Geoderma 232234 (2014) 538546

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Geoderma

j ourna l homepage: www.e lsev ie r .com/ locate /geodermaNano-structural and chemical characterization of charred organic matterin a fire-affected ArenosolSvetlana Filimonova a,, Andr Hilscher a, Ingrid Kgel-Knabner a,b

a Lehrstuhl fr Bodenkunde, Department fr kologie und kosystemmanagement, Technische Universitt Mnchen, D-85350 Freising-Weihenstephan, Germanyb Institute for Advanced Study, Technische Universitt Mnchen, Lichtenbergstrasse 2a, D-85748 Garching, Germany Corresponding author.E-mail address: filimono@wzw.tum.de (S. Filimonova

http://dx.doi.org/10.1016/j.geoderma.2014.06.0100016-7061/ 2014 Elsevier B.V. All rights reserved.a b s t r a c ta r t i c l e i n f oArticle history:Received 21 July 2013Received in revised form 29 March 2014Accepted 13 June 2014Available online xxxx

Keywords:Pyrogenic organic materialMicroporesAdsorptionNuclear magnetic resonanceOrgano-mineral interactionsThe objective of this studywas to characterize the formation and properties of black carbon (BC) domainswithinpyrogenic organic material (PyOM) of an Arenosol derived from a fire-affected site in Portugal (Algarve, Faro).Both the particulate organic matter (POM) and organo-mineral particle size fractions were investigated bymeans of 13C CPMAS NMR (composition of the soil organic matter, SOM) and 129Xe NMR of adsorbed Xe atoms(nano-structure of PyOM). Fire-induced alteration of the soil mineral phasewas revealed by the X-ray diffractionand chemical extractionmethods. An enhanced contribution of aromatic species combinedwith depletion in car-bohydrates and alkyl functionalities (13C NMR)was observed in the surface soil horizon (Ah), suggesting that thedehydration of carbohydrates and thermal breakdown of n-alkanes was mainly responsible for the formation ofBC species. As a measure of a charring degree of PyOM, we suggested to use the relative intensity of the down-field 129Xe NMR signal (130160 ppm) characterizing micropores (b2 nm) that were formed by the fused aro-matic units of BC. The revealed microporous BC regions and the low-dense domains with larger pores (suppos-edly formed by the alkyl functional groups) exist within the same PyOM particle, as this was supported by thecalculatedmean Xe displacement distance between these two regions. The ascertainedmicropore heterogeneitywithin the BC domains is supposedly caused by the varying size of aromatic clusters that approach each other at avarying distance. This nano-scale heterogeneity may have important implications for the specific adsorption be-havior and persistence of BC in a soil.In the organo-mineral soil fractions, the BC species mainly occurred in a highly dispersed (mineral-associated)form. Supposedly, the association PyOMsoil minerals did not necessarily require the aromatic ring opening,as we detected rather high content of aromatic 13C NMR moieties in these fractions. An increased content ofthe short-range order Fe- and Al oxyhydroxides and mixed layer silicates (supposedly fire-induced) appearedto promote the interactions PyOMsoil minerals via an enhanced extent of hydroxylated mineral surfaces.

2014 Elsevier B.V. All rights reserved.1. Introduction

Increasing interest in the black carbon (BC) research has been gener-ated recently due to its potential importance for the global carbon cycle.The term black carbon is generally used to describe an impure formof the element produced by incomplete combustion of fossil fuels orbiomass that contains over 60% carbon (Goldberg, 1985). BC specieshave been observed in numerous Chernozems (Ponomarenko andAnderson, 2001; Schmidt et al., 1999), Cambisols (Knicker et al.,2005a), and volcanic ash soils (Dmig et al., 2008; Golchin et al.,1997). Vegetation fire is an important source of the soil BC, as the latteris contained in the pyrogenic organic material (PyOM) generated under).the impact of high temperatures (reviewed by Ballard, 2000;Bento-Gonalves et al., 2012; Knicker, 2007). PyOMrepresents a contin-uum from partly charred plant material through char and charcoal tosoot and graphite particles (Schmidt et al., 2001). BC species can residein soils for hundreds to thousands of years, being relatively resistant tobiological and chemical breakdown (Hammes et al., 2008; Preston andSchmidt, 2006). These species have been found in all particle size frac-tions of soils, particularly in the light fractions (e.g., Hilscher andKnicker, 2011; Ponomarenko and Anderson, 2001). Occurrence of BCin the light soil fractions supports its intrinsic chemical recalcitrancedue to the aromatic character. At the same time, BC can also be stabi-lized in microaggregates (b250 m) and/or in organo-mineral com-plexes, e.g. with Fe or Al oxides and clay minerals (Brodowski et al.,2005; Eusterhues et al., 2005). Despite high interest in the various as-pects of BC and PyOM, the knowledge about their interaction with soilminerals remains sparse. One potential mechanism is the oxidation of

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539S. Filimonova et al. / Geoderma 232234 (2014) 538546the functional groups at the surface of PyOM favoring interactions withsoil organic and mineral fractions (Brodowski et al., 2005; Cheng et al.,2008).

