Effect of cotton straw-derived materials on native soil organic carbon

Science of the Total Environment 663 (2019) 38–44

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Effect of cotton straw-derived materials on native soil organic carbon Xiangyun Song a,b, Yan Li a, Xin Yue a, Qaiser Hussain c, Jinjing Zhang d, Qinghua Liu a, Shengai Jin a, Dejie Cui a,b,⁎ a

College of Resources and Environment, Qingdao Agricultural University, Qingdao 266109, PR China Qingdao Engineering Research Center for Rural Environment, College of Resources and Environment, Qingdao Agricultural University, Qingdao 266109, PR China Institute of Soil Science, Pir Mehr Ali Shah Arid Agriculture University, Rawalpindi, Pakistan d College of Resources and Environment, Jilin Agricultural University, Changchun 130118, PR China b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Biochar, compost, and straw derived SOC contents were 50.84%, 41.03%, and 38.55%. • The increase of SOC was maximum in the biochar amendment. • Straw and biochar treatments had high carbohydrate C and methoxyl C contents. • Phenolic C and alkyl C contents were predominant in compost treatment. • SOC in biochar treated soil was more stable than straw and compost treatments.

a r t i c l e

i n f o

Article history: Received 29 October 2018 Received in revised form 8 January 2019 Accepted 24 January 2019 Available online 25 January 2019 Editor: Baoliang Chen Keywords: 13 C NMR spectroscopy Humic substances SOC stabilization Biochar Straw

a b s t r a c t Different types of crop straw and their derived biochars and compost treatments have huge potential for carbon sequestration to sustain crop productivity. In this study, cotton straw (straw), cotton straw-derived compost (compost) and cotton straw-derived biochar (biochar) with equivalent carbon (C) content were added to soil and incubated for 30 and 180 days. The C sequestration potential of these organic materials was determined by 13C isotope trace method. The structural characteristic of soil organic carbon (SOC) was analyzed by solidstate 13C NMR spectroscopy. The SOC concentration was measured by wet oxidation and dry combustion methods. The results showed that 50.84%, 41.03% and 38.55% of native SOC were replaced by biochar, compost, and straw, respectively. The carbohydrate C and methoxyl C contents were significantly higher in straw and biochar amendments respectively, while phenolic C and alkyl C were high in compost amendment and a higher proportion of aryl C occurred in biochar treatment. These findings revealed that straw material was easier to be decomposed, but compost and biochar showing better stability. © 2019 Published by Elsevier B.V.

1. Introduction Soil organic carbon (SOC) and active SOC contents will increase when crop straw is returned to the field (Liu et al., 2014a; Song et al., 2012; Wang et al., 2015; Zhu et al., 2015). No-tillage with straw mulch ⁎ Corresponding author at: College of Resources and Environment, Qingdao Agricultural University, Qingdao 266109, PR China. E-mail address: [email protected] (D. Cui).

https://doi.org/10.1016/j.scitotenv.2019.01.311 0048-9697/© 2019 Published by Elsevier B.V.

resulted in higher organic carbon (OC) content and stocks in dryland farming systems (Si et al., 2018). Continuous application of straw mulching over time can increase soil C sequestration by increasing non-labile C fractions while decreasing labile fractions in the Loess Plateau of China (Wang et al., 2018). Straw mulching could simultaneous increases the SOC content and improves C sequestration in soil (Liu et al., 2017; Liu et al., 2014b; Xu et al., 2011). Incubation experiment showed that crop residues incorporation in the soil could increase SOC content up to 43% (Li et al., 2016), while straw removal can reduce

