.05 0.97 20BW-1 % 0.06 0.98 17BW-3 % 0.06 0.95 17BM-1 % 0.05 0.99 20BM-3 % 0.02 0.99 50

Plant Soil (2014) 374:871882 877

estimate of respiratory activity and, thus, increasing theMRT of C.

Our results are comparable to values found in otherlaboratory incubation experiments. For instance,Hilscher et al. (2009) noticed that MRT of biochar inthe B horizon of a Cambisol varied between 19 and56 years depending on the feedstock of the charredmolecules. Using 14C labeled biochar, Kuzyakovet al. (2009) roughly estimated that the decompositionof biochar under field conditions in a temperate climate(mean annual temperature = 7 C, mean annual precipi-tation = 600700 mm) is 10 times slower compared tooptimal conditions in a laboratory (20 C, 70 % waterholding capacity, loamy texture, Ap horizon of HaplicLuvisol). Since our study was conducted under similaroperational conditions, it can be expected that theMRTofbiochar in various treatments under field conditions will

be about 140 to 500 years. Although many other factorsmay differ in the field compared to laboratory conditions(e.g. freezing/thawing cycles, drying/wetting, rhizo-sphere processes and both soil mixing and aggregatedestruction by biota)(Kuzyakov et al. 2009), the extrap-olated MRTs of biochar in soil were within therange of ages of biochar in the field which are usuallyreported to lie in the hundreds to thousands of years(Lehmann 2007b; Lehmann et al. 2009). The MRTof Cin biochar-amended soils is nevertheless most like-ly underestimated in our study for two main rea-sons: (1) the MRT of C in biochar-amended soil isaffected by non-biochar organic material (Chenget al. 2008b), and (2) short-term incubation experimentslead to systematic overestimation of long-term biochardecay rate because the oxidation of the surface ofbiochar particles, on which the decay is initiated, occurs

Fig. 4 Representative scan-ning electron microscope(SEM) images of particles ofbiochar from (a, b) coffeehusks, (c, d) woody material,and (e, f) Miscanthus(Miscanthus giganteus)straws. Red points on theSEM-EDS element dot-mapindicate the presence of Si

878 Plant Soil (2014) 374:871882

rapidly (i.e. within a few months), but is restricted to theouter areas of particles, even after hundreds of years insoils (Lehmann 2007a).

Biochar as a source of Si

In the experimental conditions of the Fraysses study(mixed-flow reactors at far from equilibrium condi-tions at 2pH12), the dissolution rates of bamboophytoliths from soil are 17 times higher than those ofquartz (Fraysse et al. 2006). These dissolution rates areone order of magnitude higher than those for soil clayminerals and are similar among different types of plantspecies despite various morphologies and differentspecific surface area (Fraysse et al. 2009). The solubil-ity of silica polymorphs is essentially constant betweenthe pH limits of 2 and 8.5, but increases rapidly nearpH 9 since the weakly acidic H4SiO4 dissociates ap-preciably (Dove 1995). The adsorption of Al and otherbi- and trivalent metals affects the surface properties ofphytoliths and decreases their rates of dissolution(Dove 1995). Here, the higher Si bio-availability ob-served in BM-3 % treatment cannot be attributed to apH-effect on the dissolution of phytoliths since BC-3 %, not characterized by a significant increase in Sirelease, had the same pH variation as BM-3 %(Table 2). The chemical composition (i.e., metal sorp-tion on phytoliths) and the accessibility of phytoliths inthe structure of biochar can partly control the Si solu-bility in biochar. Biochar fromMiscanthus (BM) actedas a potential source of bio-available Si, likely because,being a C4 grass-derived biochar, it contains substan-tially higher amounts of Si in the form of phytolithsthan any other plant-derived biochar (Krull et al. 2003).

