SOILS, SEC 3 • REMEDIATION AND MANAGEMENT OF CONTAMINATED OR DEGRADED LANDS •

RESEARCH ARTICLE

Changes in biochar properties in typical loess soil under a 5-yearfield experiment

Lianshuai Tan1,2& Cengceng Sun1,2

& Ying Wang1,2& Tongtong Wang1,2

& Gao-Lin Wu2,3& Honghua He2,3

&

Jiyong Zheng1,2

Received: 7 March 2019 /Accepted: 8 July 2019 /Published online: 3 August 2019# Springer-Verlag GmbH Germany, part of Springer Nature 2019

AbstractPurpose After biochar is applied to soil as a sustainable soil amendment and a carbon (C) sequestration technique, its physico-chemical properties change over time. However, few studies have reported on the changes of biochar properties over the agingprocess under field conditions. An understanding of such changes can help us to make full use of biochar as a sustainable soilamendment and C sequestration technique.Materials and methods We used apple tree branches as the raw material to produce biochar and studied the changes in thephysicochemical properties of the biochar over a 5-year field experiment. Scanning electron microscopy (SEM), specific surfacearea (SSA) and micropore area, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) spectroscopy, elemen-tal analysis, and X-ray photoelectron spectroscopy (XPS) were performed.Results and discussion After 5 years of aging, the SSA of the biochar had increased by 23.91% relative to that of fresh biochar;however, the SSA initially decreased over the first 3 years and then increased over the next 2 years. The reasons for the initialdecrease were the destruction and clogging of the existing pore structure, whereas new micropore formation was responsible forthe subsequent increase, as verified by micropore area, SEM and XPS analyses. The C content of the biochar was stable over the5 years, but the surface O content and quantity of oxygen-containing functional groups increased relative to those of freshbiochar, which impacts the adsorption capacity of the biochar.Conclusions Our findings illustrate that the SSA of the biochar varied with time during the aging process. The stability of the Cillustrated the potential of biochar as a C sequestration technique. The increase in oxygen-containing functional groups of thebiochar was responsible for the process of nutrient adsorption.

Keywords Aging . Biochar . Dynamic change . Field experiment . Loess soil . Physicochemical properties

1 Introduction

Biochar is the carbon (C)-rich product produced by heatingbiomass under limited or zero oxygen supply (Demirbas2004; Lehmann et al. 2006). Biochar has a strong adsorptioncapacity and strong C sequestration ability because of its largespecific surface area (SSA), porous structure, and high stabil-ity (Lehmann et al. 2008; Laird 2008; Trigo et al. 2014).Numerous studies have shown that biochar can be applied asa soil amendment to improve soil properties, such as waterholding capacity, and reduce soil nutrient leaching (Lehmannet al. 2003; Liang et al. 2006; Basso et al. 2013; Herath et al.2013; Liu et al. 2014; Wang et al. 2018). However, thesebenefits can be altered due to the changes in biochar propertiesduring the aging process (Singh and Cowie 2014; Heitkötterand Marschner 2015; Li et al. 2016; Ren et al. 2016).

Responsible editor: Yong Sik Ok

* Jiyong Zhengzhjy@ms.iswc.ac.cn

1 College of Natural Resources and Environment, Northwest A & FUniversity, Yangling 712100, China

2 State Key Laboratory of Soil Erosion and Dryland Farming on theLoess Plateau, Northwest A& FUniversity, Yangling 712100, China

3 State Key Laboratory of Soil Erosion and Dryland Farming on LoessPlateau, Institute of Soil and Water Conservation, Chinese Academyof Sciences and Ministry of Water Resources,Yangling 712100, Shaanxi, China

Journal of Soils and Sediments (2020) 20:340–351https://doi.org/10.1007/s11368-019-02398-0

Numerous studies have shown that the properties of bio-char change over time (aging), but most of these studies haveemployed artificial agingmethods to simulate or accelerate theaging process of biochar in soil under laboratory conditions.These methods have included the exposure of biochar to theenvironment for a certain period of time at different tempera-tures (Singh and Cowie 2014; Shi et al. 2015; Wang et al.2017), using acid solvents such as H2O2, HCl, HNO3, andH2SO4 to accelerate the biochar oxidation process (Qianet al. 2015; Ghaffar and Abbas 2016; Lawrinenko et al.2016; Mia et al. 2017a), using physical methods of freeze-thaw cycles and dry-wet cycles (Hale et al. 2011) and/ormixing soil and biochar and incubating the mixture for extend-ed periods (Guo et al. 2014; Mukherjee et al. 2014; Zhanget al. 2016; Rechberger et al. 2017; Khorram et al. 2017;Zhelezova et al. 2017; Ren et al. 2018b; Huang et al. 2018).However, studies have shown that changes in biochar can beinduced by multiple factors (Li et al. 2014; Singh and Cowie2014; Mia et al. 2017b). Thus, indoor simulation experimentscannot fully reveal the integrated effects of natural factors(e.g., temperature, water, light, oxidation, biotic factors) onbiochar in soil. Therefore, it is necessary to study the changesin biochar properties in soil under natural conditions.