X. Song et al. / Science of the Total Environment 663 (2019) 38–44

soil C content as low as 10.9% (Saffihhdadi and Mary, 2008) which indicates that straw returning into the field is an effective way of sequestering SOC for the long term. Moreover, compost derived from cattle manure, organic waste, green waste or sewage sludge is also an effective way of improving SOC and soil quality (Lehtinen et al., 2017; Mi et al., 2016; Peltre et al., 2015). Compost application of 45 Mg ha−1 year−1 could increase SOC by 16% (Willekens et al., 2014) and there is a significant positive correlation between the application rate of the manures and the SOC content (Hemmat et al., 2010). The stabilization rate of SOC was only 1.5% in mineral fertilizer-treated soil while this value will exceed 8.7% in compost-amended (Fan et al., 2014). For the purpose of improving the fertilizer efficiency of the straw, crop straw can be converted into biochar, which will increase SOC content significantly and maintain soil quality (Grunwald et al., 2017; Hansen et al., 2017). For an instance, application of biochar derived from rice straw enhanced C sequestration in paddy soil (Wu et al., 2016) due to high recalcitrant carbon content in biochar. The labile fraction of biochar C was stable for 108 days with a pool size of 3%, but recalcitrant biochar C could last for 556 years with a pool size of 97% (Wang et al., 2016). Previous studies have analyzed the structure characteristics of SOC or OC fractions in straw, manure or biochar amended soils (De la Rosa et al., 2018; Song et al., 2012; Song et al., 2018). However, the structural composition and turnover rates of SOC are not well understood in soils treated with the straw and its derived compost and biochar. Straw incorporation into the soil could affect the chemical structure, different fractions and relative content of native SOC. In this study, it is hypothesized that structural characteristics of SOC change with the application of cotton straw and its derived compost and biochar, which may enhance soil C sequestration. The SOC derived from organic materials amendments was verified by 13C isotope trace method, and structural characteristics of SOC was analyzed by 13C polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy.

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1:1 (CSB2) and 2:1 (CSB3) and cotton straw-derived compost (CC) combined with CB in the ratio of 1:2 (CCB1), 1:1 (CCB2) and 2:1 (CCB3) were mixed with soil in a plastic jar and incubated at 20 °C for 30 and 180 days. The C/N ratio of the soil was adjusted before incubation using a solution of (NH4)2SO4. The moisture content was maintained at 60% of field capacity. A control treatment (no amendment) was also incubated for comparison. 2.4. Determination and purification of SOC The SOC was determined by dichromate wet oxidation method (Nelson and Sommers, 1982). In addition, the soil samples incubated for 180 days were also measured for SOC by dry combustion method at 1140 °C using CNS analyzer (Elementar Analysensysteme GmbH, Germany, 2008). The soil was treated by 30% HCl + 30% HF solution for 24 h at room temperature in order to disperse the silicate and iron and then centrifuged at 3552g/min for 10 min. The supernatant was discarded after centrifuging and the procedure was repeated six times. Then the samples were washed with distilled water till pH 6–7. After air drying, samples were passed through a 0.25 mm sieve for solid-state 13C NMR spectroscopy analysis. 2.5. Measurement and calculation of δ 13C The measurement of soil δ13C was carried out based on the fact that SOC has an isotopic composition and corresponds closely to the vegetation cover from where it originates (Nissenbaum and Schallinger, 1974). The δ13C values of SOC were determined by an isotope ratio mass spectrometer (Finnigan MAT-251) at Institute of Soil Science, Chinese Academy of Science, Nanjing, China. The 13C abundance is expressed in delta (δ) units and calculated as follows: 


 Rsample =Rstandard –1  1000

2. Materials and methods

δ13 C‰ ¼

2.1. Soils

where R was the isotope ratio 13C/12C, and the standard (PDB) was a belemnite carbonate from the Pee Dee formation of North Carolina. As the studied soils were under vegetation of maize with C4 signatures, contents of C derived from cotton straw derived organic materials (C3) were calculated as follows:

