Supportive Information
Experimental evidence for sequestering C with biochar by avoidance of CO 2 emissions
from original feedstock and protection of native soil organic matter
H.M.S.K. Herath1,2, M. Camps-Arbestain1, M.J. Hedley1, M.U.F. Kirschbaum3, T. Wang1, R.
Van Hale4
1
New Zealand Biochar Research Centre, Soil and Earth Sciences Group, Institute of Natural
Resources, Private Bag 11 222, Massey University, Palmerston North 4442, New Zealand
2
Department of Export Agriculture, Faculty of Animal Science and Export Agriculture, Uva
Wellassa University, Badulla 90000, Sri Lanka
3
Landcare Research, Private Bag 11052, Palmerston North 4442, New Zealand
4
Iso-trace Research, Department of Chemistry, University of Otago, PO Box 56
Union Street West, Dunedin 9054, New Zealand
Correspondence: Saman Herath
Email: samanherath@uwu.ac.lk
Telephone: +94 55 3566522
Fax: +94 55 2226470
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Characterisation of feedstock and biochar
Total C, H, N and S of biochars and feedstock were determined using a TruSpec CHNS
analyser (LECO Corp. St. Joseph, MI). The ash content was determined by the thermo
gravimetric analysis (TGA) using a TA instrument (Alphatech, SDT Q600, TA Instruments,
Melbourne, Australia) as described by (Calvelo Pereira et al., 2011). Oxygen content was
estimated by O = 100 – (C + H + N + S + ash) (all wt %).
The pH values of the biochars were determined in a suspension of a 1% (wt/wt basis) biochar
in deionised water heated in a water bath to 80 °C under stirring and cooled down before pH
measurement (Ahmedna et al., 1998).
The fraction of OC in the amendments oxidisable by dichromate oxidation (Cdichro) was
determined following the Walkley-Black method as modified by Calvelo Pereira et al. (2011)
after Wolbach and Anders (1989) and Knicker et al. (2007). The organic hydrolysable C
fraction (hydro-C, non-hydro-C) was determined using 6 M HCl acid following Silveira et al.
(2008).
Solid state
13
C MAS NMR with cross polarisation (CP) was used to characterise the initial
(T0) biochars (produced at 350 and 550 °C) and the biochars manually collected after 295 d
(T295) and 510 d (T510) of incubation. The hand-picked biochar particles were repeatedly
washed with deionised water until the electrical conductivity became nearly the background
of distilled water following Cheng et al. (2008). Then, the particles were dried at 65 °C for 24
hours before using for the analysis. The NMR analysis was conducted in a Brucker
(Rheinstetten, Germany) DRX 200MHz horizontal bore magnet. Finely ground samples were
packed into a 7 mm diameter cylindrical zirconia rotor with Kel-F end caps and spun at
speeds of 5.0 ± 0.2 kHz in a dual resonance magnetic angle spinning (MAS) probe from Doty
Scientific. During acquisition the sample temperature was maintained at 20 °C. Solid-state
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13
C MAS NMR spectra were obtained at a
13
C frequency of 50.3 MHz on a Bruker
(Rheinstetten, Germany) DRX 200 MHz spectrometer. Free induction decays FIDs were
acquired with a sweep width of 30 KHz; 960 data points were collected over an acquisition
time of 30 ms. The CP-MAS spectra were acquired with a 1H 90° pulse for 5.5 µs, a crosspolarisation contact time of 1000 µs, an acquisition time of 30 ms, relaxation time of 2 s and
4 k scans. All spectra were zero filled to 4 k data points and processed with a 0.005 s
Gaussian broadening. Chemical shifts were externally referenced with glycine.
Characterisation of soil
Initial soils were characterised for pH in water at a soil: solution ratio of 1:2.5, and Olsen P
(available P) extracted in 0.5 M sodium bicarbonate at a soil: solution ratio of 1:20 and
determined colorimetrically following Blakemore et al. (1987). Available N was also
determined following the method given in Blakemore et al. (1987). Cations (K, Ca, Mg, and
Na) were extracted by leaching soil samples with 1 M ammonium acetate (pH 7.0) at a soil:
solution ratio of 1:50 and determined by atomic absorption spectroscopy (AAS) (Blakemore
et al., 1987). Effective cation exchange capacity (ECEC) was determined following the
method given by Matsue & Wada (1985). Briefly, 1 g of finely ground soil was dissolved in
30 mL of 0.01 M SrCl2, and five washings were done with the same SrCl2 solution, following
consecutive centrifugations and shakings. After the last washing, 30 mL of 0.5 M HCl was
added, and the Sr concentration of supernatant was then determined with AAS after 24 h
shaking followed by centrifugation.
Total OC, C oxidisable with potassium dichromate (dichro-C), and C hydrolysable with 1 M
HCl (hydro-C) was determined similar to the methods presented under ‘characterisation of
feedstock and biochar’. Sodium pyrophosphate extractable-C, -Al and -Fe (Cp, Alp, Fep) were
extracted shaking 1 g soil (air-dried, <2 mm) with 100 mL 0.1 M sodium pyrophosphate
reagent on an end-over-end shaker at 50 r.p.m (Bascomb, 1968) over night (16 h). The Cp
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was determined taking 10 mL of the extract and oxidising with 0.17 M K2Cr2O7, following
the same method as for the biochar. The Alp and Fep were determined using the AAS.
Determination of δ13CO2
C isotope ratios of the trapped CO2-C were measured by reaction with phosphoric acid to
form CO2 gas, which was introduced into a stable isotope ratio mass spectrometer (IRMS).
100 µL of 1M NaOH trap solution was reacted with 200 µL of acid for 18 hours at 25 ± 0.1
°C before sampling of the headspace in a 12 mL exetainer. Eight replicate 100 µL aliquots of
CO2+helium headspace were injected into a Thermo Advantage IRMS (Thermo, Bremen)
bracketed by pulses of CO2 reference gas. Three standard carbonate materials were analysed
at the beginning and end of each batch of 96 samples (LSVEC, lithium carbonate: –46.50 ‰;
IRU-Bicarb, sodium bicarbonate: –5.03 ‰; IRU-Marble, calcium carbonate: 2.47 ‰); control
materials were analysed at every 12th position. The results of the standard materials were used
to correct the raw delta values (vs. ref gas) to the international scale (vs. VPDB) by 3-point
calibration.
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Figure Captions
Fig. S1: Measured and modelled total C mineralisation rates (mg C kg–1 soil h–1) for the
treatments: (a) Alfisol amended with fresh CS, (b) Andisol amended with fresh CS, (c)
Alfisol amended with CS-350 biochar, (d) Andisol amended with CS-350 biochar, (e) Alfisol
amended with CS-550 biochar and (f) Andisol amended with CS-550 biochar. Measured vs.
modelled data are plotted in the inner boxes with the model efficiency (EF).
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