De la Rosa et al. 2018 reported the changes in the stabilityof five types of biochar after 2 years. Sorrenti et al. (2016)reported the changes in the physicochemical properties ofpeach and grapevine chipped biochar after 4 years, andDong et al. (2017) reported the changes in the quantity andquality of mushroom production waste biochar after 5 years.All of these studies compared the properties of biochar beforeand after application to the soil to illustrate the changes withaging. However, few studies of such changes under naturalconditions have been conducted. An understanding of the dy-namic changes in biochar with aging is needed.

In this study, we applied apple tree branch-derived biocharto natural soil in June 2012 andmeasured the characteristics ofthe biochar isolated from the soil in June 2015 (after 3 years)and June 2017 (after 5 years) to study the changes in thebiochar properties during the aging process. Our objectiveswere (1) to clarify how the biochar properties changed in loesssoil under field conditions, (2) to explore the change trendsduring the aging process, and (3) to determine whether bio-char is suitable as a soil amendment and for carbon (C) se-questration over time from the perspective of physical andchemical property changes.

2 Materials and methods

2.1 Study site

The experiment was conducted at the Guyuan EcologicalExperimental Station of the Chinese Academy of Sciences.

The station is located in the central west part of the LoessPlateau of Yuanzhou District, Ningxia, China (106° 15′–106° 30′ E, 35° 59′–36° 03′ N). The average altitude is1773.5 m, and the geomorphology is that of a typical beam-like hilly area on the Loess Plateau. The average annual tem-perature in this area is 6.2 °C; the hottest month (July) has anaverage temperature of 18.9 °C and the coldest month(January) has an average temperature of − 8.3 °C. The recordhigh temperature is 34.6 °C, and the record low temperature is− 28.1 °C. The average annual sunshine duration is 2518 h,and the average annual rainfall is 472 mm. The soil type in thetest area is loess soil developed from loess parent materials.The basic properties of the soil are described in Table 1. Themain types of vegetation cover in the region are shrubs, grass-lands, and agricultural land.

2.2 Experimental design and sampling

The biochar was provided by the Shanxi Yi-xin BioenergyCompany (Shanxi, China). Apple tree branches were used asthe raw material, and the biochar was produced by a high-temperature (550 °C) pyrolysis method. The biochar wascrushed and passed through a 2-mm sieve for use. The prop-erties of the biochar are shown in Table 1.

To study the integrated effects of biochar addition into thesoil in this area, a field plot experiment was conducted inJune 2012. The biochar addition ratios of the treatments were1%, 3%, and 5% (w/w), with three replicates. The plot sizewas 2.4 m × 2.4 m (5.76 m2), and the plots were separatedfrom one another by aluminum-plastic panels (Fig. 1).Potatoes were planted in May and harvested in October everyyear. The biochar was added as follows: First, weeds and otherimpurities were cleared from the experimental plots; then,20 cm of the surface soil was stripped and stockpiled. The

Table 1 The basic properties of the 0–20-cm soil layer and biochar inthe study

Item Soil Biochar

Sand (%) 45.85 /

Slit (%) 34.47 /

Clay (%) 19.68 /

Texture Sandy loam /

pH 8.43 ± 0.02 9.52 ± 0.02

Organic matter (g/kg) 17.13 ± 1.14 866.25 ± 9.05

Total nitrogen (g/kg) 0.76 ± 0.02 8.77 ± 0.20

Nitrate nitrogen (mg/l) 2.21 ± 0.01 1.14 ± 0.05

Ammonium nitrogen (mg/l) 3.25 ± 0.11 –

EC (μs/cm) 125.1 ± 0.15 808 ± 1.00

CEC (cmol/kg) 10.57 ± 0.09 20.14 ± 0.05

“–” cannot be detected

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biochar was fully mixed with the stripped soil using a blender,and then, the soil was backfilled to its original location. Asample of unapplied biochar (here termed “fresh,” marked asBC2012) was stored in a fully sealed 5 L wide-mouth bottle atroom temperature.

In June 2015 and June 2017, samples from the 0–20 cmsoil layer were collected using a soil auger. Sampling wasconducted at three randomly selected locations in all threereplicate plots of each treatment, and then, the samples ofeach set of replicate plots were uniformly mixed. Althoughthe additive proportions of biochar into the soil were dif-ferent, the changes in the physicochemical propertiesshould vary similarly (Dong et al. 2017). Therefore, wecollected one replicate sample from each 2015 treatmentfor mixing. Then, we chose either of the two remainingreplicate samples from each 2015 treatment for mixing.Finally, the remaining replicate sample of each 2015 treat-ment was mixed; ultimately, three 2015 samples of mixed1%, 3%, and 5% concentrations were obtained, which wereconsidered as 3 replicates and marked as BC2015. Samplesin 2017 were treated in the same way and marked asBC2017. After the soil samples were air-dried and visibleimpurities, such as plant roots, were removed, the soil sam-ples were sieved through a 2-mm sieve for further use.

2.3 Separation of the aged biochar particles

Tweezers were used to collect the visible biochar fromBC2015 and BC2017 (Fig. 2). The biochar particles weresuspended in deionized water, and the solution was shakenslightly to remove all adhering soil particles. The biocharwas subsequently rinsed three times with deionized waterand dried at 60 °C for 8 h in preparation for the measurements.Because the biochar in the soil had been frequently subjectedto rainfall over the 3- and 5-year periods, the process of wash-ing did not influence its properties.

