Pyrogenic carbon in Australian soils

STOTEN-22001; No of Pages 9 Science of the Total Environment xxx (2017) xxx–xxx

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Pyrogenic carbon in Australian soils Fangjie Qi a,b, Ravi Naidu a,b,⁎, Nanthi S Bolan a,b, Zhaomin Dong a,b, Yubo Yan a,c, Dane Lamb a,b, Thomas D. Bucheli d, Girish Choppala e, Luchun Duan a,b, Kirk T Semple f a

Global Centre for Environmental Research, ATC Building, Faculty of Science and Information Technology, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia Cooperative Research Centre for Contamination Assessment and Remediation of Environment (CRC CARE), The University of Newcastle, PO Box 18, Callaghan, NSW 2308, Australia Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China d Agroscope Reckenholz-Tänikon Research Station ART, Reckenholzstrasse 191, 8046 Zürich, Switzerland e Southern Cross GeoScience, Southern Cross University, PO Box 157, Lismore 2480, NSW, Australia f Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, United Kingdom 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

• The chemo-thermal oxidation method (CTO-375) was applied to quantify pyrogenic carbon (PyC) in Australian soils. • Principle component analysis - multiple linear regression (PCA-MLR) gave a good prediction for soil PyC levels. • PyC was a key fraction of TOC and chemically recalcitrant organic C accounts for a significant proportion of soil TOC. • Chemically recalcitrant organic carbon was an order of magnitude greater than that of thermally stable organic carbon.

a r t i c l e

i n f o

Article history: Received 20 November 2016 Received in revised form 6 February 2017 Accepted 7 February 2017 Available online xxxx Editor: Simon Pollard Keywords: Soil Pyrogenic carbon (PyC) Content Distribution Soil properties

a b s t r a c t Pyrogenic carbon (PyC), the combustion residues of fossil fuel and biomass, is a versatile soil fraction active in biogeochemical processes. In this study, the chemo-thermal oxidation method (CTO-375) was applied to investigate the content and distribution of PyC in 30 Australian agricultural, pastoral, bushland and parkland soil with various soil types. Soils were sampled incrementally to 50 cm in 6 locations and at another 7 locations at 0–10 cm. Results showed that PyC in Australian soils typically ranged from 0.27–5.62 mg/g, with three Dermosol soils ranging within 2.58–5.62 mg/g. Soil PyC contributed 2.0–11% (N = 29) to the total organic carbon (TOC), with one Ferrosol as high as 26%. PyC was concentrated either in the top (0–10 cm) or bottom (30–50 cm) soil layers, with the highest PyC:TOC ratio in the bottom (30–50 cm) soil horizon in all soils. Principal component analysis - multiple linear regression (PCA-MLR) suggested the silt-associated organic C factor accounted for 38.5% of the variation in PyC. Our findings suggest that PyC is an important fraction of the TOC (2.0–11%, N = 18) and chemically recalcitrant organic C (ROC) obtained by chemical C fractionation method accounts for a significant proportion of soil TOC (47.3–84.9%, N = 18). This is the first study comparing these two methods, and it indicates both CTO-375 and C speciation methods can determine a fraction of recalcitrant organic C. However, estimated

Abbreviations: CEC, cation exchange capacity; CTO-375, chemo-thermal oxidation method; IC, inorganic carbon; MA, microbial activity; MLR, multiple linear regression; NPOM, nonPyC organic matter; PCA, principal component analysis; PyC, pyrogenic carbon; ROC, recalcitrant organic carbon; SOM, soil organic matter; TOC, total organic carbon. ⁎ Corresponding author at: Global Centre for Environmental Research, ATC Building, Faculty of Science and Information Technology, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia. E-mail address: [email protected] (R. Naidu).

http://dx.doi.org/10.1016/j.scitotenv.2017.02.064 0048-9697/© 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Qi, F., et al., Pyrogenic carbon in Australian soils, Sci Total Environ (2017), http://dx.doi.org/10.1016/ j.scitotenv.2017.02.064

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F. Qi et al. / Science of the Total Environment xxx (2017) xxx–xxx

chemically recalcitrant organic carbon pool (ROC) was approximately an order of magnitude greater than that of thermally stable organic carbon (PyC). © 2017 Elsevier B.V. All rights reserved.