Various approaches exist to quantify BC in soils and sediments, in-cluding chemical and thermal oxidation (Calvelo Pereira et al., 2011;Golchin et al., 1997; Schmidt et al., 1999), microscopic, spectroscopictechniques (Ding and Rice, 2012; Knicker et al., 2005a, 2005b, 2013)and molecular marker methods (Brodowski et al., 2007; Hammeset al., 2008). A recent comparative study of some of these methods(Hammes et al., 2007) has demonstrated that their results may differby an order of magnitude, as suggested, mainly due to the complex het-erogeneous structure of BC. On the other hand, particle size, morpholo-gy, and the presence of a mineral matrix may lead to an over- orunderestimation of the content of PyOM and BC (Preston and Schmidt,2006). Specifically, the non-consisting results given by the methodsbased on chemical and thermal oxidation (they rely on the principlethat BC ismore resistant to breakdown than other forms of organicmat-ter) are caused by the varying nature of attacking species. The widelyused 13C solid-state NMR with cross-polarization (CP) may poorlydetect highly condensed aromatic structures, so that a more time-consuming Bloch decay technique must be used (e.g., Knicker et al.,2005a). Bearing in mind these challenges, BC has been assigned to themost uncharacterized part of the total organic carbon (Hedges et al.,2000).

One promising approach to examine C-rich materials is 129Xe NMRspectroscopy of adsorbed Xe atoms, as the 129Xe resonance parameters(chemical shift, line width and relaxation rates) are sensitively influ-enced by the surface structure, porosity, crystallinity, the presence ofcations and functional groups (reviewed by Bonardet et al., 1999;Raftery, 2006; Springuel-Huet et al., 1999). This technique has been ap-plied for studying porosity in both the synthetic carbons (Simonov et al.,1999; Suh et al., 1991; Tsiao and Botto, 1991) and natural charcoals(Filimonova et al., 2011; Radovic et al., 1997). 129Xe NMR allows thesensitive determination of the pores of micro- and mesopore range(b2 nm and 250 nm) (IUPAC, 1996), in contrast to the commonlyused N2 adsorption method, which does not give reliable data forPyOM (e.g. Hilscher et al., 2009), due to the slow diffusion of N2 intosmall pores at 77 K (Rodriguez-Reinoso et al., 1982). In our recent129Xe NMR study (Filimonova et al., 2011), we have revealed specificpore structures in the charcoals, i.e. an occurrence of constricted poreopenings of the order of one, or two diameters of the Xe atom (0.440.88 nm). Such constrictions supposedly cause the so-called bottleneck effect upon adsorption of various species onto charcoals.

In the present study, we attempted to investigate the physico-chemical properties of PyOM and BC domains in a natural soil, byusing a combination of methods that allow linking the nano-structureand the chemistry of PyOM, and to get an insight into interactionsPyOMsoil minerals. We have chosen a fire-affected Arenosol. Asthis sandy soil possesses a low water capacity, a pronounced impact offire on soil constituents was expected. On the other hand, we also ex-pected an enhanced mobility of PyOM species. The Reference SoilGroup of Arenosol consists of sandy soils developed after weatheringof old, usually quartz-rich soil material or rock, and soils developed inrecently deposited sands as occur in deserts and beach lands (IUSSWorking Group RB, 2006). Accumulation of organic carbon (OC) insandy soils occurs in the particulate organic matter (POM), but to a sub-stantial proportion also in associations with soil minerals (Kgel-Knabner et al., 2008). We applied 129Xe NMR to reveal the nano-structural properties of PyOM and BC domains, whereas 13C CPMASNMR was used to assess their chemical nature. Fire-affected alterationof the soil minerals was revealed by the X-ray powder diffraction andchemical extraction methods. Both the POM and organo-mineral parti-cle size fractions derived from the surface (Ah) and sub-surface (B) soilhorizons were analyzed. For comparison, we examined a few modelcharcoals and amodel organo-mineralmixture that compositionally re-sembled the studied Arenosol. We focused on the following mainobjectives: (1) understanding whether the fire-induced alterations ofsoil minerals and organic matter may promote organo-mineral interac-tions; (2) revealing the ability to identify the BC domains in the soilPyOM by means of 129Xe NMR spectroscopy and to assess the extentof nano-structural heterogeneity of PyOM; (3) obtaining insights intointeraction PyOMsoil minerals. Our approach was based on the fol-lowing hypotheses: (1) organo-mineral interactions are facilitated bythe fire-impacted alteration of soil minerals and organic matter; (2)structural heterogeneity of PyOM (e.g. pore variety) is controlled bythe variety of the chemical structures; and (3) BC domains possess anextensive microporous network.