2.4 Characterization sampling and measurement

2.4.1 Scanning electron microscopy

The surface morphologies of the fresh (BC2012) and agedbiochar samples (BC2015 and BC2017) were observed by aJSM-6510LV scanning electron microscope (JEOL, Japan)operating with a 20 kV scan voltage. Before the scanningelectronmicroscopy (SEM) imaging, all biochar samples weresputter coated with gold to improve the sample conductivityand enhance the image quality.

2.4.2 Specific surface area and micropore area

Approximately 0.5 g of biochar sample was degassed for3 h at 125 °C. Then, the SSA and micropore area weredetermined via a V-Sorb 2800P specific surface area andpore size analyzer (GAPP, China) using N2 as the adsor-bate at 77 K under a relative pressure of 0.05–0.20. The

Fig. 1 Images of the experimental site in 2012, 2015, and 2017

Fig. 2 Separation of the visible biochar from the soil-biochar mixture

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SSA and micropore area of the biochar sample were ana-lyzed using the Brunauer-Emmett-Teller method (Brunaueret al. 1938) and t-plot method (De Boer et al. 1966),respectively.

2.4.3 Fourier transform infrared spectroscopy

The infrared spectra were measured with the KBr pellet meth-od. The surface functional groups of the fresh and aged bio-char samples were identified using a Vertex 70 Fourier trans-form infrared (FTIR) spectrometer (Bruker, USA). Sixteenscans were collected over a range of 400–4000 cm−1 at aresolution of 2 cm−1. The functional groups were identifiedaccording to published references (Pastorova et al. 1994; Guoand Bustin 1998; Ellerbrock and Gerke 2004; Chen et al.2008; Rre et al. 2008; Simkovic et al. 2008; Yang et al.2008; Singh et al. 2016).

2.4.4 X-ray diffraction spectroscopy

The mineral species on the biochar surfaces were identifiedusing a D/max2400 X-ray powder diffractometer(RIGAKU, USA). The biochar samples were ground asthin as possible, and the sample powder was added asevenly as possible into the sample holder to enable thesample to spread out over the window hole. The squashmethod was utilized to make the test piece; the test piecewas vertically compressed by nonrotatable glass. It wasnecessary for the prepared sample test piece to have a flatsurface and be in the same plane as the reference plane ofthe sample holder. The test piece was placed into the in-strument sample holder, and the measurement conditionswere as follows: scan step size, 0.02 degree; scan speed,2 deg.·min−1; receiving slit width, 0.15 mm; and operatingvoltage and current, 30–40 kV and 30–40 mA, respective-ly. The X-ray diffraction (XRD) patterns were analyzedusing Jade 6.5 and the related literature (Zama et al.2018) and JADE 6.5 PDF cards were used to determinethe mineral compositions of the fresh and aged biocharsamples.

2.4.5 Elemental analysis

The C, H, O, N, and S contents of the fresh and aged biocharwere measured using a Vario EL cube element analyzer(German Element, GRE) with argon as the carrier gas. Theoxidation degrees of the different types of biochars were esti-mated by the O/C molar ratio. The H/C molar ratio was usedas an indicator of the aromaticity and carbonation level ofbiochar, and the biochar polarity was estimated by the (O +N)/C molar ratio (Huang et al. 2018).

2.4.6 X-ray photoelectron spectroscopy analysis

Fresh and aged biochar samples were scanned with anESCALAB 250 Xi X-ray photoelectron spectrometer(Thermo Scientific, USA) to investigate the chemical statesof the main elements and functional groups on the surface.The X-ray source was a monochromated Al Kα (1486.6 eV)source. The experimentally obtained elemental electron bind-ing energy was corrected with C1s (284.8 eV). In the X-rayphotoelectron spectroscopy (XPS) spectrum, the abscissa in-dicates the binding energy, and the ordinate indicates the elec-tronic counts. The wide scans of the XPS spectra were ana-lyzed using Thermo Avantage v5.979 to determine the surfaceelemental content. The surface O/Cmolar ratio was calculatedto evaluate the surface oxidation degree and hydrophilicity ofthe biochar, and the surface (O + N)/C molar ratio was calcu-lated to evaluate the surface polarity of the biochar (Huanget al. 2018). The narrow scans of the C1s peak were fitted bythe same program to analyze and quantify the current state ofC. The chemical bonds were identified by reference to pub-lished studies (Zielke et al. 1996; Yao et al. 2010; Azargoharet al. 2014; Singh et al. 2014; Kumar et al. 2018).

2.5 Data analysis

The data were analyzed using SPSS v.23, and significant dif-ferences between means were determined using ANOVAfollowed by Tukey’s HSD post hoc test (p < 0.05).Figures were drawn by using Origin 2016.

3 Results

3.1 SEM

SEM images of the three biochar samples from each year areshown in Fig. 3. In BC2015, the structures of some of thepores had been destroyed, and in BC2015 and BC2017, someof the pores had been filled by small particles. The number ofsmall holes increased with time after biochar application to thesoil.