1. Introduction Pyrogenic carbon (PyC), alternatively termed black carbon, char, charcoal or biochar (Bird and Ascough, 2012; Cusack et al., 2012; Zimmermann et al., 2012) depending on different aspects of focus, is the residue from incomplete combustion of fossil fuel and biomass (Bird et al., 2015; Goldberg, 1985). As a part of the inert fraction of soil organic matter (SOM), it can resist to a high degree microbial degradation and persist for centuries to millennia in the natural environment (Baker et al., 2011; Singh et al., 2012). PyC is widespread in soils and sediments (Kasozi et al., 2010; Schmidt and Noack, 2000) due to its recalcitrant nature and the common occurrence of combustion (Gustafsson and Gschwend, 1998; Reisser et al., 2016; Schellekens et al., 2017). It plays an important role in a wide variety of biogeochemical processes including acting as a fire history tracer in sediments and ice cores (Schmidt and Noack, 2000; Wang, 2010), a permanent carbon sink for climate change mitigation (Lehmann et al., 2005; McBeath and Smernik, 2009), a soil conditioner that increases soil nutrient-holding and cation exchange capacity (Liang et al., 2006; Qu et al., 2016) and a geo-sorbent for potentially hazardous contaminants in soils and sediments (Pignatello et al., 2006; Semple et al., 2013; Wang et al., 2010). Accurate quantification of soil PyC content is essential in order to parameterize its role in biogeochemical processes (Falloon and Smith, 2000; Falloon et al., 2000). However, unified and standardized PyC analytical approaches are currently lacking. This is because PyC is not a unique C substance but a continuum including char, charcoal, soot and graphite (Schmidt et al., 2001; Thevenon et al., 2010), while varying methods often measure different fractions of the PyC continuum relying on operational definitions with clear-cut boundaries (Agarwal and Bucheli, 2011a; Hammes et al., 2007; Ponomarenko and Anderson, 2001). Chemo-thermal oxidation at 375 °C (CTO-375) initially reported by Gustafsson and Gschwend (1997) has been widely applied in PyC quantification and PyC relevant studies since its inception. This method has enabled findings of significant correlations between PyC and heavy metals including Cd, Cu, Pb, Zn, Cr, V, Mo, Sc (Cai et al., 2011; Chen et al., 2010; Wang, 2010) as well as organic contaminants such as polyaromatic hydrocarbons (PAHs), polychlorinated benzenes (PCBs) and polychlorinated dibenzo-dioxins and furans (PCDD/Fs) in soils and sediments (Gustafsson and Gschwend, 1997; Lohmann et al., 2004; Persson et al., 2002). Further, the CTO-375 method has successfully demonstrated the PyC - non-PyC organic matter (NPOM) dual sorption model improved quantitative prediction of solid-water distribution, mass transfer, bioavailability and bioaccessibility of various organic contaminants (e.g. PAHs, PCDD/Fs, PCBs) compared to the bulk TOC only sorption model used in numerous studies (Accardi-Dey and Gschwend, 2002; Apell and Gschwend, 2014; Bucheli and Gustafsson, 2001; Gustafsson et al., 1996; Koelmans and Jonker, 2011; Lohmann et al., 2004; Moermond et al., 2005; Oen et al., 2006; Pee et al., 2015; Werner et al., 2010). However, in Australia, where PyC levels can vary from 0 to 82% of TOC (Lehmann et al., 2008) due to repeated historical burning of grasslands, open woodlands and agricultural crop residues by indigenous people (Skjemstad et al., 2002), PyC quantification has been assessed by ultraviolet photo-oxidation + solid-state 13C NMR spectroscopy methods (Skjemstad et al., 2001; Skjernstad et al., 1999; Smernik et al., 2000). The CTO 375 method has only been applied in 8 Australian soils in a comparison with five other methods (Schmidt et al., 2001). Hence, it is essential to introduce the CTO-375 method into PyC quantification for more Australian soils. The CTO-375 estimates the stable pool of organic carbon in soil using combustion. Similarly, chemical fractionation methods may estimate the stable pool of organic

C (Bolan et al., 2012; Hedley et al., 2004). Hence, it is essential to introduce the CTO-375 method into PyC quantification of more Australian soils, and to compare chemical and thermal estimates of the stable pools of carbon. In addition, previous studies on PyC in Australian soils covered only a limited number of soils. Studies on the distribution of PyC in Australian soil profiles in varying land-uses, as well as the relationship of PyC with major soil properties, are lacking. In this study, we aim to (a) evaluate PyC levels in Australian agricultural, pastoral, bushland and parkland soils as influenced by soil depths, soil types and land-uses, (b) characterize potential correlations between soil properties and PyC content by applying multivariate statistics, (c) identify common groups of soil properties that influence PyC levels via principal component analysis (PCA), and (d) investigate the relationship between stable organic carbon pool obtained by CTO-375 method and the chemical C fractionation method. This study represents the first detailed interrogation of soil stable organic carbon content by both CTO-375 and chemical C fractionation methods in such a wide variety of Australian soils. 2. Methodology 2.1. Soil sampling and analysis In total 30 soil samples were used in this study. These soils were sampled from South Australia (SA), New South Wales (NSW), Victoria (VIC) and Queensland (QLD), Australia. Among them, 23 soil samples were from 5 locations with depths of 0–10, 10–20, 20–30 and 30– 50 cm and another location with depths of 0–20, 20–30, 30–50 cm. Another seven soil samples at depths of 0–10 cm were also collected to cover a wide range of soil types and soil properties. The soil types included Calcarosols, Dermosols, Ferrosols, Sodosol, Tenosol and Vertosols (Isbell, 1996). These 30 soil samples were air dried and sieved through a 2 mm sieve and assessed for pH, cation exchange capacity, soil texture and microbial activity analysis. Their b 425 μm counterparts were used for measurement of total carbon (TC), total organic carbon (TOC), PyC, active/amorphous Fe, Al and Mn oxides. Soil pH and electric conductivity (EC) was determined using a 1:5 soil to water ratio. Soil texture was determined by the hydrometer method (Gee et al., 1986). Basic cation exchange capacity (CECB) of the soils was measured by the BaCl2/NH4Cl compulsive exchange method as described by Gillman and Sumpter (1986). Amorphous Al, Mn and Fe (Aloxa, Mnoxa, Feoxa) was extracted by 0.2 M ammonium oxalate/ oxalic acid following Rayment and Higginson (1992). Microbial activity (MA) was obtained in the form of CO2-C emission during a 20-day incubation period following Cheng et al. (2008). Soil TC was determined by combusting 0.2 ± 0.05 g soils at 1100 °C in a Leco TruMac CNS elemental analyzer. TOC was determined the same way as TC except a preceding 1 M hydrochloric acid (HCl) treatment process was applied to remove soil inorganic carbon (IC). Soil IC was estimated by the difference of TC and TOC. Dissolved organic carbon (DOC) was measured by shaking 2 g soil in 10 ml Milli-Q water for 16 h, following detection of soluble OC by a TOC analyzer (TOC-LCSH, Shimadzu). Measurement of PyC by the CTO-375 method was carried out in Switzerland by the same laboratory and procedure as Agarwal and Bucheli (2011b). This method involved combustion of soils (b425 μm) at 375 °C for 24 h under air atmosphere to remove soil NPOM, followed by removal of carbonates using acid fumigation for 4 h by 12 M HCl. The residual C content was lastly determined by a CHN elemental analyser. The carbon fractionation method adapted from Hedley et al. (2004) and Bolan et al. (2012) was applied to 18 selected soils to examine the recalcitrant fraction of organic

