Vertical distribution of charcoal in a sandy soil: Evidence from DRIFT spectra and field emission scanning electron microscopy

Abstract

This study uses diffuse reflectance infrared Fourier Transform (DRIFT) spectrometry and field emission scanning electron microscopy to investigate the vertical distribution of charcoal in a sandy soil from SE Australia. The soil was sampled to bedrock (120 cm) at varying depths and bulk samples were fractionated into three particle-sizes: macro- (2000-200 μm), micro- (200-60 μm) and mineral-associated organic matter (MAOM, <60 μm). Charcoal was isolated from 0-30 and 30-60-cm depths. Soil charcoal was detected by using a DRIFT band centred at 1590 cm-1 and scanning electron microscopy combined with energy dispersive spectroscopy. Charcoal content as a proportion of soil organic carbon (SOC)was estimated with linear regressions of cumulative DRIFT bands. At 0-30 cm, charcoal content as a portion of SOC did not differ significantly between particle-size fractions, constituting 5-26% of SOC. At a depth of 30-60 cm, charcoal constituted 19-39% of SOC in the fractions. At 60-100 cm, charcoal was only detectable in the mid-sized fraction, where it constituted about 17% of SOC. These results support our previous hypothesis of charcoal enrichment in the micro-fraction inducing a greater SOC stability in this fraction as inferred from radiocarbon ages (Hobley et al., 2013). Our findings indicate that DRIFT spectra can be used to detect the presence and amount of charcoal in soil, which may prove to be a simple and low-cost alternative to more laborious and costly detection methods.