A representative soil of Typic Hapludoll (43°48′53.00 N, 125°19′1.00 E) was collected from cropland where maize was grown in monoculture for a long-term in Jilin Province, North China (Soil Survey Staff, 2014). The mean annual temperature and rainfall are 5.5 °C and 582 mm, respectively. The plant debris and gravels in the soil samples were picked out, and then the samples were air dried at room temperature and passed through a 2 mm sieve. The SOC and total nitrogen were 16.10 g kg−1 and 1.35 g kg−1, respectively. 2.2. Organic amendments Cotton straw (straw), cotton straw-derived compost (compost) and cotton straw-derived biochar (biochar) were air dried at room temperature and then passed through a 1 mm sieve. Biochar used in the study was produced by pyrolysis of cotton straw at 500 °C in a Muffle furnace for 4 h under oxygen deficient condition. Cotton straw was collected after harvest. Compost was prepared by composting straw for 6 months in the field. The SOC and TN contents of organic materials analyzed by wet oxidation method and the dry combustion method, the data are shown in Table S1. 2.3. Incubation experiment The soil samples were kept in a plastic box for 1 week at 20 °C before incubation in order to minimize variations of microbial activities. All organic materials (cotton straw, cotton straw-biochar, and compostbiochar) with equivalent C (2%, dry-weight basis) were mixed into the soil. Cotton straw (CS) combined with CB in the ratio of 1:2 (CSB1),

ð1Þ

C3 =C or f ¼ ðδ−δC4 Þ=ðδC3 −δC4 Þ

ð2Þ

C4 =C or 1− f ¼ ðδC3 −δÞ=ðδC3 −δC4 Þ

ð3Þ

where δ was the δ13C value of soil sample with cotton straw derived materials; δC4 was the δ13C value of control treatment which was −21.34‰; δC3 was the average δ13C value of cotton straw (−27.67‰), compost (−27.53‰) and biochar (−27.85‰). The δ13C value of soil decreases when cotton straw-derived organic materials are added to the soil. 2.6. Solid-state 13C NMR spectroscopy The characteristic investigation of SOC by solid-state 13C NMR spectroscopy was quantified by Song et al. (2018). The 13C CPMAS spectra were recorded on a Bruker AVANCE III 400 WB spectrometer equipped with a 4 mm standard bore CPMAS probehead whose X channel was tuned to 100.62 MHz for 13C and the other channel was tuned to 400.18 MHz for broadband 1H decoupling, using a magnetic field of 9.39 T at 297 K. The dried and fine powdered samples were packed in the ZrO2 rotor closed with Kel-F cap which was spun at 5 kHz rate. The experiments were conducted at a contact time of 2 min. A total of 10,000 scans were recorded with 6 s recycle delay for each sample. All

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13

C CPMAS chemical shifts are referenced to the resonances of adamantane (C10H16) standard (dCH2 = 38.5). The overall chemical shift range of 13C CPMAS NMR spectra of SOC was divided into the following main resonance regions: alkyl C (0–45 ppm), methoxyl C or N-C (45–60 ppm), oxygen alkyl C (O-alkyl-C) (60–110 ppm), aromatic-C (110–160 ppm) including aryl C (110–145 ppm) and phenol C (145–160 ppm), and carboxyl- and carbonyl-C (160–190 ppm). 2.7. Statistical analysis All data were analyzed by EXCEL 2010 and the significance of difference was determined using t-test. The statistical analysis including ANOVA test and principal components analysis were conducted using JMP statistical software, Version 11.2 (SAS Institute, Cary, NC, USA, 2013). 3. Results 3.1. SOC concentrations analysis The concentrations of SOC in straw, compost, and biochar amended soils were 19.14 g kg−1, 22.59 g kg−1, and 31.92 g kg−1, respectively (Table 1). After 180 days of incubation, the SOC contents (wet-oxidation method) increased to 23.18 g kg−1, 26.62 g kg−1 and 35.22 g kg−1 while SOC contents determined with dry-combustion method increased to 25.37 g kg−1, 27.70 g kg−1 and 30.03 g kg−1 in straw, compost, and biochar treatments, respectively (Table 1). The amount of SOC, analyzed both by wet oxidation and dry combustion methods, in biochar amended treatment was higher than compost and straw treatments. The SOC content was high in soil amended with combine application of biochar and straw or compost compared to the single application of cotton straw or compost. However, biochar alone application caused a decrease of SOC content (Table 1). Compared to the wet oxidation method, the SOC concentrations measured by dry combustion method were higher in soils treated with individual organic materials or their combinations with biochar (Table S1 and 1). Similarly, compared to wet oxidation method, dry combustion method showed higher SOC contents in cotton straw or compost-amended soils, but lower SOC content was recorded in biochar treatment (Table 1). 3.2. δ 13C analysis of SOC The δ 13C values or C distribution of C3 and C4 are shown in Table 2 and Fig. 1. The δ 13C value of soil for long-term monoculture maize (C 4 derived) was −21.31‰ which was higher than soil amended with cotton straw (−23.78‰), compost (−23.88‰) and biochar (−24.65‰) (C3 derived). Compared to control, the δ13C value of SOC, decreased to 2.47‰, 2.57‰ and 3.34‰ in CS, CC, and CB treatments,