Please cite this article as: Qi, F., et al., Pyrogenic carbon in Australian soils, Sci Total Environ (2017), http://dx.doi.org/10.1016/ j.scitotenv.2017.02.064

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Statistical analyses were conducted using IBM SPSS Statistics 19. Firstly, Pearson's analysis was conducted to assess the correlations between soil PyC content and other soil parameters including pH, EC, CECB, TC, TOC, DOC, clay, silt and sand content, amorphous Fe, Al, Mn and MA to evaluate whether a significant (p b 0.05) correlation existed between PyC and each of these parameters. For those significantly correlated parameters, linear regression analyses were performed with PyC as the dependent variable. Principal component analysis (PCA) was applied to soil parameters other than PyC so that soil properties could be grouped into smaller data sets that were linearly combined from separate soil parameters. To have a better definition of the principal components (PCs), a varimax rotation method was applied when performing PCA. Multiple linear regression (MLR) was applied together with PCA to further confirm the relationships of derived PCs with PyC.

were also observed by Brodowski et al. (2007), highlighting the input of PyC into top soils through aeolian, dry deposition, surface runoff and fluvial transport (Kuhlbusch and Crutzen, 1996; Masiello, 2004; Santín et al., 2016). On the other hand, higher PyC levels below the top soils has also been reported (Hammes et al., 2008; Leifeld et al., 2007; Rodionov et al., 2006; Zhan et al., 2013). For instance, PyC in Russian steppe Chernozems (Hammes et al., 2008) and German arable soils (Brodowski et al., 2007) maximized at the depth of 30–50 cm and 87– 114 cm, respectively, with the latter study showing increased PyC content with increasing depth. PyC concentrations in the Swiss soil monitoring network (NABO) were also reported to be highest at 100– 130 cm, while being smallest in the surface layer (Agarwal and Bucheli, 2011a). The presence of PyC in depth in soils developed in situ above bedrock indicate vertical movement of PyC down to deeper soil horizons. This movement can be possibly through mass movement with clay and silt or biological activity (Dai et al., 2005; Zhan et al., 2013), ploughing (Rodionov et al., 2006) or other physical disturbances. Moreover, higher concentration of PyC in deeper soil horizons can also be due to less microbial degradation given generally more anaerobic conditions in lower soil layers (Lamb et al., 2012; Leifeld et al., 2007). The comparison of the PyC distribution trends involved studies using the CTO-375 method (Agarwal and Bucheli, 2011a) and benzene polycarboxylic acids (BPCA) method (Brodowski et al., 2007; Hammes et al., 2008; Rodionov et al., 2006).

3. Results and discussion

3.1.3. TOC and PyC among soil types and land uses

3.1. Soil TOC and PyC contents and distribution

3.1.3.1. TOC as influenced by soil types and land uses. Overall, the TOC concentration decreased with soil depth irrespective of land use (Fig. 1). Nevertheless, when we look at TOC concentrations in top 0–10 cm soils, the virgin bushland soil (S4) showed a significantly higher TOC level than other land uses and the other virgin soil (parkland, S6) had the second highest level of TOC. This finding was similar to Fang et al. (2012), who found that shrubland and natural grassland had notably higher TOC levels than soils under other land uses. This observation is possibly because these two virgin soil types get more TOC input from aboveground plants (Agarwal and Bucheli, 2011a; Zhan et al., 2013) and less mechanical disturbance from human activities. The most alkaline soil (sheep grazing/rice growing, S3) contained a clearly lower TOC concentration than other soils, especially when compared to S4 (bushland) and S5 (rice field) soils that had similar properties but different pH values (Table 1). Given that soil texture and pH values often determine the decomposition rates of SOM (Glaser and Amelung, 2003) and the similar soil textures of S4 and S5, this may suggest higher pH is more favourable for TOC degradation. This point was also noted by Skjemstad et al. (1996), who suggested lower pH is more favourable for TOC stabilization. Also, Torri et al. (2003) found that slightly acid soils retained more sewage sludge carbon than soils with a higher pH. This principle may have worked for the soil TOC stabilization in our studied soils. Moreover, the significant negative correlation between TOC and pH (Fig. A1 in supporting information, p b 0.03, N = 27) could further support this point.

carbon. Here, the exchangeable ionic forms of C and conceptually labile fractions of C were extracted by anion and cation exchange membranes (AMI-7001S and CMI-7000S; Membrane International Inc., USA) and 1 M sodium hydroxide (NaOH), respectively. The organic C fraction extractable with neither resin nor NaOH was regarded as the recalcitrant organic carbon (ROC). All of the analyses were conducted in duplicate and adjusted to values for oven dry soil (105 °C). 2.2. Statistical analyses