2. Materials and methods

2.1. Samples

The soil investigated was a fire-affected Arenosol sampled underPinus pinaster and Quercus suber located in Southern Portugal (Algarve,Faro, moderate Mediterranean climate) at a fire-affected site which ex-perienced a first fire two years- and a second fire one year before sam-pling (Table 1). Soil classificationwas based onWorld reference base forsoil resources (IUSSWorking Group RB, 2006). The parent material wassands and gravels of Pliocene to Quaternary age with a thickness from 8to 50 m (Stigter et al., 1998). The soil pH (CaCl2) was moderate (5.36.5), being somewhat higher in the Ah horizon, as compared to the B1and B2 horizons (Table 1) that accords with the fire-impacted addingof calcite and other salts in plant ash after the fire (Yusiharni andGilkes, 2012). A fewmodel charred samples were investigated for com-parison. The barbeque charcoal was a commercially available sample.The model charred samples were obtained by charring of milled sprucewood and wheat straw in a porcelain crucible in a muffle furnace at350 C for 10 min under oxic conditions. To roughly model our studiedArenosol, we prepared the organo-mineral mixture via incubation ofthe minerals (illite, quartz sand, ferrihydrite, Ca-montmorillonite)with organic components including the PyOM particles manually sepa-rated from the SOM (Table 2). These constituents were mixed withwater (weight proportions are indicated in Table 2) to a suspension,with a following incubation in the darkness at 20 C during 20 days.At the end of the incubation, themixturewas freeze dried. The resultingOC content in this mixed sample was ca. 180 mg g1.

2.2. Chemical treatments and analyses

Determination of the soil texture was performed by combiningweighing and analysis with a sedigraph (SediGraph 5100, Micromeritics)after adding sodium pyrophosphate. Prior to the analysis, decompositionof organic matter by H2O2 was carried out according to Kunze and Dixon(1986).

To obtain the soil POM fractions, fine soil aliquots (each 30 g) weresubjected to a density fractionation with a Na-polytungstate solution(150ml, density 1.8 g cm3). The POM fractionswere recovered asma-terial floating on the Na polytungstate solution after centrifugation(10 min at 3000 rpm) and separated by using a 20-m sieve. The extrac-tionwas repeated 3 times and both the light fraction and themineral res-idue were washed with deionized water to remove remaining salt. Saltremoving was complete when the conductivity of the washing waterdropped below 50 S cm1 for the mineral residue and 3 S cm1 forthe POM. The mineral residue was subjected to a particle size fraction-ation, which combined wet sieving (sand and coarse silt fractions) andgravity sedimentation (medium-,fine silt and clay fractions), after disrup-tion of soil aggregates by ultrasound (Branson Sonifier 250 with a poweroutput of 200Wand a frequency of 20 kHzused for 3 min). Subsequently,the obtained fractionswere freeze dried. As these soil fractions containedmineral-bound organic material, we called them organo-mineralparticle size fractions.

Table 1Arenosol texture and OC content in the bulk soil (b2 mm).

Horizon Depth (cm) pH (CaCl2) Sand (200063 m) Coarse silt (6320 m) Medium silt (206.3 m) Fine silt (6.32 m) Clay (b2 m) OC (mg g1)

Ah 02 6.5 82 3 5 3 7 37.0B1 27 5.3 90 2 2 2 4 4.8B2 714 5.6 90 2 2 3 3 5.0

540 S. Filimonova et al. / Geoderma 232234 (2014) 538546Determination of the total carbon and nitrogen contents was per-formed in duplicate using dry combustionwith a Vario EL elemental an-alyzer (Elementar Analysen Systeme GmbH, Hanau, Germany). Sinceour soil samples did not contain inorganic carbon in a formof carbonate,the OC concentration was equal to the total carbon content. The wellcrystalline and short-range order Fe- and Al minerals in the soilfractions were evaluated by chemical extraction with dithionitecitratebicarbonate (DCB) (Mehra and Jackson, 1960) and oxalate solu-tions (Schwertmann, 1964). The dithionite- and oxalate extracts wereanalyzed for Fe (Fed and Feo) and Al (Ald and Alo) with an ICP-OES(Varian Vario-Pro).

Oxidative removal of SOM was performed by the treatment of thesoil organo-mineral fractionswith a 30% (w/w) H2O2 solution. The sam-ples were allowed to react at 60 C until gas development ceased (1012 h). The procedure was repeated 3 to 5 times, to achieve up to 90%loss of OC in the Ah horizon, and 5060% in the B horizon.

2.3. 13C CPMAS NMR

Composition of the SOM in the studied soil was evaluated by 13CNMR spectroscopy (Bruker DSX 200, resonance frequency for 13C50.32 MHz) with the cross polarization magic angle spinning (CPMAS)technique at a spinning speed of 6.8 kHz. A ramped 1H pulse was ap-plied during a contact time of 1 ms. Pulse delay was chosen to be 0.8 s(soil materials) and 2 s (model charred samples). The number of accu-mulated scans (NS) varied between 103 and 105, and a 100 Hz Gaussianline broadeningwas applied before Fourier transformation. For improv-ing the sensitivity of the analysis, soil organo-mineral fractions weredemineralized with HF according to Goncalves et al. (2003). Quantifica-tion of organic functional groups was performed via signal intensity in-tegration in the respective chemical shift regions according to Knickeret al. (2005a). The following assignment of the chemical shifts was con-sidered: 245185 ppm: carbonyl C; 185160 ppm: carboxyl/amide C;160110 ppm: olefins and aromatic C; 160140 ppm: substituted aro-matic C; 11045 ppm: O/N-alkyl C; 45-0 ppm: alkyl C. Chemical shiftswere referred to an external tetramethylsilane.