3.2 Specific surface area and micropore area

The results showed that the SSA of BC2012 was 1.213 ±0.067 m2 g−1 (Table 2) and that the SSA of BC2015 wassignificantly decreased to 1.032 ± 0.029 m2 g−1 (p < 0.05).However, the SSA of BC2017 significantly increased andeven exceeded that of BC2012, reaching 1.503 ±0.034 m2 g−1 (p < 0.05). The micropore area could only bemeasured in BC2017, and the value was 0.255 ± 0.018m2 g−1.

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3.3 Elemental analysis

The biochar contents of C, O, N, and H in the biochar weredifferent after 3 and 5 years (Table 3), but the differences werenot significant. The S content was 0.23 ± 0.05% for BC2012;it had significantly decreased to 0.10 ± 0.03% (p < 0.05) after3 years and 0.07 ± 0.03% (p < 0.05) after 5 years. In addition,the O/C molar ratio was increased after 3 and 5 years of aging,with values of 0.15 in 2012, 0.19 in 2015, and 0.17 in 2017.The same change trends were also observed in the H/C and

(O + N)/C molar ratios, both of which showed an increasetrend after 3 and 5 years of aging.

3.4 FTIR spectroscopy

The BC2012, BC2015, and BC2017 samples all contained thefollowing peaks (Fig. 4): 3425 cm−1 (stretching of the O-Hbond in the hydroxyl groups), 1570 cm−1 (C=O stretching inthe aromatic ring), 1430 cm−1 (symmetric C-O stretching ofCOO- or stretching and OH deformation), and 874 cm−1 (out-of-plane bending of the aromatic). Additionally, two newpeaks were exhibited at 1081 cm−1 and 466 cm−1 in the scanspectrum of BC2017, indicating the C-O stretching vibrationsof the C-O-C groups and the absorbances of the Si-O-Si vi-brations, respectively. The peak intensities differ among theBC2012, BC2015, and BC2017 samples. Specifically, thepeaks at 3425 cm−1 and 1570 cm−1 were of greater intensityfor BC2015 and BC2017, suggesting that the aged biocharcontained more -OH and C=O groups than the fresh biochar.However, the intensity at 874 cm−1 exhibited the oppositeresult, suggesting that the fresh biochar contained more aro-matic structures than the aged biochars.

Fig. 3 Scanning electronmicrographs of BC2012 (a, b),BC2015 (c, d), and BC2017 (e, f)

Table 2 Specific surface area and micropore area of the fresh and agedbiochar samples

Type Specific surface area (m2 g−1) Micropore area (m2 g−1)

BC2012 1.213 ± 0.067b –

BC2015 1.032 ± 0.029c –

BC2017 1.503 ± 0.034a 0.255 ± 0.018

“–” cannot be detected. Different letters denote significant differencesamong the biochars (p < 0.05)

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3.5 XRD spectroscopy

The occurrence of various mineral phases was evident in theXRD spectra of BC2012, BC2015 and BC2017 (Fig. 5). Allthree biochar samples exhibited the same peaks, indicatingthat the surface mineral species had not changed over time.The peaks occurring at 2θ = 26.6° were attributable to SiO2,and the peaks occurring at 2θ = 29.5°, 39.4°, 44.3°, 47.6°,48.6°, and 64.6° indicated a one-to-one correspondence tothe (104), (113), (202), (018), (116), and (300) crystal facesof CaCO3.

3.6 XPS analysis

The XPS data for the relative concentrations of each elementon the surfaces of the biochar samples are shown in Table 4.Five elements, i.e., C, O, N, Ca, and Si, were detected in allthree biochar samples. The C and O elements were the pre-dominant elements at the biochar surface of the biochar, andthe change in the content of each element was not regular

during the two periods from BC2012 to BC2015 and fromBC2015 to BC2017. The other elements also showed differenttrends in the two phases. However, only the contents of C andO showed significant differences after aging, with C contentdemonstrating a downward trend, and O content showing anincreasing trend. In addition, both the O/C molar ratio and the(O +N)/C molar ratio were increased after 3 and 5 years ofaging. In detail, the O/C molar ratio of biochar was 0.10 in2012 and increased to 0.19 and 0.17 in 2015 and 2017, re-spectively. The (O +N)/C molar ratio of biochar was 0.13 in2012, and it increased to 0.23 and 0.21 in 2015 and 2017,respectively. The chemical bond compositions (%) of carbon(C1s) of the fresh and aged biochar samples are shown inTable 5. The C-C content significantly decreased from52.44 ± 1.87 to 47.40 ± 0.70% (p < 0.05) between BC2012and BC2015 and then to 36.90 ± 1.18% (p < 0.05) inBC2017, and the contents of oxygen-containing functionalgroups, including C-O and C=O, were increased over time.In addition, we detected C-OOR, a new oxygen-containingfunctional group, in BC2017.