3.1.1. Soil TOC and PyC contents TOC content of the studied agricultural, pastoral, bushland and parkland soils ranged from 3.94 to 80.2 mg/g (mean 17.9 mg/g, median 10.4 mg/g, N = 30). The three Dermosols had the highest TOC values (58.4–80.2 mg/g) (Table 1). Correspondingly, PyC levels in these soils varied from 0.27 to 6.06 mg/g (mean 0.96 mg/g, median 0.47 mg/g, N = 30) with the three Dermosols within 2.75–6.06 mg/g (Table 1). Compared to published works for top soils using the same method, our TOC (4.67–80.2 mg/g, median 18.6 mg/g, N = 13) contents in the top soils (0–10 cm) were higher than that of Delhi soils (0–20 cm) (TOC 3–25 mg/g, median 8 mg/g) (Agarwal and Bucheli, 2011b), but less than that in Swiss soils (0–20 cm) (TOC 11–392 mg/g, median 50 mg/g, N = 105) (Agarwal and Bucheli, 2011b) and global background soils (0–5 cm) from 6 continents (TOC 11.9–406 mg/g, median 62.8 mg/g, mean 92.3 mg/g, N = 27) (Nam et al., 2009). Within these continents, the UK and Norway had a background TOC level of 54– 460 mg/g (mean 256 mg/g) (Nam et al., 2008). In contrast, PyC contents in our 0–10 cm soils (0.3–1.6 mg/g, mean 1.60, median 0.99 mg/g, N = 13) were comparable with that in Delhi soils (0–20 cm) (PyC 0.58– 2.1 mg/g, median 1.25 mg/g; N = 36) (Agarwal and Bucheli, 2011b), Swiss soils (0.41–4.75 mg/g, median 1.13 mg/g, N = 104) (Agarwal and Bucheli, 2011b) and global background soils (0–5 cm) (PyC 0.2– 5.1 mg/g, mean1.43 mg/g, median 1.13 mg/g, N = 27) (Nam et al., 2009) including the UK and Norway (PyC 0.45–3.13 mg/g, Mean 1.27, N = 52) (Nam et al., 2008). Hence, we can see that PyC levels in Australian soils are comparable with those in soils from other continents in the world. 3.1.2. Soil TOC and PyC in the soil profiles Overall, TOC concentrations in the studied soils decreased with increasing soil depths (0–50 cm) (Fig. 1). This trend is common in soil TOC studies (Agarwal and Bucheli, 2011a; Fang et al., 2012) given that TOC originates from plants or animals (Lehmann and Kleber, 2015; Maia et al., 2011) and is firstly deposited on surface soils. Whereas, PyC concentration was higher in either the top (0–10 cm, S1, S3, S4 and S6) or bottom soil layers (30–50 cm, S2), with significant variation between the soil depths (Fig. 2). Enrichment of PyC in top soil layers

3.1.3.2. PyC as influenced by soil types and land uses. From Fig. 2, we can see that the highest PyC level was found in the top (0–10 cm) parkland soil (S6) followed by the top (0–10 cm) bushland soil (S4). Same reason as for soil TOC, this can be due to more PyC input and less anthropogenic disturbance of these two virgin soils. Besides, for parkland soil (Calcarosol, S6), the presence of a large amount of CaCO3 may also help store more PyC by the formation of Ca-organic bonds (Baldock and Skjemstad, 2000). Clough and Skjemstad (2000) also noticed more C stabilization occurred in calcareous compared to non-calcareous soils. When comparing soils S1 and S2, which are located approximately 10 m and 500 m from a farm road respectively, higher levels of PyC were observed in S1 down to 30 cm. This may be due to higher fossil

Please cite this article as: Qi, F., et al., Pyrogenic carbon in Australian soils, Sci Total Environ (2017), http://dx.doi.org/10.1016/ j.scitotenv.2017.02.064

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F. Qi et al. / Science of the Total Environment xxx (2017) xxx–xxx

Table 1 Information and properties of studied soils. Land-use/depth (cm)/soil type/label

Location

PyC TOC PyC ROC ROC pH (mg/g) (mg/g) (%TOC) (mg/g) (%TOC)

Barley growing 0–10, Ferrosol, S1 Barley growing 10–20, Ferrosol, S1 Barley growing 20–30, Ferrosol, S1 Barley growing 30–50, Ferrosol, S1 Cow grazing 0–10, Ferrosol, S2 Cow grazing 10–20, Ferrosol, S2 Cow grazing 20–30, Ferrosol, S2 Cow grazing 30–50, Ferrosol, S2 Sheep grazing/rice 0–10, Vertosol, S3 Sheep grazing/rice 10–20, Vertosol, S3 Sheep grazing/rice 20–30, Vertosol, S3 Sheep grazing/rice, 30–50, Vertosol, S3 Bushland 0–10 Vertosol, S4 Bushland 10–20 Vertosol, S4 Bushland 20–30, Vertosol, S4 Bushland 30–50, Vertosol, S4 Rice field 0–20, Vertosol, S5 Rice field 20–30, Vertosol, S5 Rice field 30–50 Vertosol, S5 Foreshore park 0–10 Calcarosol, S6 Foreshore park 10–20 Calcarosol, S6 Foreshore park 20–30 Calcarosol, S6 Foreshore park 30–40 Calcarosol, S6 Cattle grazing 0–10 Dermosol