Results of SEM-EDS analysis (Fig. 4) confirmed thepresence of phytoliths in BM, as evidenced by thespread of bilobate-shaped structures (Fig. 4e), whichare typical of Miscanthus species (Lu and Liu 2003),at the surface of biochar particles. Moreover, EDS anal-ysis indicates that these structures are rich in Si (Fig. 4f).The presence of phytolyths could not be observed in BCand BW (Fig. 4b, d). The very high Si content of BW(Table 1) was likely due to the contamination of thebiomass by impurities such as unintentionally admixedsoil minerals (e.g. aluminosilicates) or atmospheric de-posits of Si-rich particles which may be retained in largeamounts by tree surfaces (Catinon et al. 2009, 2011).This is supported by the higher presence of other ele-ments (Al, K, Ca) in Si-concentrated areas of BW rela-tive to BM, as revealed by SEM analysis (Table 4). Thelowest Si concentrations in CaCl2-extract systematicallymeasured in BW-3 % treatment were likely due to thehigher soil pH (Table 2) since adsorption of dissolvedsilica by soils is strongly pH-dependent and increasesthroughout the reaction range pH 4 to pH 9 (McKeagueand Cline 1963; Beckwith and Reeve 1963).

The kinetics of Si release was adequately describedby Elovich equations for all treatments, as indicated bythe high coefficients of determination (R2) (Table 5).This is consistent with results from Qiu et al. (2010)showing that the Elovich equation successfully de-scribed Si adsorption onto soil particles. Similar tostudies focusing on the release of other elements suchas K and P (Jalali 2006; Toor and Bahl 1999), theconstant b was used here as an index of Si release rates.The larger the b value, the more Si is released from thesoil. Among all treatments, BM-3 % exhibited thehighest rate of Si release. This result is of particularinterest in so far as the possibility of using biochar as asustainable Si amendment is concerned. It indicates thatBM-3 % not only enriches the soil in bio-available Si

Table 4 Semi-quantitative element concentration (wt%) mea-sured by SEM-EDS of the area in red spots shown in Fig. 4d(BW) and Fig. 4f (BM)

Content in red spots of Fig. 4 (wt%)

BW BM

C 28.6 55.2O 41.9 30.5Al 3.3 0.2Si 8.1 11.9K 7.0 1.3Ca 10.7 0.6P 0.2 /Mg 0.2 0.1

Table 5 Parameters of the Elovich model used to describe thekinetics of Si release into 0.01 M CaCl2

a (mg kg1) b (mg kg1 ln(min)1) R2

Soil 77.6 17.5 1.00BC-1 % 72.3 16.2 0.99BC-3 % 89.6 19.6 0.99BW-1 % 88.0 19.6 0.99BW-3 % 105.3 20.0 0.96BM-1 % 86.1 19.0 0.99BM-3 % 120.8 25.8 0.99

Plant Soil (2014) 374:871882 879

but also increases its ability for re-generating bio-available Si.

Conclusions

The present study aimed at determining the potential ofbiochar to be used as an amendment for Si-high accumu-lator plants while increasing the soil fertility and C seques-tration. Comparing biochars produced from three differentplant-derived feedstocks, our findings revealed thatbiochar from Miscanthus (BM) turned out to be the mostefficient amendment since its incorporation at 3%not onlyimproved several soil fertility parameters (pH, availablecations) but also combined the highest mean residencetime (MRT= 50 years) of C in soil with the highest releaserate of bio-available Si (25.8 mg kg1 ln(min)1). In addi-tion to its interest for enhancing both soil C sequestrationand soil fertility, the promising results of this study suggestthat biochar could also be regarded as a potential bio-available Si source. It is nevertheless crucial to conductfurther long-term and field studies in order to improveour understanding of various essential parameters (e.g.feedstock, pyrolysis conditions, rate of application, typeof extractant for determining bio-available Si) influencingthe Si bio-availability in biochar amended soils.