Fig. 5 XRD spectroscopy of BC2012, BC2015, and BC2017. The black,red, and blue lines represent BC2012, BC2015, and BC2017, respectively

Fig. 4 FTIR spectroscopy of BC2012, BC2015, and BC2017. The black,red, and blue lines represent BC2012, BC2015, and BC2017, respectively

Table 3 Element content (%) of fresh and aged biochar samples

Type C O H N S O/C H/C (O +N)/C

BC2012 72.53 ± 2.24a 14.85 ± 1.31a 2.49 ± 0.13a 1.49 ± 0.23a 0.23 ± 0.05a 0.15a 0.41a 0.17a

BC2015 75.60 ± 4.64a 18.83 ± 4.79a 2.99 ± 0.15a 1.07 ± 0.08a 0.10 ± 0.03b 0.19a 0.47a 0.20a

BC2017 74.04 ± 1.35a 16.96 ± 2.48a 2.82 ± 0.37a 1.28 ± 0.20a 0.07 ± 0.03b 0.17a 0.46a 0.19a

Different letters denote significant differences among the biochars (p < 0.05)

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4 Discussion

4.1 Variation in the structure and specific surface areaof biochar with aging

Biochar is a porous material with a large SSA (Breweret al. 2014; Zhang et al. 2015). However, after biochar isapplied to the soil, its properties may change for a varietyof reasons (Hale et al. 2011; Beesley et al. 2011). In ourstudy, the SSA of the biochar after 3 and 5 years of agingwas decreased and increased, respectively, relative to thatof fresh biochar. In addition, the micropores could be mea-sured only after 5 years of aging. The changes in SSA andmicropores could be explained by the changes apparent inthe SEM images. For the biochar that was aged in soil for3 years (BC2015), the destruction of pore structure and theclogging of pore spaces both led to a decrease in SSA(Uchimiya et al. 2012; Liu et al. 2013; Xu et al. 2014;Ren et al. 2018a). The increase in SSA in BC2017 wasaccompanied by a change in the SEM results, whichshowed an increase in the number of small pores. Studieshave shown that biochar micropores mainly form throughthe loss of C during the pyrolysis process and the fractureof the C frame (Uchimiya et al. 2010; Martin et al. 2012;Ding et al. 2014). Although macropores may act as precur-sors for micropores, micropores contribute most to theSSA of biochar (Lehmann 2007a). Our SEM data also sup-port this conclusion. In the SEM images, only a few smallpores in the fresh biochar were formed by the pyrolysisprocess, but as the aging time increased, the number ofsmall pores increased. The small pores formed during the

aging process may be caused by (1) the labile fraction oforganic C being digested by soil microbes or leached out asdissolved organic C (Dong et al. 2017) and (2) the fractur-ing of the C skeleton, which can be proven by the signif-icant decrease in the C-C bond content in the XPS data(Martin et al. 2012; Ding et al. 2014). All these observa-tions indicate that the increase in SSA in BC2017 was dueto the increase in the number of micropores. The contribu-tion of micropores to surface area is significant; thus, de-spite some plugging of the pore structure, the surface areasignificantly increased over time. The same SSA resultalso appeared in studies by Kupryianchyk et al. (2016)and Dong et al. (2017), in which relatively large SSAsfor naturally aged chars (aged for 150 and 2000 years)and biochar 5 years after soil application. Dong et al.(2017) also found that small pores appeared on the surfaceof biochar after 5 years of aging. However, Mia et al.(2017b) speculated that the SSA of naturally aged biocharmight decrease due to the blocking of pores and thedestruction of pore structure. Moreover, Mukherjee et al.(2014) reported that the SSAs of biochars made from oak,pine, and grass all showed decreasing trends after15 months of field aging. It has been hypothesized thatthe reduction in surface area due to pore structure destruc-tion that reduces the surface area may be greater than theeffect of the micropore formation that increases the surfacearea, and the differences in the type of biochar (raw mate-rial, pyrolysis temperature, etc.) and soil properties mightbe the main causes of this change (Obia et al. 2017). In thepresent study, the SSA of biochar made from apple treebranches showed a dynamic change in a relatively shorttime (compared with 100 or even 1000 years) followingthe application of the biochar to the soil. There were twomain reasons for this change: (1) the pore structure becom-ing broken and clogged by minerals, soil organic matter,and microbial biomass (Hockaday et al. 2007; Ren et al.2018a) and (2) the formation of micropores (Dong et al.2017). Biochar properties vary widely among different bio-char types (Obia et al. 2017). Because there have beenlimited studies on the SSA of naturally aged biochar, wecannot fully determine all of the biochar trends for the SSAfrom our data. The changes in SSA and their causes requirefurther research.

Table 4 XPS results for the surface elemental contents (%) of the fresh and aged biochar samples

Type C O N Si Ca O/C (O +N)/C

BC2012 83.62 ± 2.02a 11.67 ± 0.95b 2.64 ± 0.76a 1.25 ± 0.49a 0.81 ± 0.11a 0.10b 0.13b

BC2015 74.79 ± 3.06b 19.36 ± 1.97a 3.47 ± 0.62a 1.15 ± 0.35a 1.23 ± 0.41a 0.19a 0.23a

BC2017 76.88 ± 2.48b 17.69 ± 1.92a 3.09 ± 0.69a 1.52 ± 0.56a 0.82 ± 0.28a 0.17a 0.21a

Different letters denote significant differences among the biochars (p < 0.05)