Mt Bygalore, NSW S 33°39.88′E 146°49.07″

0.50

16.3

3.05

9.10

55.9

6.14 50.2

11.6

14.2 30.9 4.37

1.31

0.19

0.09

122

0.061

0.46

9.10

5.10

6.61

72.6

6.42 26.5

14.6

19.0 26.9 6.07

1.77

0.27

0.10

95.6

0.020

0.47

8.78

5.36

6.13

69.8

6.96 29.1

21.4

42.4 21.9 8.54

2.54

0.23

0.09

86.5

0.016

0.41

7.63

5.40

5.76

75.4

7.33 46.5

32.3

52.8 21.3 7.89

2.23

0.48

0.08

98.0

0.013

0.30

15.1

2.01

7.66

50.6

6.13 40.7

14.0

23.7 38.4 5.95

1.38

0.60

0.11

102

0.033

0.41

9.23

4.49

–

–

6.78 27.7

19.5

33.5 32.6 8.70

1.74

0.81

0.06

56.3

0.009

0.27

8.23

3.29

–

–

7.27 44.6

32.0

58.5 14.8 9.74

2.58

0.53

0.17

69.2

0.008

0.48

11.4

4.20

–

–

7.58 56.9

32.3

54.6 17.4 10.57

3.88

0.36

0.27

61.2

0.004

0.52

12.8

4.04

10.9

84.9

8.09 179

28.4

61.6 19.3 1.70

1.79

0.27

0.25

117

0.043

0.44

6.44

6.81

–

–

8.56 200

29.7

64.0 17.6 1.56

1.78

0.24

0.25

62.5

0.030

0.34

5.25

6.57

–

–

8.69 225

29.6

63.6 19.8 1.79

2.07

0.31

0.34

50.4

0.024

0.41

3.94

10.4

–

–

9.00 364

25.7

60.8 17.8 1.64

1.76

0.26

0.27

58.8

0.023

0.81

25.8

3.14

15.2

59.0

6.31 94.5

23.4

51.8 20.7 2.23

1.26

0.37

0.22

286

0.068

0.49

13.7

3.56

8.77

63.8

6.67 71.9

24.9

58.3 23.2 2.31

1.45

0.48

0.11

165

0.031

0.46

7.18

6.40

5.63

78.4

7.19 70.3

26.0

62.1 22.0 2.12

1.50

0.48

0.09

122

0.024

0.38

6.97

5.45

5.32

76.3

7.87 105

31.6

62.3 20.7 2.05

1.50

0.46

0.07

97.6

0.018

0.36

14.3

2.50

9.51

66.6

6.26 108

20.0

49.7 26.1 3.79

1.41

0.27

0.13

95.2

0.041

0.41

13.0

3.17

–

–

7.05 90.1

22.5

51.9 23.6 3.80

1.29

0.31

0.15

71.9

0.029

0.39

7.33

5.27

–

–

8.26 281

25.5

51.4 15.5 2.94

1.46

0.32

0.30

100

0.038

1.02

19.5

5.21

15.3

78.1

8.47 129

14.5

21.9 16.9 0.63

0.91

0.07

0.65

134

0.045

0.75

9.11

8.25

–

–

8.69 148

13.1

30.4 12.7 0.49

0.75

0.025

1.07

149

0.020

0.84

10.4

8.02

–

–

8.66 134

13.1

29.8 15.2 0.48

0.75

0.027

1.07

178

0.021

0.73

8.47

8.66

–

–

8.73 132

11.5

33.0 14.0 0.49

0.76

0.027

1.08

161

0.020

2.75

79.0

3.44

60.5

76.6

7.87 264

37.8

10.1 38.9 14.92

2.29

0.41

1.32

415

0.143

6.06

80.2

7.33

64.8

80.8

7.63 365

48.7

36.4 39.1 1.19

3.30

0.065

0.84

322

0.132

Canola growing 0–10 Dermosol

Mt Bygalore, NSW S 33°30.77′E 146°46.22′

Leeton, NSW S 34°35.59′E 146°21.77′

Leeton, NSW S 34°36.59′E 146°21.38′

Leeton, NSW S 34°36.61′E 146°21.30′

Yorke Peninsula Foreshore park, SA S 34°25.33′E 137°55.30′

Mt Shank, SA S 37°56.78′E 140°44.59′ Millicent, SA S 37°34.98′E

Clay Silt EC CECB (μs/cm) (cmol/kg) (%) (%)

Feoxa Aloxa Mnoxa IC DOC MA (μg (g/kg) (g/kg) (g/kg) (mg/g) (mg/kg) CO2/ g soil/min)

Please cite this article as: Qi, F., et al., Pyrogenic carbon in Australian soils, Sci Total Environ (2017), http://dx.doi.org/10.1016/ j.scitotenv.2017.02.064

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Table 1 (continued) Land-use/depth (cm)/soil type/label Fallow/cattle grazing/canola 0–10, Dermosol Rice growing 0–10, Sodosol

Grassland 0–10, Vertosol

Crop land 0–10, Ferrosol

Mallee bush 0–10, Tenosol

Location

140°22.51′ Penola, SA S 37°23.47′E 140°46.73′ Yenda, NSW S 34°36.81′E 146°25.20′ Plenty Gorge, VIC, S 38°24.59′ E 144°59.03′ Kingaroy, QLD S 26°33.25′E 151°51.28′ Dublin, SA S 34°49.78′E 138°373.25′

PyC TOC PyC ROC ROC pH (mg/g) (mg/g) (%TOC) (mg/g) (%TOC)

EC CECB Clay Silt (μs/cm) (cmol/kg) (%) (%)

Feoxa Aloxa Mnoxa IC DOC MA (μg (g/kg) (g/kg) (g/kg) (mg/g) (mg/kg) CO2/ g soil/min)

3.94

58.4

6.42

45.5

77.9

7.01 185

45.5

51.3 35.7 0.94

2.63

0.024

0.19

374

0.063

0.38

18.6

2.12

8.78

47.3

5.56 75.8

8.56

24.8 26.8 5.05

0.98

0.107

0.00

121

0.039

1.34

30.9

4.43

19.9

64.2

6.10 113

28.7

39.2 47.0 3.94

1.46

0.17

0.03

371

0.088

1.17

4.67

25.6

–

–

4.99 32.6

7.22

56.1 35.6 4.16

4.76

0.003

0.06

99.2

0.003

1.60

14.6

11.2

7.68

53.1

7.49 245

11.3

10.9 7.00 0.72

0.78

0.097

0.14

497

0.038

“–” indicates no data available (did not measure).