The aqueous chemistry of Si is regulated by a numberof linked processes: dissolution of the Si-bearing solidphases, precipitation and neoformation of authigenic Si-bearing solid phases, Si adsorption/desorption on vari-ous solid phases, Si absorption by vegetation and micro-organisms, and preservation of resistant Si forms in thesoil profile. Further studies are therefore necessary tobetter assess the impact of soil type and biochar on thedynamic of Si released in soil solution as monosilicicacid. Then, studies must be performed to confirm theresults of the lab studies in the field with Si-high accu-mulator plants. The ready availability of Si from biocharamendment should also be compared with Si-sourceusually used for soil applications, such as calcium sili-cates (wollastonite, CaSiO3).

Acknowledgments We thank P. Populaire, M. Marteleur, A.Iserentant and C. Givron (UCL) for laboratory assistance. CynthiaRozewicz is gratefully acknowledged for proofreading the manu-script. D. Houben was supported by the Fonds pour la formation la Recherche dans lIndustrie et dans lAgriculture (FRIA) ofBelgium. J-T. Cornelis is supported by Fonds National de laRecherche Scientifique (FNRS) of Belgium. This research wasalso supported by the Fonds Spcial de Recherche of the UCL.

References

Baldock JA, Smernik RJ (2002) Chemical composition andbioavailability of thermally altered Pinus resinosa (Redpine) wood. Org Geochem 33(9):10931109

Beckwith RS, Reeve R (1963) Studies on soluble silica in soils. I.The sorption of silicic acid by soils and minerals. Soil Res1(2):157168

Beesley L, Moreno-Jimnez E, Gomez-Eyles JL, Harris E, Rob-inson B, Sizmur T (2011) A review of biochars potentialrole in the remediation, revegetation and restoration ofcontaminated soils. Environ Pollut 159(12):32693282.doi:10.1016/j.envpol.2011.07.023

Blanger RR, Benhamou N, Menzies JG (2003) Cytologicalevidence of an active role of silicon in wheat resistance topowdery mildew (Blumeria graminis f. sp. tritici). Phytopa-thology 93(4):402412. doi:10.1094/phyto.2003.93.4.402

Berthelsen S, Noble A, Garside A (2001) Silicon reasearch downunder: past, preseent, and future. In: Datnoff LE, SnyderGH, Korndrfer GH (eds) Silicon in agriculture. ElsevierSciences, Amsterdam, pp 241255

Berthelsen S, Noble A, Kingston G, Hurney A, Rudd A, GarsideA (2003) Improving yield and ccs in sugarcane through theapplication of silicon based amendments. Final report onSRDC Project CLW009

Bolan NS, Kunhikrishnan A, Choppala GK, Thangarajan R,Chung JW (2012) Stabilization of carbon in composts andbiochars in relation to carbon sequestration and soil fertility.Sci Total Environ 424:264270. doi:10.1016/j.scitotenv.2012.02.061

Brndli RC, Hartnik T, Henriksen T, Cornelissen G (2008)Sorption of native polyaromatic hydrocarbons (PAH) toblack carbon and amended activated carbon in soil.Chemosphere 73(11):18051810. doi :10.1016/ j .chemosphere.2008.08.034

Brewer C, Unger R, Schmidt-Rohr K, Brown R (2011) Criteriato select biochars for field studies based on biochar chem-ical properties. BioEnergy Res 4(4):312323. doi:10.1007/s12155-011-9133-7

Bruun EW, Ambus P, Egsgaard H, Hauggaard-Nielsen H (2012)Effects of slow and fast pyrolysis biochar on soil C and Nturnover dynamics. Soil Biol Biochem 46:7379. doi:10.1016/j.soilbio.2011.11.019

Catinon M, Ayrault S, Boudouma O, Asta J, Tissut M, Ravanel P(2009) The inclusion of atmospheric particles into the barksuber of ash trees. Chemosphere 77(10):13131320