Table 5 XPS results for the chemical bond compositions (%) of carbon(C1s) of the fresh and aged biochar samples

Type BC2012 BC2015 BC2017

C-C 52.44 ± 1.87a 47.40 ± 0.70b 36.90 ± 1.18c

C-O 29.08 ± 0.93b 30.78 ± 1.45b 34.60 ± 0.68a

C=O 18.49 ± 2.14a 21.82 ± 0.79a 19.04 ± 0.77a

C-OOR – – 9.47 ± 0.75

“–” cannot be detected. Different letters denote significant differencesamong the biochars (p < 0.05)

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4.2 Variation in the elemental contents of biocharwith aging

Numerous studies have shown that biochar can be oxidized inthe soil, which leads to changes in its elemental contents(Kasin and Ohlson 2013; Ghaffar et al. 2015; Sigmund et al.2017). However, our results did not show any significant dif-ferences in the contents of C, O, N, and H over the 5 years ofaging, which suggested that the C element content of the appletree branch-derived biochar was relatively stable after 5 years.Despite this, the low H:C of all of the biochars suggested highlevels of carbonation aromaticity. The fresh biochar showed alower H:C than did the aged biochars, suggesting that it had agreater proportionate aromatic C content than did the agedbiochars. Similarly, Ghaffar et al. (2015) found that the H/Catomic ratio of peanut shell biochar was increased after oxi-dation with a HNO3/H2SO4 mixture, and Huang et al. (2018)reported that the H/C molar ratio of rice husk biochar in-creased over 100, 200, and 300 days of aging. In general, anH:C atomic ratio lower than 0.4 (i.e., an H:C molar ratio lowerthan 4.8) indicates a high degree of aromatization. In the pres-ent study, the H:C molar ratios of both the fresh biochar andaged biochars were less than 4.8, which indicated that thearomaticity of the biochar remained high after aging and thatits C element content of the biochar used in our research wasrelatively stable. A high H:C molar ratio may be caused by thefollowing: (1) the inclusion of aliphatic organic C due to sorp-tion (Mia et al. 2017b) and (2) the introduction of H or O byfunctional groups on the biochar surface during aging process,resulting in a decrease in the relative content of C. In thepresent study, the O:C molar ratio of the fresh biochar was0.15; after 3 and 5 years of aging, the O:C molar ratio in-creased to 0.19 and 0.17, respectively. These results indicatedthat with aging, the hydrophilicity of the biochars was en-hanced and that the oxygen content increased, i.e., that theoxidation degree was enhanced. The enhanced hydrophilicityresulted from the increased oxidation of the biochar: such anincrease leads to an increase in the number of oxygen-containing functional groups, such as carboxyl groups andhydroxyl groups. The FTIR and XPS data both indicated anincrease in the number of polar functional groups, which re-sulted in the enhanced hydrophilicity of the biochar.Furthermore, the increase in the (O +N)/C molar ratio of bio-char with aging process also supports our theory, which sug-gested that the polarity of biochar after the aging process wasenhanced. The observed changes in the O/C and (O +N)/Cmolar ratios are consistent with the findings of Mia et al.(2017b) and Ghaffar et al. (2015). The contents of elementalC and O on the surface showed downward and upward trends,respectively, over time. The C content significantly decreasedfrom 83.62 ± 2.02% in BC2012 to 74.79 ± 3.06% (p < 0.05)and 76.88 ± 2.48% (p < 0.05) in BC2015 and BC2017, re-spectively. In addition, the O content significantly increased

from 11.67 ± 0.95% in BC2012 to 19.36 ± 1.97% (p < 0.05)and 17.69 ± 1.92% (p < 0.05) in BC2015 and BC2017. Thesurface C contents of the aged biochars were lower than thesurface C content of the fresh biochar. Similarly, Sorrenti et al.(2016) reported that the surface C content had decreased andthe surface O content had increased in peach and grapevinechipped biochar after 4 years of aging. A decrease in surface Cand an increase in surface O are signs of oxidation (Chenget al. 2006, 2008; Hale et al. 2011; Mia et al. 2017a), whichsuggests that biochar aging is mainly a surface phenomenoninvolving alterations of interfacial properties (Ahmad et al.2012). In the present study, the surface O/C and (O +N)/Cmolar ratios were calculated to investigate the changes in theoxidation degree, hydrophilicity, and polarity of the biocharsurface. The ratio changes were consistent with results of theelemental analysis, with both ratios increasing with aging.This result indicates that the polarity, hydrophilicity, and oxi-dation of the biochar surface increased with aging. However,whereas the surface O/C molar ratio was significantly in-creased by 90% and 70% after 3 and 5 years, respectively,the elemental analysis results indicated only 26.67% and13.33% increases after 3 years and 5 years, respectively.Furthermore, whereas the surface (O +N)/C molar ratio wassignificantly increased by 76.92% and 61.54% after 3 and5 years, respectively, the elemental analysis results indicatedonly 17.65% and 11.76% increase after 3 and 5 years, respec-tively. As for the reasons for this difference, we consider thatthere are two main points: (1) biochar aging is mainly a sur-face phenomenon and (2) the aging layer, that is, the protec-tive layer covering the biochar surface formed by the environ-ment during the aging process, prevents changes to the bio-char. The results are consistent with the findings of Huanget al. (2018), who reported that a layer formed during theaging process. In addition, the crystal structures on the biocharsurface vary depending on the raw materials used in the pro-duction process. It will be leached out as dissolved organicmatter during aging, thus affecting the biochar’s adsorptionperformance (Ren et al. 2018a). Using XPS, we detected Caand Si on the biochar surface, which was consistent with theXRD results. However, the XRD results indicated that thebiochar surface mineral species had not changed with time.We suggest there are two main reasons for this result: (1) thebiochar used in the experiment was relatively stable and (2)the aging time was too short and cannot affect the biochar’scrystal structure. The crystal structure changes that occur inbiochar with aging process are poorly understood, and furtherresearch is needed.