fuel PyC input into S1 from road traffic emissions. Previously, Glaser et al. (2005) also reported that PyC caused by highway traffic can be transported up to 30 m from the highway. Again when S3, S4 and S5 are compared, the cultivated soils (S3, S5) had lower PyC levels than virgin bushland soil (S4) within 0–30 cm. This was Quénéa et al. (2006) also noted that more charcoal and soot were lost in cultivated soils than a forest soil. Besides, the rice field soil (S5) that was tilled down to 1 m and revealed an almost even PyC level along the soil profile (0– 50 cm). In a field study, Major et al. (2010) found very little of amended PyC was leached down profile, and most losses were via surface runoff. Native PyC, however, would have a substantially greater time for the breakdown of coarse fragments, allowing PyC to migrate to layer soil horizons. Hence, our study reflects cultivation/tillage can accelerate the mixing of PyC in soil profiles. Rodionov et al. (2006) also suggested that the act of tillage resulted in the breakdown of large fragments of PyC, resulting in small fragments being moved downward by water and gravity. During tillage, destruction of soil aggregates and exposure of SOM to microbial attack can facilitate co-metabolic decomposition of PyC with SOM (Joseph et al., 2010). Hence, the content and

Fig. 1. TOC distribution in soils. S1: Barley growing, Ferrosol; S2: cow grazing, Ferrosol; S3: sheep grazing/rice growing, Vertosol; S4: Bushland, Vertosol; S5: rice field, Vertosol; S6: Parkland, Calcarosol.

distribution of soil PyC are determined jointly by its input and movement caused by anthropogenic activities (Rodionov et al., 2006). 3.1.4. Contribution of soil PyC to TOC Considering the contribution of PyC to TOC along soil profiles (Fig. 3), an overall increasing trend was observed with soil depth, with the maximum PyC:TOC ratio in bottom (30–50 cm) soils or other sub-surface soils. This PyC distribution trend was also observed by Zhan et al. (2013) for Chinese Loess plateau soils. The ratio may increase with depth as a consequence of recent additions of non-PyC at the surface, while PyC remains largely constant with soil depth. When looking at all 30 studied soils, the PyC:TOC ratio in this study ranged within 2.0– 11% (mean 6.06%, median 5.21%, N = 29) with an outlier of around 26% in an acidic Ferrosol. Compared with studies which have used the same quantification method, this PyC level is comparable to that of the Swiss soil monitoring network soils which ranged from 1 to 9% TOC (median 3%, N = 104), but slightly lower than that of Delhi (India) soils in which PyC:TOC ranged from 6 to 23% TOC (median 13%, N = 36) (Agarwal and Bucheli, 2011b). The highest PyC:TOC of 26% is identical to a previous report for four Australian soils (including one black

Fig. 2. PyC distribution in soils. S1: Barley growing, Ferrosol; S2: cow grazing, Ferrosol; S3: sheep grazing/rice growing, Vertosol; S4: Bushland, Vertosol; S5: rice field, Vertosol; S6: Parkland, Calcarosol.

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F. Qi et al. / Science of the Total Environment xxx (2017) xxx–xxx

minerals. A previous report also found different inorganic soil properties such as pH, silt and clay content had little effect on PyC levels (Glaser and Amelung, 2003). Soil parameters showing significant correlations with PyC included organic C relevant indexes TC (p b 0.0001), TOC (p b 0.001), DOC (p b 0.001) and MA (p b 0.001). The significant correlation of PyC with TOC (p b 0.001, R2 = 0.74, Fig. 4) was also observed by previous studies using the same (Agarwal and Bucheli, 2011b; Bucheli et al., 2004; Nam et al., 2008) and other (Cusack et al., 2012; Glaser and Amelung, 2003; He and Zhang, 2009) detection methods. This correlation was robust (p b 0.03) in our study even when the three highest PyC levels were excluded, and indicated that PyC could contribute significantly to soil TOC content. In addition, the significant correlation of TC, DOC and MA with PyC are within expectations as these three parameters were closely related to soil TOC.

Fig. 3. Contribution of PyC to TOC in soils. S1: Barley growing, Ferrosol; S2: cow grazing, Ferrosol; S3: sheep grazing/rice growing, Vertosol; S4: Bushland, Vertosol; S5: rice field, Vertosol; S6: Parkland, Calcarosol.

earth soil) (Skjemstad et al., 1996), in which PyC derived by the ultraviolet photo-oxidation + solid-state 13C NMR spectroscopy method was found to be up to 30% TOC. A comparison of the PyC:TOC values (2– 11%, 26%) in this study with that of worldwide soils (Table 2), shows that this ratio is similar to ratios observed in most soils around the world, but lower than typical PyC rich soils like the Black Chernozem in Western Canada (Ponomarenko and Anderson, 2001) and the Anthrosols from the central Amazon (Liang et al., 2008). Besides, Cornelissen et al. (2005) reported a median soil PyC:TOC ratio of 4% after reviewing works based on 90 soil samples from locations worldwide. This value is similar to the median PyC:TOC ratio of 6% in this study with the exception of PyC rich soils, which may have up to 80% TOC (Lehmann et al., 2008). The average PyC:TOC ratio in Australian soils are not significantly higher than that of other continents. 3.2. Correlations of PyC with soil parameters From Pearson's correlation of PyC with each single soil parameter, EC (p b 0.003), CECB (p b 0.001) and silt (p b 0.05) were found to be significantly correlated to PyC. However, these correlations were not robust as they were driven by the 3 soils with the highest PyC values (Fig. A2 in Supporting information). Amorphous Fe, Al and Mn are regarded as the most reactive soil mineral components, and were found positively associated with TOC and PyC levels in a previous study (Cusack et al., 2012). However, we cannot see this in our study. Hence, based on soil data obtained in this study there are no quantifiable relationships between PyC and soil EC, CECB, clay, silt percentage and amorphous