CatinonM, Ayrault S, Spadini L, Boudouma O, Asta J, Tissut M,Ravanel P (2011) Tree bark suber-included particles: along-term accumulation site for elements of atmosphericorigin. Atmos Environ 45(5):11021109

Chao TT, Sanzolone RF (1992) Decomposition techniques. JGeochem Explor 44:65106

Chapman HD (1965) Cation exchange capacity. In: Black CA(ed) Methods of soil analysis: Part 1, physical and mineral-ogical methods. American Society of Agronomy and SoilScience Society of America, Madison, pp 891901

Cheng C-H, Lehmann J, Thies JE, Burton SD, Engelhard MH(2006) Oxidation of black carbon by biotic and abioticprocesses. Org Geochem 37(11):14771488. doi:10.1016/j.orggeochem.2006.06.022

880 Plant Soil (2014) 374:871882

Cheng C-H, Lehmann J, Engelhard MH (2008a) Natural oxida-tion of black carbon in soils: changes in molecular form andsurface charge along a climosequence. GeochimCosmochim Acta 72(6):15981610. doi:10.1016/j.gca.2008.01.010

Cheng C-H, Lehmann J, Thies JE, Burton SD (2008b) Stability ofblack carbon in soils across a climatic gradient. J GeophysRes 113(G2), G02027. doi:10.1029/2007jg000642

Chien SH, Clayton WR (1980) Application of Elovich equationto the kinetics of phosphate release and sorption in soils.Soil Sci Soc Am J 44(2):265268

Cornelis JT, Titeux H, Ranger J, Delvaux B (2011) Identificationand distribution of the readily soluble silicon pool in atemperate forest soil below three distinct tree species. PlantSoil 342:369378

Cross A, Sohi SP (2011) The priming potential of biochar prod-ucts in relation to labile carbon contents and soil organicmatter status. Soil Biol Biochem 43(10):21272134. doi:10.1016/j.soilbio.2011.06.016

Crutzen PJ, Andreae MO (1990) Biomass burning in the tropics:impact on atmospheric chemistry and biogeochemical cy-cles. Science 250(4988):16691678

Datnoff LE, Seedbold KW, Correa VFJ (2001) The use of siliconfor integrated disease management: reducing fungicide ap-plications and enhancing host plant resistance. In: DatnoffLE, Snyder GH, Korndrfer GH (eds) Silicon in agriculture.Elsevier Sciences, Amsterdam, pp 171184

Datnoff LE, Rodrigues FA, Seebold KW (2007) Silicon andplant disease. In: Datnoff LE, Elmer WH, Huber DM(eds) Mineral nutrition and plant disease. American Phyto-pathological Society Press, St. Paul, pp 233246

Dove PM (1995) Kinetic and thermodynamic controls on silicareactivity in weathering environments. In: White AF,Brantley SL (eds) Chemical weathering rates of silicateminerals. Review in mineralogy. Mineralogical Society ofAmerica, Washington, pp 235290

Drees LR, Wilding LP, Smeck NE, Sankayi AL (1989) Silica insoils: quartz and disordered silica polymorphs. In: DixonJB, Weed SB (eds) Minerals in soil environments. SoilScience Society of America, Madison

Epstein E (1994) The anomaly of silicon in plant biology. ProcNatl Acad Sci U S A 91(1):1117

Epstein E (1999) Silicon. Annu Rev Plant Physiol Plant Mol Biol50(1):641664. doi:10.1146/annurev.arplant.50.1.641

Epstein E (2001) Silicon in plants: facts vs. concepts. In: DatnoffLE, Korndrfer GHS (eds) Silicon in agriculture (Studies inplant science), vol 8. Elsevier, pp 115. doi:10.1016/s0928-3420(01)80005-7

Fraysse F, Pokrovsky OS, Schott J, Meunier J-D (2006) Surfaceproperties, solubility and dissolution kinetics of bamboophytoliths. Geochim Cosmochim Acta 70(8):19391951.doi:10.1016/j.gca.2005.12.025