4.3 Variation in the functional groups of biocharwith aging

The FTIR results showed that the changes in the functionalgroup contents over time were inconsistent; i.e., contents did

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not consistently rise or fall. Compared to that of fresh biochar,the intensity of the -OH stretching peak at 3425 cm−1 of 3-year-aged biochar was increased. After 5 years of aging, theintensity was lower than that at 3 years but than that of freshbiochar. These results suggest that BC2017 contained morehydroxylic groups than BC2012 but fewer than BC2015.Similar trends were observed for the intensity of the C=Ostretching peak at 1570 cm−1; the results suggested thatBC2015 contained the most carbonylic groups, followed byBC2017, and finally BC2012. However, the intensity of out-of-plane bending of the aromatic peak at 874 cm−1 indicatedthat BC2012 containedmore aromatic structures than BC2015and BC2017. This change in aromatic structure was supportedby the H:C molar ratio results of the elemental analysis, whichsuggested that the aged biochars had few aromatic structures.The findings ofMia et al. (2017a) provide insight into why thehydroxylic and carbonylic groups increased and why the aro-matic structures decreased with aging. In biochar, the aromaticparts are linked by the aliphatic parts, and aliphatic parts alsooccur as side chains of the aromatic layers. When biochar isapplied to soil, the disconnected aromatic moieties can bereleased through microbial decomposition (Kuzyakov et al.2014) of aliphatic C, leading to a decrease in aromatic struc-tures. The breaking points of the aromatic parts will then beoxidized, with functional groups, such as hydroxylic groupsand carbonylic groups, forming at the breaking points. Theseobservations are consistent with our findings of increases inhydroxylic groups and carbonylic groups with aging. The de-crease in hydroxylic groups may have been due to the dehy-droxylation of the biochar in the soil environment. However,the contents of oxygen-containing functional groups (hydrox-ylic groups at 3425 cm−1 and carbonylic groups at 1570 cm−1)in the aged biochars were consistently higher than those in thefresh biochar, consistent with most other related studies(Lehmann et al. 2005; Mukome et al. 2014; Guo et al.2014). An increase in the content of oxygen-containing func-tional groups causes an increase in the O content; this obser-vation is consistent with the elemental analysis results. Withthe increase in the content of oxygen-containing functionalgroups and the decrease in aromatic structures, the polarityof biochar is increased, which can affect the capacity of bio-char to adsorb adsorbates, such as phosphate and ammonium(Mia et al. 2017b) and atrazine and phenanthrene (Ren et al.2018a). However, Liu et al. (2013) found that only straw bio-char showed increases in the quantity of oxygen-containingfunctional groups, while oak and bamboo biochar did notshow significant changes due to their stability. The same con-clusion was drawn by Singh and Cowie (2014). Thus, weconclude that change trends may vary due to variation in theenvironment and in the raw materials and methods used toproduce biochar. In BC2017, there are two new peaks at1081 cm−1 (C–O stretching vibrations) and 466 cm−1 (absor-bances of the Si–O-Si vibrations). The peak at 1081 cm−1 was

supported by the XPS results for BC2017, presented inTable 5, which indicated a significant increase in C-O fromBC2012 to BC2017. The peak at 466 cm−1 indicated the pres-ence of silicon-containing substances. This result is supportedby the XRD spectroscopy results, which showed that the bio-char contained SiO2. The peak at 466 cm

−1 in the FTIR spec-troscopy analysis may have been due to the exposure of thesilicon-containing substance in the biochar through oxidationand the action of water scouring, such as by rain. Apart fromthe FTIR spectroscopy analysis, an increasing number of stud-ies suggest that biochar aging is mainly a surface phenomenon(Ahmad et al. 2012; Sorrenti et al. 2016; Wang et al. 2017;Kumar et al. 2018). Thus, it is very important to use XPS tostudy the changes in the functional groups at the biochar sur-face during the aging process. Mia et al. (2017a) suggestedthat most of the functional groups develop at the biochar sur-faces rather than inside the core of aromatic moieties. OurXPS data, presented in Table 5, support this conclusion. Asthe aging time increased, the content of C-C continued todecrease significantly and the total amount of oxygen-containing functional groups continued to increase, whichwas considered to be a significant sign of aging (Pereiraet al. 2014; Huff and Lee 2016; Sorrenti et al. 2016). Thesignificant reduction in the C-C bond content with aging canbe attributed to the fracturing of the C skeleton due to micro-bial decomposition and other processes during the biocharaging process. In addition, the increase in the oxygen-containing functional groups, including C-O, C=O, and C-OOR, suggested that the oxidation effect on the biochar sur-face was strong. Similar results have been reported by Yaoet al. (2010), Pereira et al. (2014), and Ghaffar et al. (2015).Furthermore, the increase in the surface oxygen-containingfunctional groups of biochar with aging will also affect thebiochar’s adsorption capacity. Based on the FTIR and XPSresults, we speculate that the C-C content of biochar will de-crease and the total amount of oxygen-containing functionalgroups will increase after biochar is applied to the soil. Therates of increase of different oxygen-containing functionalgroups during different stages will vary, but the total amountof oxygen-containing functional groups in the aged biocharwill not be lower than that in the fresh biochar, which will leadto an impact on the biochar’s adsorption capacity.