3.3. PCA-MLR analysis To obtain a more integrated relationship between PyC and soil properties, PCA analysis was conducted on the major soil parameters without PyC. The Kaiser-Meyer-Olkin measure of sampling adequacy was 0.537, which was higher than 0.5, suggesting that the variables were sufficiently independent to allow PCA to be conducted (Zhao et al., 2013). Concomitant probability, based on the Bartlett Test of Sphericity, was 0.000, which was lower than the 0.05 significance level, so the null hypothesis was rejected and PCA was acceptable (Zhao et al., 2013). PCA grouped the investigated soil parameters into four PCs, together accounting for 86% of the total variance of the data set (Table 3). Hence, these four PCs covered 86% of the total variation represented by the 14 soil parameters. The first component (PC1) was the organic C associated factor, which included TC, TOC, DOC, MA and silt. This group also indicated that organic C levels were positively related to silt-sized minerals, suggesting that the medium-sized silt fraction of soils is likely to be the most important physical protection fraction for TOC stabilization in soils. The second component (PC2) was relevant to the soils' inorganic aspects including alkalinity (pH), salt concentration (EC) and inorganic carbon (IC). As low pH, metal ion binding and adsorption to clay surfaces have also received considerable attention for C stabilization (Skjemstad et al., 1996), PC2 could influence PyC stabilization and amount in soils. PC3 was composed of amorphous Fe, Aland Mn concentrations that may also be responsible for PyC storage (Cusack et al., 2012). This grouping suggests that these active soil mineral components had similar geochemical behaviours. PC4 consisted of clay content and CECB that represented the cation exchange capacity of soils. To elucidate the influence of the four PCs on PyC concentrations, a multiple linear regression (MLR) analysis was performed to PyC, with the principal component scores on PC1 to PC4 as dependents. The

Table 2 PyC levels in selected studies. BC deposition soils

Method

Content (% TOC)

References

Australian agricultural and pastoral soils Swiss soil monitoring network (0–10 cm) 65 soil samples worldwide Four soils from Queensland and South Australia Germany chernozemic soils Mixed grass savannah in German southern Great Plains A horizons of Brazilian black soils Western Canada black Chernozem soils American agricultural soils Surface soils of native North American prairies Russian Chernozems 0–10 cm/50–60 cm Anthrosols from the central Amazon, Brazil Black soils in Germany (0–10 cm) Major grassland ecosystems of the world

CTO-375 CTO-375 CTO-375 UV photooxidation +NMR UV-oxidation + NMR BPCAs BPCAs UV-oxidation + NMR UV-oxidation + NMR BPCAs BPCAs NMR-CP/MAS BPCAs BPCAs

2–11%, 26% 1–6% 12.5% Up to 30% 3–45% 5–13% Up to 35% 50–70% 10–35% 4–18% 20% 72–90% 11.9–13.2% 5%–30%

This study Bucheli et al. (2004) Reisser et al. (2016) Skjemstad et al. (1996) Schmidt et al. (1999) Dai et al. (2005) Glaser et al. (2000) Ponomarenko and Anderson (2001) Skjemstad et al. (2002) Glaser and Amelung (2003) Rodionov et al. (2006) Liang et al. (2008) Brodowski et al. (2007) Rodionov et al. (2010)

NMR-CP/MAS - cross polarisation with magic angle spinning nuclear magnetic resonance, BPCAs - benzenecarboxylic acids, UV - ultraviolet.

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charcoal into the top soils in Terra Preta, and this process was aided with silt size soil particles. Hence, considering the dominant role of PC1 on soil PyC content and that silt content was included into PC1, the role of silt on PyC stabilization cannot be ruled out based on the results of this study. Work to test this needs to be undertaken in the future. 3.4. Comparison of CTO-375 with the C chemical fractionation method

Fig. 4. Correlation of soil PyC with TOC.

following equation was derived from MLR: P y C ¼ 0:912 þ 1:005 Factor score 1

ð1Þ

Based on Eq. (1), PyC was controlled by PC1, and PC2, PC3 and PC4 did not show an influence on PyC levels. Therefore combining PCA analysis (Table 3), PC1 accounted for 38.5% of the total variance of all variables investigated.” The value of PyC predicted from Eq. (1) was compared with measured PyC (CTO-375) levels. A significant relationship was found between PyC-predicted and CTO-375 (R2 = 0.76) (Supporting information Fig. A3). This indicated that the PCA-MLR analysis was reasonable, and that PC1 (covering organic carbon source and silt percentage) plays a dominant role on determining soil PyC content. What deserves mention is that PC1 covers both various organic carbon indexes and silt percentage, indicating the potential importance of silt size minerals for PyC storage. Previously, PyC was found to concentrate in clay and silt fractions in Australian soils (Skjemstad et al., 1996) and surface soils across the native North American prairies (Glaser and Amelung, 2003). Brodowski et al. (2007) demonstrated that more PyC was distributed in the coarse silt, sand fractions and the heavy mineral fraction in several German arable soils, and Han et al. (2016) noted that aromatic C may be adsorbed to and preserved in minerals in the coarse silt/sand fractions. Furthermore, Rodionov et al. (2006) stated that maximum aromatic C of zonal steppe topsoils of Russia existed in the silt fraction. Also, Ponge et al. (2006) reported that earthworm Pontoscolex corethrurus was the most important organism incorporating