2.4. 129Xe NMR of adsorbed xenon

129Xe NMR spectroscopy of adsorbed Xe atoms (Bruker DMX-400,resonance frequency 110.68 MHz for 129Xe) was used to assess the po-rosity and O-containing functional groups in the studied materials.Xenon gas of 99.98% purity with a natural abundance of the 129Xe iso-tope (26.4%) was used as an adsorbate. Before Xe adsorption, the soilTable 2Composition of the organo-mineral mixture modeling the studied Arenosol.

Component (weight proportion) Origin

OM (0.25) O-horizon of a Podzola

Soil PyOM (0.10) Manually separated from the soil oIllite (0.23) Inter-ILI Mrnki Iroda, HungaryCaMM (0.15) Ca-montmorillonite, Wyomingc

FH (0.02) 2-Line ferrihydrited

QS (0.25) Quartz sand

a Sampled under Norway spruce (Picea abies (Karst.) L.) in Bavary (Germany).b Not determined.c Source clay minerals repository.d Produced according to Cornell and Schwertmann (2003).samples and model chars were put into NMR glass tubes (10 mmouter diameter) and pre-treated in air at 70 Cwith the following evac-uation at this temperature for 15 to 24 h. Xe gas was introduced intoevacuated samples and allowed to equilibrate until no further drop inxenon pressure was detected; afterwards the sample tubes wereflame-sealed. Most of the spectra were acquired at the equilibrium Xepressure PXe = 500600 mbar. For the controlling of the presence ofmicropores, the pressure dependence of the chemical shift, , was mea-sured. The radio frequency pulse length was 20 s (/2), the delay be-tween pulses varied between 1 and 5 s. For all spectra, 512 data pointswere accumulatedwith a single-pulse sequence, and a 2030 HzGauss-ian line broadening was applied before Fourier transformation. NS var-ied between 5 102 and 4 104 to achieve an acceptable signal-to-noiseratio. 129Xe chemical shifts were referenced to the extrapolated shift ofbulk Xe gas at P=0 using an external reference (zeolite NaYwith pre-adsorbed xenon). Downfield shifts were considered as positive.

The assignment of the 129Xe resonance peaks was based on the factthat the more down-field chemical shifts correspond to the higheradsorption potential of adsorbing surfaces. The lattermay imply adsorp-tion in micropores and/or interaction with the so-called strong ad-sorption sites, or paramagnetic ions. This is in line with the commonequation representing the detected chemical shift, , as a sum of differ-ent variables corresponding to perturbation from which may suffer(Ito and Fraissard, 1980):

ref s Xe SAS E M : 1

Here ref is the shift of the gaseous Xe typically regarded to be zero, scharacterizes Xepore surface interactions, Xe corresponds toXeXe in-teractions and increases with the local Xe density, SAS results from in-teraction with strong adsorption sites, and E and M describe theinfluence of the electric and magnetic fields created by cations.

2.5. N2 adsorption

N2 adsorption (77 K) was used to estimate the specific surface areaof the samples, SBET, using a conventional volumetric apparatus and anAutosorb 1 surface area analyzer (Quantachrome Corporation). Priorto the N2 adsorption, the samples were outgassed under vacuum at70 C for 24 h. The BrunauerEmmetTeller equation (Brunauer et al.,1938) was applied to the data points in the p/p0 range between 0.05and 0.30, and a cross-sectional area of 16.2 2 for the N2 molecule wasassumed in the calculation of SBET.Fraction SBET (m2 g1)

b20 m 4rganic material n.d.b 45

b63 m 30b2 m 34n.d. n.d.b200 m 1

Q

Q

B2

Ah

Ch IK

Mixed

ChI

Ch

FVCh

V

VG

F

Fig. 1. X-ray diffractograms of clay fractions derived from the Ah- and B2 horizons of thestudied Arenosol: Chl = chlorite, F = feldspar, G = goethite, I = illite, K = kaolinite,Mixed = mixed layer silicates, Q = quartz, and V = vermiculite.