Fraysse F, Pokrovsky OS, Schott J, Meunier J-D (2009) Surfacechemistry and reactivity of plant phytoliths in aqueoussolutions. Chem Geol 258:197206

Gascho GJ (2001) Silicon sources for agriculture. In: DatnoffLE, Korndrfer GHS (eds) Silicon in agriculture (Studies inplant science), vol 8. Elsevier, pp 197207. doi:10.1016/s0928-3420(01)80016-1

Glaser B, Lehmann J, Zech W (2002) Ameliorating physical andchemical properties of highly weathered soils in the tropics

with charcoal - a review. Biol Fertil Soils 35(4):219230.doi:10.1007/s00374-002-0466-4

Grove JH, Sumner ME, Syers JK (1981) Effect of ime onexchangeable magnesium in variable surface charge soils.Soil Sci Soc Am J 45(3):497500

Guntzer F, Keller C, Meunier J-D (2012) Benefits of plant siliconfor crops: a review. Agron Sustain Dev 32(1):201213.doi:10.1007/s13593-011-0039-8

Haynes RJ, Belyaeva ON, Kingston G (2013) Evaluation of indus-trial wastes as sources of ferilizer silicon using chemical ex-tractions and plant uptake. J Plant Nutr Soil Sci 176:238248

HaysomMBC, Chapman LS (1975) Some aspects of the calciumsilicate trials at Mackay. Proc Austr Sugar Cane Technol42:117122

Hilscher A, Heister K, Siewert C, Knicker H (2009)Mineralisationand structural changes during the initial phase of microbialdegradation of pyrogenic plant residues in soil. Org Geochem40(3):332342. doi:10.1016/j.orggeochem.2008.12.004

Hodson MJ, Evans DE (1995) Aluminium/silicon interactions inhigher plants. J Exp Bot 46(2):161171

Houben D, Evrard L, Sonnet P (2013) Mobility, bioavailabilityand pH-dependent leaching of cadmium, zinc and lead in acontaminated soil amended with biochar. Chemosphere92(11):14501457. doi:10.1016/j.chemosphere.2013.03.055

Houben D, Evrard L, Sonnet P (in press) Beneficial effects ofbiochar application to contaminated soils on the bioavail-ability of Cd, Pb and Zn and the biomass production ofrapeseed (Brassica napus L.). Biomass Bioenerg. doi:10.1016/j.biombioe.2013.07.019

Jalali M (2006) Kinetics of non-exchangeable potassium releaseand availability in some calcareous soils of western Iran.Geoderma 135:6371. doi:10.1016/j.geoderma.2005.11.006

Korndrfer GH, Lepsch I (2001) Effect of silicon on plant growthand crop yield. In: Datnoff LE, Korndrfer GHS (eds) Siliconin agriculture (Studies in plant science), vol 8. Elsevier,pp 133147. doi:10.1016/s0928-3420(01)80011-2

Krull ES, Skjemstad JO, Graetz D, Grice K, Dunning W, CookG, Parr JF (2003) 13C-depleted charcoal from C4 grassesand the role of occluded carbon in phytoliths. Org Geochem34(9):13371352. doi:10.1016/s0146-6380(03)00100-1

Kuhlbusch TAJ (1998) Black carbon and the carbon cycle.Science 280(5371):19031904

Kuzyakov Y, Subbotina I, Chen H, Bogomolova I, Xu X (2009)Black carbon decomposition and incorporation into soil mi-crobial biomass estimated by 14C labeling. Soil BiolBiochem 41(2):210219. doi:10.1016/j.soilbio.2008.10.016

Laird DA (2008) The charcoal vision: a win-win-win scenariofor simultaneously producing bioenergy, permanently se-questering carbon, while improving soil and water quality.Agron J 100(1):178181