Table 2 Changes of δ 13C value and C distribution in soils with cotton straw derived organic materials amendments. Treatments

δ 13C (‰)

Decreased δ 13C (‰)

C3 (%)

C4 (%)

Control CS CC CB CSB1 CSB2 CSB3 CCB1 CCB2 CCB3

−21.31 ± 0.15 −23.78 ± 0.33 −23.88 ± 0.45 −24.65 ± 0.23 −24.52 ± 0.19 −24.47 ± 0.26 −24.24 ± 0.60 −24.06 ± 0.28 −24.36 ± 0.47 −24.25 ± 0.30

˗ 2.47 ± 0.33 2.57 ± 0.45 3.34 ± 0.23 3.21 ± 0.19 3.16 ± 0.26 2.93 ± 0.60 2.75 ± 0.28 3.05 ± 0.47 2.94 ± 0.30

˗ 38.55 ± 5.22 41.03 ± 7.27 50.84 ± 3.52 49.53 ± 2.92 48.75 ± 4.00 45.12 ± 9.35 42.78 ± 4.42 47.51 ± 7.34 45.88 ± 4.79

˗ 61.45 ± 5.22 58.97 ± 7.27 49.16 ± 3.52 50.47 ± 2.92 51.25 ± 4.00 54.88 ± 9.35 57.22 ± 4.42 52.49 ± 7.34 54.12 ± 4.79

respectively. However, SOC concentration was 38.55%, 41.03% and 50.84% in CS, CC and CB treatments, respectively. Moreover, δ 13C value decreased to 3.21‰–2.93‰ in soil amended with a combination of biochar and straw and to 2.75‰–3.05‰ in biochar-compost mix treatment. Biochar and straw mix treatment had 45.12%–49.53% of SOC while 42.78%–47.51% SOC was measured in biochar and compost combined treatment. 3.3. Functional groups of SOC The functional groups of organic materials and SOC for 30 and 180 days of incubation were analyzed by solid-state 13C NMR spectra and are shown in Tables S2 and 3; Figs. S1 and 2. The details of functional groups of SOC assignment were described by Song et al. (2018). The signal at 25 ppm was assigned tentatively to short-chain polymethylene (Zhang et al., 2011) and linked to aryl C (Li et al., 2003). The broad peaks centered at about 30 ppm and 33 ppm were longchain polymethylene (Almendros et al., 1996; Barancíková et al., 1997). There were also amorphous – (CH2) – and crystalline – (CH2) – groups (Li et al., 2003; Mao et al., 2011) representing aliphatic compounds (Chen and Chiu, 2003; Schöning et al., 2005), respectively. The peak at 55 ppm was assigned to methoxyl-to-α-amino (Tinoco et al., 2004) and overlapped with intensity derived from N-alkyl that had their chemical shift region between 46 and 67 ppm (González Pérez et al., 2004). A peak at about 65 ppm was assigned to anomeric C6 carbon (Spaccini and Piccolo, 2007). The peak at about 72 ppm corresponded with the overlapping resonances of C2, C3 and C5 carbons in the pyranoside structure of cellulose and hemicellulose, and the signal at 82–85 ppm (shoulders) and 104 ppm were assigned to the anomeric C4 carbons and C1 carbon, respectively (Spaccini and Piccolo, 2007). In addition, broadband around 130 ppm might be related to alkyl substitutions in the p‑hydroxyl phenyl ring of cinnamic and p‑coumaric units of both lignin and suberin biopolymers as well as to both partially degraded lignin structures and condensed aromatic