The sequential C fractionation method is typically used to measure the chemically stable or recalcitrant fraction of C (Bolan et al., 2012; Hedley et al., 2004). In this current study, 12 top 0–10 cm soils and 6 sub-top soils were selected for chemical fractionation analysis. Following 1 M NaOH and ion exchange resin extraction, the residual TOC was deemed to be recalcitrant organic carbon (ROC) (Table 1). The values of ROC were in the range of 5.32–64.79 mg/g (mean 17.4 mg/g, median 3.94 mg/g, N = 18), contributing 47.3–84.9% to soil TOC. When plotting ROC with TOC, a significant correlation (P b 0.0001) was observed (Fig. 5A). Moreover, the proportion of ROC was much higher than PyC, and a significant linear relationship (p b 0.0001) was observed between them (Fig. 5B). Hence, we can see that the CTO-375 method is much harsher (i.e. removes more organic carbon) than the chemical speciation method for soil TOC. This may be because the CTO-375 method tends to remove less condensed char PyC and obtain the most condensed fraction of PyC (soot PyC) (Agarwal and Bucheli, 2011a; Currie et al., 2002; Elmquist et al., 2004), while ROC derived from chemical fractionation method may include part of the hydrophobic condensed humic polyaromatics, strongly mineral bound C (including PyC), non-reactive C (DiDonato et al., 2016; Schellekens et al., 2017) and condensed PyC. This study suggests the thermally stable organic

Table 3 Rotated component matrixa of PCA analysis. Components

% of variance pH EC CECB TC IC TOC DOC Clay Silt Aloxa Mnoxa Feoxa Microbial Activity

1

2

3

4

38.52 −0.078 0.506 0.552 0.965 0.387 0.974 0.786 −0.329 0.615 0.577 −0.262 0.084 0.947

18.10 0.949 0.624 0.173 0.157 0.722 0.034 0.002 0.022 −0.603 0.215 −0.203 −0.184 0.044

15.71 −0.095 −0.290 0.210 0.100 0.056 0.110 −0.208 −0.079 0.216 0.669 0.689 0.928 0.040

13.70 0.165 0.246 0.738 −0.082 −0.427 −0.003 −0.206 0.864 0.026 −0.131 0.378 −0.090 −0.041

Bold signifies good correlations (N0.6). a Extraction method: principal component analysis; rotation method: varimax with Kaiser normalization. Rotation converged in 6 iterations.

Fig. 5. Correlations of ROC with TOC and PyC.

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carbon is an important fraction of the TOC (2.0–11%, N = 18), while chemically stable organic carbon can account for a significant proportion of soil TOC (47.3–84.9%, N = 18). This is the first study comparing these two methods, and it indicates both CTO-375 and C fractionation methods can determine a fraction of relatively recalcitrant soil OC. 3.5. Conclusions and implications In this study, we applied the CTO-375 to quantify PyC in Australian agricultural, pastoral, bushland and parkland soils, and the chemical C fractionation method was also introduced for the first time to assess the validity of the CTO-375 method. We found that TOC is often concentrated in top soils, while PyC can be concentrated in top or bottom soils. The contribution of PyC to TOC was typically found to range from 2.0– 11% and in one acidic Ferrosol soil it reached 26%, with the highest PyC proportion in the bottom layer of all soils. Overall, PyC:TOC ratios in the Australian soils included in this study are similar to that in international soils. Among all land uses, virgin soils contained more PyC than agricultural and pastoral soils, while tilled land had a homogenous PyC distribution within soil profiles. This highlighted the importance of PyC input and human activity on final PyC content. In addition to human activities, the soil matrix itself is the key factor controlling the storage of PyC. With the aid of PCA-MLR, we found that soil PyC levels could be well predicted by the silt-associated organic C factor, which contributed 38.5% to the PyC content. This finding emphasized the importance of organic C sources and the physical protection of PyC by silt sized soil particles for PyC storage. The remaining 61.5% variance determining the PyC content are not clear yet. Calcium could be one of the most important factors that can contribute to the physical protection of both TOC and PyC through forming Ca-SOM aggregates. Further, based on CTO375 and chemical C fractionation methods, we found that CTO-375 method is a “harsh” method retaining mostly highly condensed soot PyC that is in a much smaller proportion (2.0–11%, N = 18), while chemically stable OC (ROC) accounted for a significantly higher fraction (47.3–84.9%, N = 18) of soil TOC that potentially include condensed PyC, condensed aromatic humic C and other non-reactive organic carbon. Acknowledgment We would like to acknowledge the financial support from CRC CARE (Cooperative Research Centre for Contamination Assessment and Remediation of the Environment). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2017.02.064. References Accardi-Dey, A., Gschwend, P.M., 2002. Reinterpreting literature sorption data considering both absorption into organic carbon and adsorption onto black carbon. Environ. Sci. Technol. 37, 99–106. Agarwal, T., Bucheli, T.D., 2011a. Adaptation, validation and application of the chemothermal oxidation method to quantify black carbon in soils. Environ. Pollut. 159, 532–538. Agarwal, T., Bucheli, T.D., 2011b. Is black carbon a better predictor of polycyclic aromatic hydrocarbon distribution in soils than total organic carbon? Environ. Pollut. 159, 64–70. Apell, J.N., Gschwend, P.M., 2014. Validating the use of performance reference compounds in passive samplers to assess porewater concentrations in sediment beds. Environ. Sci. Technol. 48, 10301–10307. Baker, L.L., Strawn, D.G., Rember, W.C., Sprenke, K.F., 2011. Metal content of charcoal in mining-impacted wetland sediments. Sci. Total Environ. 409, 588–594. Baldock, J.A., Skjemstad, J., 2000. Role of the soil matrix and minerals in protecting natural organic materials against biological attack. Org. Geochem. 31, 697–710. Bird, M.I., Ascough, P.L., 2012. Isotopes in pyrogenic carbon: a review. Org. Geochem. 42, 1529–1539. Bird, M.I., Wynn, J.G., Saiz, G., Wurster, C.M., McBeath, A., 2015. The pyrogenic carbon cycle. Annu. Rev. Earth Planet. Sci. 43, 273–298.

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Please cite this article as: Qi, F., et al., Pyrogenic carbon in Australian soils, Sci Total Environ (2017), http://dx.doi.org/10.1016/ j.scitotenv.2017.02.064