Many soils contain charcoal, resulting from thousands of years of burning of C-rich materials, combined with a turnover time measured in centuries or millennia (Skjemstad 2001). Charcoal generated by fires can constitute up to 0.8% C in soil and may represent up to 30% of the SOC content of Australian soils (Skjemstad et al. 1996). Charcoal is not used by micro-organisms and therefore does not contribute to soil biological activity. It follows that as POC and other reactive forms of C decline with periods of cultivation, the more stable charcoal fraction increases as a portion of total soil C. Accordingly, as soil fertililty levels decline, the need to account for charcoal is becoming more important, as most contemporary methods for total SOC cannot discriminate between charcoal-C and other SOC forms. McBeth et al. (undated) reported that charcoal-C in the 0–2 cm topsoil layer was higher at sites where vegetation had been burnt relative to adjacent non-burnt areas.
Analytical methods for charcoal-C have been described by Glaser et al. (1998) and by Skjemstad et al. (2002) but these methods are complex and time-consuming. MIR offers potential for determining soil C fractions (Merry and Janik 2001; Janik et al. 2007) but successful analysis by MIR of charcoal-C across multiple laboratories has still to be demonstrated. Therefore, the chemical method described involves gentle, relatively lengthy digestion of soil with a combination of concentrated hydrogen peroxide and dilute nitric acid, the subsequent recovery of charcoal residues, followed by quantitative determination of charcoal-C in the residue using Dumas combustion as the analytical finish (Kurth et al. 2006). The method was able to recover (see Note 1) >92% of the charcoal added (R2 = 0.99) and effectively consume most of the other forms of SOC. As recovery estimates were high at low concentrations (<0.1% charcoal-C), the authors suggested a correction multiplier of 0.70 to account for this over-estimation. Based on reported data, the method described is expected to be reasonably quantitative over the working range 0.1–5% charcoal-C (dry wt).
30% Hydrogen Peroxide
Use 30% technical or analytical grade hydrogen peroxide (H2O2). Handle and store this reagent with caution, as H2O2 is hazardous. Users should wear PVC gloves and safety glasses when using the chemical. Operations involving H2O2 should be carried out only where there is excellent ventilation.
1.0 M Nitric Acid
Dilute 66.7 mL of nitric acid, HNO3, 15 M (1.4 g/mL) to 1.0 L using deionised water.
Weigh 1.00 g of air-dry soil into a 250 mL Erlenmeyer flask, add 20 mL of 30% H2O2 and 10 mL of 1 M HNO3. Swirl the flask at room temperature for 30 min. before heating to 100°C and maintaining that temperature for 16 h. Swirl samples occasionally and observe for any continued effervescence. If the chemical reaction in the flask is still active, heating for an additional 4 h is recommended.
When digestion is complete, filter sample through Whatman No. 2 filter paper, before drying the retained material at 50°C overnight. Homogenise the residue quantitatively with a mortar and pestle if required. Determine TOC in the dried material using the analytical finish described in 6B1 or 6B3. Total C measured by Dumas combustion is reported as charcoal-C, following the assumption that all non-charcoal-C has been consumed in the digestion process. When calculating the result from the combustion furnace (or equivalent), take account of the initial sample weight and the weight of dry residue. Also, for apparent charcoal-C values of <0.1%, apply a correction multiplier of 0.70, or as independently obtained by the testing laboratory.
Report charcoal-C (% C) on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to make the oven-dry conversion. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.
1. Recovery estimates reported by Kurth et al. (2006) were made by combining known amounts of soil and charcoal to give 0, 0.05, 0.1, 0.5, 1.0, 2.0 and 5.0% charcoal-C (w/w). Soil and charcoal were mixed by grinding the amended soil to a very fine powder.