541S. Filimonova et al. / Geoderma 232234 (2014) 5385463. Results

3.1. Soil mineralogy: XRD and chemical extraction data

The studied Arenosol had a loamy sand texture (Table 1). X-ray dif-fraction analysis showed that the mineral phase was composed byquartz, feldspar, layer silicates such as chlorite, vermiculite, kaolinite, il-lite, and mixed layer silicates (Fig. 1). The latter were evident by thebroadened pattern, or plateau between 12.8 and 10.3 (2 = 810)in both the Ah- and B horizons. Due to the relatively low content of Fecontaining minerals, they were not detected in the XRD spectra, exceptfor goethite exhibiting a small shoulder at 2=25 (Fig. 1). The presenceof further Fe oxyhydroxides (not detectable by XRD) in the soil organo-mineral fractions was established by the values of dithionite- and oxa-late extractable Fe representing, respectively, well crystalline (Fed)and short-range order (Feo) minerals (Table 3). Dithionite soluble alu-minum (Ald) may correspond to feldspar, but also to those Al speciesthat are incorporated into the structure of Fe oxides, e.g. throughisomorphic substitution of Fe3+ cations (Cornell and Schwertmann,2003; Muggler et al., 2001). The observed higher content of short-range order minerals in the Ah horizon (Feo and Alo), as compared tothe B horizon, accords with the reported inhibitory effect of organiccompounds on mineral crystallization (Schwertmann, 1966). Anotherreasoning for the high content of the Fe- and Al Fe oxyhydroxides mayTable 3Characteristics of the soil organo-mineral particle size fractions.

Horizon OC (mg g1) C/N Fed Feo Ald Alo SBET (m2 g1)

(mg g1)

Ahb2 m 111 9.3 22.5 8.2 9.0 14.8 3526.3 m 121 11.0 18.7 2.3 5.2 9.5 226.320 m 59 14.8 1.4 1.8 1.3 3.4 422063 m 31 15.5 4.2 0.7 0.6 1.3 71632000 m 26 n.d.a n.d. n.d. n.d. n.d. 5

B2b2 m 54 10.8 35 3.8 3.6 2.4 5826.3 m 40 20.0 14.9 1.4 0.6 1.0 176.320 m 15 15.0 7.6 0.8 0.4 0.8 102063 m 5 n.d. 2.5 0.5 0.1 0.3 4632000 m 2.9 n.d. n.d. n.d. n.d. n.d. 3

a Not determined.be offered by the fire-induced dehydroxylation of some crystallinemineral phases.

3.2. OC content and SOM quality

Despite the sandy soil texture and rather low clay content (Table 1),we observed relatively high OC concentrations in the soil fractions. Inthe Ah horizon, the OC content approached 3.7 wt.% in the bulk soil(b2 mm), with ca. 21% of the clay-bound OC (calculated from themass-related C-distribution among particle size fractions) (Table 3). Inboth the B1 and B2 horizons, the OC content was lower (ca. 0.5 wt.%in the bulk soil), and the percentage of the clay-bound OC comprisedonly ca. 3%. In the following, we will compare the samples of the Ahand B2 horizons.

Solid state 13C CPMAS NMR spectra differed by the relative signal in-tensities between the fractions of the Ah and B2 horizons (Fig. 2 andTable 4). In the Ah horizon, a large proportion of aromatic carbon wasdetected in both the POM and organo-mineral fractions, attributed toaromatic identities of lignin and charred residues (relative intensity4359%) (Fig. 2A and C, patterns a, b). In contrast, the signal of O,N-alkyl groups (carbohydrate-derived structures) had a low intensity(1226%). The 13C NMR peak of alkyl C was rather depleted, while car-boxyl and carbonyl groups features more intense signals, as comparedto the B2 horizon (Table 4). In the B2 horizon, the relative intensityof aromatic C signal approached only 1229%, whereas the signals ofO,N-alkyl groups were more intense (2848% in the POM fractionsand 3744% in the organo-mineral soil fractions), as compared to theAh horizon (Fig. 2B, C, patterns c, d and Table 4).

The 13C NMR spectrum of the PyOM particles (manually separatedfrom the SOM)was rich in aryl C, but also displayed the peaks of carboxyland alkyl groups (Fig. 4A).

3.3. 129Xe NMR data

129Xe NMR spectra of adsorbed Xe atoms demonstrated similar pat-terns for both the silt- and clay-sized organo-mineral soil fractions,without visible differences between the Ah and B2 horizons (Fig. 3A).The signal position between 7 and 18 ppm indicated Xe adsorptionwithin relatively large pores of macropore range (N50 nm) (IUPAC,1996), supposedly formed by mineral particles. Noticeably, the 129Xechemical shift value was sensitively affected by the H2O2 oxidation: adown-field shift by 3 to 8 ppm occurred in the oxidized soil fractions,as compared to the untreated ones (Fig. 3A, patterns b, d), supposedlydue to the pore enlargement after the partial removal of SOM.

The 129XeNMR spectra of the POM fractions (Ah horizon) drasticallydiffered from those of the organo-mineral fractions. The formercontained several resonance signals in both the low-field region (150160 ppm) and the high-field region (partly overlapping peaks between10 and 15 ppm) (Fig. 3B, patterns a, b). The position of the down-fieldsignal was affected by the Xe loading, i.e. increased from ca. 120 to160 ppm upon increasing the equilibrium Xe pressure from ca. 150 to850 mbar. Such a behavior confirmed the Xe adsorption within thevoids ofmicropore range, as in the narrow confined spaces an enhancedcollision frequency of Xe atoms with pore surfaces and with neighbor-ing Xe atoms (terms s and Xe in Eq. (1)) cause the disturbance of theXe electron shell and therefore, an increase of . In contrast, in thePOM fractions of the B2 horizon, the down-field peak characterizingmi-cropores was missing (Fig. 3B, patterns c, d).