4.4 Implications of aging for biochar utilization

The application of biochar to soil can improve soil qualityby increasing the water holding capacity (Wang et al.2018), adsorbing harmful heavy metal elements (Trigoet al. 2014), and so on. In addition, Lehmann (2007a, b)suggested that biochar application to soil can lead to Csequestration and thereby slow atmospheric carbon dioxide(CO2) increases. However, an increasing number of studiessuggest that the properties of biochar change during the

348 J Soils Sediments (2020) 20:340–351

aging process (Singh and Cowie 2014; Heitkötter andMarschner 2015). These changes can affect the variousapplications of biochar described above; if not taken intoaccount, these changes may result in much lower benefitsof biochar application than expected.

Our results suggest that the properties of biochar changewith aging after its application to the soil. This finding hasseveral implications for the use of biochar as a soil amend-ment, an aquasorb, a carbon (C) sequestration technique, andso on. For instance, changes in SSA and oxygen-containingfunctional groups with age suggest that the adsorption capac-ity of aged biochar in mixed soils may differ from that of freshbiochar addition. Additionally, the changes in hydrophilic ca-pacity, evidenced by the changes in the H/C ratio, suggest thatthe impact of biochar on water retention may change withaging. The high observed stability of C content indicates thatbiochar addition has potential as a carbon (C) sequestrationtechnique in the short term; whether it can seal carbon forhundreds or thousands of years remains to be determined.

Interestingly, the changes in the various properties of bio-char with age are closely related. For example, the increase inSSA is due to the formation of micropores, and one cause ofmicropore formation is the fracturing of the C skeleton, asevidenced by our XPS data. Furthermore, the increase in thecontent of oxygen-containing functional groups is closely re-lated to the increase in O content. In addition, the change infunctional group content in FTIR analysis was verified by themolar ratio results of the element and XRD analysis.Therefore, the establishment of a long-term, dynamic tracingmethod for aging biochar properties seems feasible becausethese features are related.

An improved understanding of biochar aging can en-able the more efficient use of biochar. For example, ourresults indicate that microbial decomposition leads to thefracturing of the C skeleton and a decrease in the C-Cbond content of biochar; these changes may affect theapplication of biochar as a carbon (C) sequestration tech-nique. To address this problem, we can enhance biocharstability by optimizing the production process and othermethods. Regarding the change in adsorption capacityduring the aging process, we can take measures, such asbiochar modification, to improve its role as a soil amend-ment. Based on the above discussion, we suggest thatsome of the beneficial properties of apple tree branch-derived biochar, such as its adsorption capacity, can beenhanced with age when incorporated in typical loess soilwithin a reasonable time. However, the main factors thataffect the properties of biochar include the raw materialsused for its production and the pyrolysis temperature(Ahmad et al. 2012). In addition, environmental factors,including soil, climate, and temperate, also affect thepropert ies of biochar during the aging process.Therefore, further research is needed to improve our

understanding of the relationships between the changesin the properties of aging biochar and both the raw mate-rial and the pyrolysis temperature. In addition, a long-term, dynamic method for tracing aging biochar propertiesneeds to be established, which requires a series of studiesof different types of biochar and environments. By under-taking these measures, biochars optimized for specific tar-get applications such as carbon (C) sequestration or soilamendment can be developed.

5 Conclusions

Five years after the application of biochar to typical loess soilunder field conditions, the SSA of the biochar did not consis-tently decline or increase but fluctuated over time. This fluc-tuation was due to the destruction and blockage of the porestructure and the formation of micropores. The increases in thesurface O content and oxygen-containing functional group con-tent over the 5 years suggest that the aging of the biochar isinitiated at the surface and is mainly a surface phenomenoncharacterized by alterations in the interfacial properties. In ad-dition, the stability of the C over time illustrates the feasibilityof biochar as a C sequestration technique, and the increases inthe oxygen-containing functional groups and SSA over time areresponsible for the process of nutrient adsorption.

Acknowledgments We thank Professor Hui Shi at the Xi’an Universityof Architecture and Technology for his assistance with the sampleanalyses.

Funding information This study received funding from the National KeyResearch and Development Plan of China (2016YFC0501702,2017YFC0504504), the National Natural Science Foundation of China(41571225), and the STS project of the Chinese Academy of Sciences(KFJ-STS-ZDTP-012).

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