Table 1 Concentration of SOC incubated for 30 and 180 days, separately. Treatments

Control CS CC CB CSB1 CSB2 CSB3 CCB1 CCB2 CCB3

Wet oxidation method

Dry combustion method

30 d

180 d

180 d

15.10 ± 0.45 19.14 ± 0.64 22.59 ± 0.36 31.92 ± 0.58 28.64 ± 0.48 25.59 ± 0.78 24.52 ± 0.59 32.92 ± 0.88 28.22 ± 0.46 28.42 ± 0.53

15.33 ± 0.65 23.18 ± 0.28 26.62 ± 0.42 35.22 ± 0.85 34.12 ± 0.26 32.36 ± 0.25 31.07 ± 0.25 36.07 ± 0.63 33.49 ± 0.39 33.42 ± 0.26

15.43 ± 0.57 25.37 ± 4.44 27.70 ± 3.61 30.03 ± 4.15 32.77 ± 4.24 32.90 ± 3.29 29.63 ± 3.86 32.17 ± 2.98 32.80 ± 1.49 33.20 ± 2.88

Fig. 1. Distribution of C3 and C4 derived C.

X. Song et al. / Science of the Total Environment 663 (2019) 38–44 Table 3 Relative distribution (%) of signal area over chemical shift regions (ppm) in CPMAS 13C NMR spectra of SOC incubated 30 and 180 days. Treatments

160–190

145–160

110–145

60–110

45–60

0–45

30 d Control CS CC CB CSB1 CSB2 CSB3 CCB1 CCB2 CCB3

12.03 7.02 7.80 3.94 5.14 11.20 8.63 12.15 9.78 8.40

5.66 6.32 8.89 0.00 5.09 6.49 5.26 4.92 4.69 8.40

27.92 20.63 21.68 72.80 50.39 31.24 28.73 36.19 30.69 26.87

21.90 39.51 34.87 6.66 27.61 24.86 30.80 22.99 26.00 28.30

7.10 7.09 7.10 0.00 3.14 4.37 6.38 4.92 8.02 6.55

25.39 19.44 19.66 16.59 12.90 21.84 20.19 18.84 20.82 21.49

180 d Control CS CC CB CSB1 CSB2 CSB3 CCB1 CCB2 CCB3

13.81 7.44 7.23 5.96 8.85 6.57 7.33 7.54 6.70 9.02

4.70 7.89 9.32 0.00 6.11 9.72 7.40 6.11 6.03 8.75

28.45 17.49 17.12 66.25 40.97 30.79 27.33 37.48 35.19 24.98

20.99 43.01 40.03 9.63 23.10 29.55 32.60 23.91 26.74 30.48

5.80 6.99 7.66 0.00 3.81 5.12 6.23 4.75 4.83 6.13

26.24 17.19 18.64 18.17 17.17 18.25 19.12 20.21 20.51 20.65

olefinic carbons (Hatcher et al., 1995). Conversely, the small shoulder at about 152 ppm in the phenolic aromatic region (140–160 ppm) suggested a low content of O-substituted ring carbons, which are usually coupled to methoxy substituents in lignin components (Spaccini and Piccolo, 2009). The signal for quaternary carbons at 173 ppm was currently assigned to carboxyl groups (Spaccini and Piccolo, 2007).