The micropores dominated the 129Xe NMR spectra of the modelcharredmaterials (Fig. 3C, patterns ac). Also, the individual PyOMpar-ticles obtained by a manual separation from the SOM exhibited the129XeNMRpattern comparablewith those recorded in the Ah POM frac-tions, i.e. the broadened resonance at ca. 130 ppm combined with thenarrower peak around0 ppm(Fig. 3C, pattern d). This similarity indicat-ed that the Ah POM fractions most probably contained some charredstructures of PyOM. The 129Xe NMR pattern of the model incubated

13C chemical shift, ppm

2-6.3m

6.3-20m

2060100140180220

>20m

Table 5Charring degree in the soil POM fractions, soil PyOM and model charred samples calculated by the use of 129Xe NMR and 13C CPMAS NMR data.

Sample OC (mg g1) Imicro/Imeso,macro (129Xe NMR) Aromatic C/Alkyl C (13C NMR) SBET (m2 g1)

Ah POM N20 m 390 2.2 4.2 11Ah POM b20 m 310 0.6 5.9 8Charred spruce wood 800 8.0 11.2 50Charred wheat straw 650 11.5 16.0 16Barbeque char 750 11.5 n.d.a 8Soil PyOM 560 9.0 20.0 45

a Not determined.

544 S. Filimonova et al. / Geoderma 232234 (2014) 538546formed by the fused aromaticmoieties of BC. Comparable 129Xepatternshave been reported for carbonaceousmaterialswith graphite-like struc-tures (Filimonova et al., 2011; Radovic et al., 1997; Simonov et al., 1999;Suh et al., 1991). Also, the barbeque charcoal and individual PyOM par-ticles measured in the present study featured similar spectra (Fig. 3C).The high-field overlapping 129Xe signals between (1015 ppm) are at-tributable to relatively large pores of meso- and macropore range,most probably formed by plant residues of varying decomposition de-gree, e.g. party burnt.

The identifiedmicroporous BC domains in the POM fractions are ap-parently non-uniform, as the detected large 129Xe signal width (35KHz) is a typical indication of a pore heterogeneity. Such pore heteroge-neity ismost probably caused by the varying condensation degree of ar-omatic units that makes them approaching each other at a varyingdistance. On the other hand, these units having a natural tendency toalign parallel to each other may be prevented from doing so by the het-eroatoms at their peripheries (Radovic et al., 1997). The created micro-porous constrictionsmost probably led to the activated diffusion effectsin the N2 adsorption. Indeed, the soil POM fractions showed rather low,most probably underestimated SBET (811 m2 g1) (Table 5). The lowSBET in the model charred samples confirmed this limitation. The ab-sence of microporous regions within the POM fractions of the B2 hori-zon (129Xe NMR) combined with the low C aromaticity (13C NMR)advised to the negligible BC content, supposedly due to the low extentof SOM charring and the poor mobility and translocation of BC domainsthrough the soil profile.

Bearing in mind the concurred formation of micropores (fused aro-matic structures identified by129Xe NMR) and the enhanced relative in-tensity of aromatic moieties (13C NMR) in both the natural soil fractionsandmodel charcoals (Figs. 2 and 3),we quantitatively compared the so-called charring degree estimated by the use of these two techniques.More exactly, we compared the ratio of the relative intensities of thedown-field to the high-field 129XeNMR signals (Imicro/Imeso,macro) versusthe ratio aromatic C/alkyl C signal intensities (Table 5). For most of thesamples, both these ratios showed a similar trend, although no definitecorrelation between themwas recognized, most probably owing to am-biguities associated with the quantitative analysis of the 13C CP NMRdata. Instead, we suggest that the former ratio considering the relativeintensities of the 129Xe peaks (microporous polyaromatic structures)serves as a more reliable estimate of the charring degree in the naturalPyOM.