Laird D, Fleming P, Wang B, Horton R, Karlen D (2010) Biocharimpact on nutrient leaching from a Midwestern agriculturalsoil. Geoderma 158(34):436442. doi:10.1016/j.geoderma.2010.05.012

Lehmann J (2007a) Bio-energy in the black. Front Ecol Environ5(7):381387. doi:10.1890/1540-9295(2007)5[381:bitb]2.0.co;2

Lehmann J (2007b)A handful of carbon. Nature 447(7141):143144Lehmann J, Joseph S (2009) Biochar for environmental manage-

ment - an introduction. In: Lehmann J, Joseph S (eds)

Plant Soil (2014) 374:871882 881

Biochar for environmental management - science and tech-nology. Earthscan, London, pp 112

Lehmann J, Pereira da Silva J, Steiner C, Nehls T, Zech W,Glaser B (2003) Nutrient availability and leaching in anarchaeological Anthrosol and a Ferralsol of the Central Am-azon basin: fertilizer, manure and charcoal amendments.Plant Soil 249(2):343357. doi:10.1023/a:1022833116184

Lehmann J, Czimczik C, Laird D, Sohi S (2009) Stability ofbiochar in the soil. In: Lehmann J, Joseph S (eds) Biocharfor environmental management - science and technology.Earthscan, London, pp 183205

Liang B, Lehmann J, Solomon D, Kinyangi J, Grossman J,ONeill B, Skjemstad JO, Thies J, Luizo FJ, Petersen J,Neves EG (2006) Black carbon increases cation exchangecapacity in soils. Soil Sci Soc Am J 70(5):17191730

Liang Y, Sun W, Zhu Y-G, Christie P (2007) Mechanisms ofsilicon-mediated alleviation of abiotic stresses in higherplants: a review. Environ Pollut 147(2):422428. doi:10.1016/j.envpol.2006.06.008

Lu H, Liu K-B (2003) Morphological variations of lobatephytoliths from grasses in China and the south-easternUnited States. Divers Distrib 9(1):7387. doi:10.1046/j.1472-4642.2003.00166.x

Ma JF (2004) Role of silicon in enhancing the resistance ofplants to biotic and abiotic stresses. Soil Sci Plant Nutr50(1):1118. doi:10.1080/00380768.2004.10408447

Marschner H (1995) Mineral nutrition of higher plants, 2nd edn.Academic, London

McKeague JA, Cline MG (1963) Silica in soil solutions: II. Theadsorption of monosilicic acid by soil and by other sub-stances. Can J Soil Sci 43(1):8396. doi:10.4141/cjss63-011

Novak JM, Busscher WJ, Laird DL, Ahmedna M, Watts DW,Niandou MAS (2009) Impact of biochar amendment on fertil-ity of a southeastern coastal plain soil. Soil Sci 174(2):105112

Olson JS (1963) Energy storage and the balance of producers anddecomposers in ecological systems. Ecology 44(2):322331. doi:10.2307/1932179

Parr JF, Sullivan LA (2005) Soil carbon sequestration inphytoliths. Soil Biol Biochem 37(1):117124. doi:10.1016/j.soilbio.2004.06.013

Powlson DS, Whitmore AP, Goulding KWT (2011) Soil carbonsequestration to mitigate climate change: a critical re-examination to identify the true and the false. Eur J SoilSci 62(1):4255

Qiu LP, Zhang XC, Cheng JM, Han XN (2010) Isotherms andkinetics of si adsorption in soils. Acta Agric Scand Sect BSoil Plant Sci 60(2):157165

Rodella AA, Saboya LV (1999) Calibration for conductimetricdetermination of carbon dioxide. Soil Biol Biochem31(14):20592060. doi:10.1016/s0038-0717(99)00046-2

Sauer D, Saccone L, Conley DJ, Herrmann L, Sommer M (2006)Review of methodologies for extracting plantavailable andamorphous Si from soils and aquatic sediments. Biogeo-chemistry 80:89108