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Compared to control treatment, carboxyl C decreased for most treatments incubated for 30 days, except CCB1 treatment. However, carboxyl C decreased in all treatments incubated for 180 days. There was no peak of phenolic C in biochar, biochar mixed straw, and compost amendments, while it increased in CS and CC treatments after 30 days of incubation. However, phenolic C increased in all organic materials treatments for 180 days of incubation. The phenolic C was 6.32% and 7.89% in CS treatment and 8.89% and 9.32% in CC treatment incubated for 30 and 180 days, respectively. In addition, aryl C decreased under CS and CC treatments both in 30 and 180 days of incubation, respectively. Conversely, aryl C was a large proportion of SOC in CB treatment and increased with biochar addition when mixed with straw or compost. Compared to control treatment, O-alkyl C increased under all treatments incubated for 30 and 180 days, with the exception of CB treatment. While incubated for 180 days, the O-alkyl C increased to 43.01% and 40.03% for CS and CC treatments, respectively, Alkyl C of SOC decreased for all organic materials amendments. However, alkyl C was 19.44% and 17.19% for CS treatment, while it was 19.66% and 18.64% for CC treatment incubated for 30 and 180 days, respectively (Table 3).

3.4. Relationship of content and functional groups of SOC The SOC showed a significant positive correlation with aryl C contents (Fig. 3a), but it displayed negative correlations with carboxyl C, methoxyl C and alkyl C (Fig. 3b, c, and d). Positive correlations between C3 derived C and aryl C or amorphous were observed (Fig. 4a), and negative correlations between C3 derived C and O-alkyl C or methxyl C or crystalline C (Fig. 4b and c; Fig. 4d and e).

Fig. 2. 13C NMR spectra of SOC incubated for 30 (a and b) and 180 (c and d) days.

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Fig. 3. Correlations of amounts of SOC and functional groups of SOC.

4. Discussion 4.1. Distribution of organic materials amendments Previous studies have pointed out that straw incorporation could significantly increase active SOC pool (Wang et al., 2015). Maize straw could increase SOC mainly sequestrated as humin in long-term paddy soils (Song et al., 2012). Non-labile C fractions increased while labile fractions decreased by straw mulching (Wang et al., 2018). However, biochar could enhance C storage in soils as well as C sequestration for a long period through reduced C turnover (Grunwald et al., 2017; Hernandez-Soriano et al., 2016; Munda et al., 2018). In this study, although the straw, compost, and biochar were added at the same C concentration, the biochar sequestrated more C in soil than compost and straw treatments. Therefore, biochar mixed with straw or compost sequestrated more C than only straw or compost amendments. In addition, relative content of native SOC decreased with biochar amendments. A meta-analysis showed that mean residence times of labile and recalcitrant biochar C pools were estimated to be about 108 days and 556 years with pool sizes of 3% and 97%, respectively (Wang et al., 2016). This means only a small part of biochar is bioavailable, and most of recalcitrant biochar C contributes directly to C sequestration in the soil. A five-year field experiment indicated that biochar application decreased the contribution of a wheat residue to the native SOC, possibly by enhancing its degradation (Dong et al., 2018). When biochar was added to the composting, the increase of SOC was only due to the addition of biochar-C and not enhanced preservation of compost feedstock-C (Hagemann et al., 2018). In this study, the decreased δ13C value indicated that more organic materials derived C incorporated into SOC. Biochar mixed with straw or compost could increase more SOC content when comparing with only straw or compost were added. The concentration of SOC with organic materials amendment increased with time. Moreover, there was more aryl C (a stable form of C) sequestrated in soil amended with biochar. Mineralization of C in the soil will be inhibited when the biochar proportion of aryl C increases (Qayyum et al., 2017). The combined application of biochar and compost or straw lead to the promotion of