4.2.2. Nano-scale heterogeneity of the soil PyOMThe multi-component 129Xe NMR patterns detected in the soil POM

fractions (Fig. 3B) pointed that the microporous regions (polyaromaticBC structures) co-existed with larger pores (range of meso- andmacropores). Those larger pores are attributable to the (i) low-densedomains formed by the alkyl functional groups (Radovic et al., 1997) ad-jacent to polyaromatic sheets and (ii) litter of varying decompositiondegree. The broadness of the down-field 129Xe signal (35 KHz) advisedto the heterogeneous character of the tested micropores, which appar-ently implied their variable geometry and variable size, e.g. owing tomolecular variation of charred structures caused by the different char-ring pathways of the ligneous and cellulosic compounds. Narrowerpores are thought to result from a better alignment of more orderedcrystallites, i.e. basic structural units (polyaromatic sheets) (Schmidtet al., 2001). On the other hand, an eventual decrease in the effectivewidth of micropores may be given by the functional groups, e.g. carbox-yl or phenolic. The presence of O-containing functionalities in the stud-ied soil PyOM and model chars is supported by the large values of theobserved (130160 ppm) exceeding those reported for synthetic car-bons (50110 ppm) (Simonov et al., 1999). The latter typically possess ahigher crystallinity and a lower oxidation degree. As suggested else-where (Brodowski et al., 2005; Cheng et al., 2008), carboxyl functional-ities are mainly located on the surfaces of charred species, as comparedto entire particles, thus making them prone to interactions with othersoil components, including organic and mineral matter, and contami-nants. The long-term exposure of PyOM in soils may however result inconsiderable penetration of oxidation into an interior of char particles(Cheng et al., 2008).

The recognized nano-scale heterogeneity of pores within the soilPyOM makes it difficult to estimate the pore size via analysis of the129Xe NMR shifts, unlike materials with more regular pores such aszeolites, or silica-based samples (Terskikh et al., 2002). Moreover, incarbonaceous materials not only the pore size, but also strong adsorp-tion sites (e.g. O-containing groups), or certain preferential orientationof graphitic layers may contribute to the detected 129Xe chemical shift(Simonov et al., 1999; Suh et al., 1991). Despite these limitations, onecan calculate one valuable parameter, namely the mean displacementdistance of Xe atoms between the micro- and meso-/macroporousdomains, i.e. the distance between the fused aromatic units and thedomains formed by alkyl functional groups. In fact, the chemical shift dif-ference between the low-field and high-field 129Xe signals =140150 ppm (Fig. 3B) corresponds to the frequency exchange of Xeatoms between these two regions: = 110.68 = 15.417.6 kHz.The characteristic time for Xe exchange between adsorption centers t=1/ = 5.9 1056.5 105 s. Considering Xe diffusion to obey theformula br(t)2N = 6Dt (diffusion coefficient D = 5.71 102 cm2 s1

at P = 1 atm) (Mair et al., 1998), one obtains the mean Xe displacementbr2N ~3844 m,which is of the same order ofmagnitude as the particlesize in the coarser POM fractions (N20 m). This advises that the domainswith fused aromatic rings and those formed by the functional groups (e.g.aliphatic) may exist within the same BC particle. Such a model concurswith thedocumented occurrence of domains of varying crystallinitywith-in the same char particle (Knicker et al., 2005b). Micropores identified by129Xe NMR can thus be assigned to the regions of a higher crystallinity(more or less aligned polyaromatic units), while meso- andmacropores characterize poorly ordered low-dense regions withinthe heat altered bio-molecules. The structural nano-scale heteroge-neity of the soil PyOM is therefore controlled by the chemical differ-ences among charred structures, and it most probably translatesinto the heterogeneity of PyOM on a larger scale. Indeed, a great va-riety of PyOM particles may exist in soils, i.e. differing by shape andsurface properties, starting from large pieces embedded within themineral soil matrix to very fine particulate material within soil ag-gregates (Brodowski et al., 2005; Ponomarenko and Anderson,2001). For the laboratory charred plant materials, Keiluweit et al.(2010) explained the observed structural diversity by the molecularvariation in different char categories resulting in their varyingcrystalline- versus amorphous character.

545S. Filimonova et al. / Geoderma 232234 (2014) 538546The peculiar co-existence of micro- and meso-/macroporous areaswithin the same PyOM particle may have implications for the specificsorption character onto soil PyOM, e.g. selective non-reversible sorptionof bio-macromolecules (Schmidt et al., 2001; Yu et al., 2011), whichmost probably geometrically fit to the porous nano-structure of PyOM.On the other hand, the heterogeneity of micropores within the BC do-mains (formed by the aromatic clusters of varying size) may help to ra-tionalize the differences in decomposition rates between various BCforms (Baldock and Smernik, 2002).

4.3. Interaction PyOMsoil minerals

The organo-mineral soil fractions of both the Ah- and B2 horizonswere characterized by the relatively small 129Xe chemical shift values(718 ppm) (Fig. 3A), assignable to the relatively large pores(Terskikh et al., 1993), e.g. those formed by agglomerated mineral par-ticles. Indeed, pureminerals (quartz, illite) exhibited 129Xe signals in thesimilar chemical shift range (Fig. 4B). Therefore, the 129XeNMRpatternsof the soil organo-mineral fractions may be used as indicators or theirmineralogical composition. For comparison, smectite rich soil would ex-hibit 129Xe resonances in the lower field (at ca. 25 ppm) characteristicfor the expandable clay silicates (Filimonova et al., 2004), while Al richsoils feature signals at ca. 100160 ppm typical for allophane andAl-humus compounds (Filimonova et al., 2011). On the other hand,the 129Xe chemical shift appears to be sensitive to the mineral pore en-largement following the SOM destruction (Fig. 3A).