Schmidt MWI, Noack AG (2000) Black carbon in soils andsediments: analysis, distribution, implications, and currentchallenges. Global Biogeochem Cycles 14(3):777793.doi:10.1029/1999gb001208

Schmidt MWI, Skjemstad JO, Gehrt E, Kgel-Knabner I (1999)Charred organic carbon in German chernozemic soils. Eur JSoil Sci 50(2):351365

Schulz H, Glaser B (2012) Effects of biochar compared toorganic and inorganic fertilizers on soil quality and plantgrowth in a greenhouse experiment. J Plant Nutr Soil Sci175(3):410422

Skjemstad JO, Clarke P, Taylor JA, Oades JM, McClure SG(1996) The chemistry and nature of protected carbon insoil. Soil Res 34(2):251271

Smith JL, Collins HP, Bailey VL (2010) The effect of young biocharon soil respiration. Soil Biol Biochem 42(12):23452347.doi:10.1016/j.soilbio.2010.09.013

Steinbeiss S, Gleixner G, Antonietti M (2009) Effect of biocharamendment on soil carbon balance and soil microbial ac-tivity. Soil Biol Biochem 41(6):13011310. doi:10.1016/j.soilbio.2009.03.016

Thies JE, Rillig MC (2009) Characteristics of biochar: biologicalproperties. In: Lehmann J, Joseph S (eds) Biochar for envi-ronmental management. Earthscan, London, pp 85102

Tiessen H, Cuevas E, Chacon P (1994) The role of soil organicmatter in sustaining soil fertility. Nature 371(6500):783785

Toor GS, Bahl GS (1999) Kinetics of phosphate desorptionfrom different soils as influenced by application of poul-try manure and fertilizer phosphorus and its uptake bysoybean. Bioresour Technol 69(2):117121. doi:10.1016/s0960-8524(98)00179-5

Vatehov Z, Kollrov K, Zelko I, Richterov-Kuerov D,Bujdo M, Likov D (2012) Interaction of silicon andcadmium in Brassica juncea and Brassica napus. Biologia67(3):498504. doi:10.2478/s11756-012-0034-9

Vermeire M-L, Kablan L, Dorel M, Delvaux B, Risde J-M,Legrve A (2011) Protective role of silicon in the banana-Cylindrocladium spathiphylli pathosystem. Eur J PlantPathol 131(4):621630. doi:10.1007/s10658-011-9835-x

Wardle DA, Nilsson MC, Zackrisson O (2008) Fire-derivedcharcoal causes loss of forest humus. Science 320:629

Yuan J-H, Xu R-K, Qian W, Wang R-H (2011) Comparisonof the ameliorating effects on an acidic ultisol betweenfour crop straws and their biochars. J Soils Sediments11:741750

Zech W, Senesi N, Guggenberger G, Kaiser K, Lehmann J,Miano TM, Miltner A, Schroth G (1997) Factors control-ling humification and mineralization of soil organic matterin the tropics. Geoderma 79(14):117161. doi:10.1016/s0016-7061(97)00040-2

Zimmerman AR, Gao B, Ahn M-Y (2011) Positive and negativecarbon mineralization priming effects among a variety ofbiochar-amended soils. Soil Biol Biochem 43(6):11691179. doi:10.1016/j.soilbio.2011.02.005

882 Plant Soil (2014) 374:871882

Biochar from Miscanthus: a potential silicon fertilizerAbstractAbstractAbstractAbstractAbstractIntroductionMaterials and methodsSoilBiocharSubstrate preparationSoil respiration measurementsPhysico-chemical characterizationBio-available Si evaluationStatistical analysis

ResultsSoil respiration and chemical characteristicsBio-available Si

DiscussionImpact of biochar on soil fertility and respirationBiochar as a source of Si

aaConclusionsReferences