C sequestration, which may be the reason of mineralization inhibition caused by a high aryl C content in soil, and C mineralization might play a critical role in C sequestration. In addition, highest Cr (VI) reduction was observed with biochar mixed compost application (Mandala et al., 2017). In this study, SOC contents in soils treated with and without organic materials amendments were higher when analyzed with a dry combustion method as compared to a wet oxidation method. The SOC contents were a little higher when soil amended with biochar analyzed by a wet oxidation method than a dry combustion method. Possibly, more Cr (VI) is reduced by biochar which induced a little higher SOC content analyzed by a wet oxidation method as compared to a dry combustion method. 4.2. Characteristics of SOC amended with different organic materials Selective protection of hydrophobic C, including lignin and lipids, was observed in maize straw amended soils with prolonged incubation (Song et al., 2013). Long-term fertilization experiment also showed that application of pig manure favors alkyl C sequestration in humic acid (Song et al., 2018). However, farmyard manure amendment caused a decrease in O, N-alkyl-C, and alkyl C from macroaggregate to silt-clay fractions, suggesting an advanced state of humic component degradation (Simonetti et al., 2012). In this study, compared to straw amendment, there was less O-alkyl C and more phenolic C or alkyl C sequestrated in SOC under the compost amendment. In addition, soil amended with biochar was highly resistant to decomposition which enhanced C sequestration in soils through reduced C turnover on a long-term basis (Hernandez-Soriano et al., 2016; Jiang et al., 2016). The biochars are heterogeneous in nature and contained a lot of aromatic functional groups (Lehmann and Joseph, 2009; Xiao and Chen, 2017). Although the aromatic fraction of C helps to stabilize SOC, in this study, structural stability of SOC was different among biochar, straw and compost amendments. The contents of phenol C and alkyl C were high in soil amended with straw or compost, compared to biochar amendment. Conversely, aryl C content was high in biochar or biochar combined with straw and compost compared with only straw or

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Fig. 4. Correlations of C3 derived C and functional groups of SOC. Notes: amorphous C was the peak at about 25 and 30 ppm, and crystalline C was the peak at about 33 ppm.

compost treatment. The significant correlations between aryl groups and SOC concentration or C3 plant-derived C showed that biochar mainly affected the relative increase of aryl C fraction in SOC, while amorphous C of alkyl functional groups was affected by all organic materials amendments. Because the biochar contained a low content of carboxyl C, methxyl C, and alkyl C, there were negative correlations between SOC concentration and these functional groups. In addition, the negative correlations between C3 derived C and O-alkyl C or methxyl C or crystalline C were also because of biochar structure characteristics. These findings indicate that soil amended with straw or compost protected more phenolic C and lipids C, while biochar favored aryl C sequestration. As there was a lot of aryl C sequestrated in SOC with biochar amendment, soil amended with biochar was more stable than straw or compost treatments under the same conditions. 5. Conclusions The biochar with and without compost or straw could sequester more OC than raw straw or compost amendments. In addition, relative content of native SOC decreased with increasing biochar amendments. Soil amended with cotton straw and compost mainly contained OAlkyl C and phenolic C, respectively. The aryl C was predominant in biochar amended soil. Soil amended with biochar was more stable than straw or compost treatments because of the large proportion of aryl C of biochar sequestrated in the soil.

Acknowledgements This work was partly supported by the National Natural Science Foundation of China (Grant No. 41501246), Modern Agricultural Industry and Technology System for Innovation Team of Cotton Domain in Soil and Fertilizer Post (Grant No. SDAIT-03-06). Thanks for the assist of Dr. Zijiang Jiang in the measurement and analysis of 13C NMR spectra. Declarations of interest None. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.01.311.

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