Despite the comparable relative intensity of the aromatic 13C NMRsignal in the POM and organo-mineral fractions (Fig. 2), the latter frac-tions did not evidence the fragments of polyaromatic units by the 129Xesignals (Fig. 3).We explain the lack of these signals by the occurrence ofcharred species in a highly dispersed form (e.g. aromatic domains of avery small cluster size), most probably due to the association with soilminerals. The association PyOMsoil minerals was confirmed in theexperiment with the laboratory prepared organo-mineral mixturemodeling our studied Arenosol. In fact, this model mixture containedthe natural PyOM species, together with the pure minerals and organicrich material (Table 2) that were allowed interacting with each otherduring the incubation. After completion of the incubation, the resulting129Xe NMR spectrum evidenced the lack of polyaromatic regions,despite the presence of PyOM. Themineral attachment to PyOM speciesoccurs rather fast (i.e. within the incubation time period of 20 days),most probably via a ligand exchange between the carboxyl groupson the charred surfaces (evidenced by the 13C NMR) (Fig. 4A) andhydroxylated mineral surfaces of the short-range order Al- and Feoxyhydroxides. On the other hand, the abundance of Fe oxyhydroxidesin our studied soil may facilitate electrostatic interactions between aro-matic -systems of PyOMand the surface Fe3+ cations. The binding roleof Al and Fe at the interface between the char species and mineralphases has been recognized elsewhere (Keiluweit and Kleber, 2009;Nguyen et al., 2008). According to our 13C NMR results, the aromaticring opening may not be required for the interactions PyOMsoil min-erals to proceed, as the aromatic C contribution was rather high in thesoil organo-mineral fractions (Ah horizon). Hockaday et al. (2007) ob-served, in contrast, the greater abundance of non-aromatic carbon fol-lowing the aging of soil charcoals. The particular transformationpathways of PyOM in soils may therefore be dependent on several spe-cific factors, such as the origin of PyOM, the nature of mineral matrixes,and environmental conditions.

5. Conclusions

In this study, an approach was developed to identify and character-ize the black carbon (BC) domains in a natural pyrogenic soil organicmatter (PyOM). We applied 129Xe NMR spectroscopy of adsorbed Xeatoms to differentiate between the specific structural domains of BC inthe POM fractions and to elucidate interactions between the PyOMand soil minerals in the organo-mineral fractions of a fire-affectedArenosol. The occurrence of BC domains (fused aromatic rings) in thePOM fractions was revealed by the characteristic 129Xe NMR signal(130160 ppm). We suggest using the relative intensity of this signalas a reliable estimate of the charring degree of the natural PyOM. Onthe other hand, our approach sensitively reveals the nano-structuralheterogeneity in the PyOM and BC domains (multi-component 129XeNMR pattern), apparently resulting from the chemical variation ofcharred structures. Micropores (fused aromatic units) co-exist withthe larger pores (e.g. formed by the alkyl groups) within the samePyOM particle, as it was judged from the calculated Xe displacementdistance between these regions. This specific pore network may haveimportant implications for the persistence of PyOM in the soil and be-havior as a sorbent.

In the organo-mineral soil fractions, aromatic domains occurred in ahighly dispersed form (decreasing of the aromatic cluster size), assuggested owing to the association with soil minerals. An increased(fire-induced) proportion of short-range order minerals and mixedlayer silicates most probably facilitates PyOM mineral interactionsvia an enhanced extent of hydroxylated surfaces.Acknowledgments

This study was financially supported by the DeutscheForschungsgemeinschaft (DFG, Bonn) (KN 463/8-1). Heike Knicker(Instituto de Resursos Naturales y Agrobiologia de Sevilla, Spain) isgratefully acknowledged for providing us with the soil materials.We are indebted to Werner Husler (Lehrstuhl fr Bodenkunde,TU Mnchen) for the XRD-analysis. Carsten Mller and GeertjePronk (Lehrstuhl fr Bodenkunde, TU Mnchen) are thankfullyacknowledged for supplying us with the model charred materials andcommercial barbeque charcoal. We are also indebted to the anonymousreviewer for the helpful comments and suggestions.References

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Nano-structural and chemical characterization of charred organic matter in a fire-affected Arenosol1. Introduction2. Materials and methods2.1. Samples2.2. Chemical treatments and analyses2.3. 13C CPMAS NMR2.4. 129Xe NMR of adsorbed xenon2.5. N2 adsorption

3. Results3.1. Soil mineralogy: XRD and chemical extraction data3.2. OC content and SOM quality3.3. 129Xe NMR data3.4. N2 adsorption data

4. Discussion4.1. Fire-induced alterations of soil minerals and organic matter4.2. PyOM in the fire-impacted soil: nano-structure and extent of heterogeneity4.2.1. Identification of microporous BC domains generated by fire in the POM fractions4.2.2. Nano-scale heterogeneity of the soil PyOM

4.3. Interaction PyOMsoil minerals

5. ConclusionsAcknowledgmentsReferences