Mobility of black carbon in drained peatland soils

Abstract

Abstract. Amount, stability, and distribution of black carbon (BC) were studied at four sites of a large peatland ("Witzwil") formerly used as a disposal for combustion residues from households to derive BC displacement rates in the profile. Possible artefacts from thermal oxidation with Differential Scanning Calorimetry (DSC) on BC quantification of C-rich deposits were inferred by choosing three sites from a second peatland with no historical record of waste disposal as a reference ("Seebodenalp"). All sites were under grassland at time of sampling, but were partially cropped in the past at Witzwil. Mean BC contents in topsoils of Witzwil ranged from 10.7 to 91.5 (0-30 cm) and from 0.44 to 51.3 (30-140 cm) mg BC g −1 soil, corresponding to BC/OC ratios of 0.04 to 0.3 (topsoil) and 0.02 to 0.18 (deeper soil). At three sites of Seebodenalp, BC was below the detection limit of 0.4 mg g −1 organic soil, indicating negligible formation of BC during thermal oxidation of peat. 13 C NMR spectra corroborated the high BC contents at Witzwil. The data support a considerable vertical transport of BC given that soils were ploughed not deeper than 30 cm since abandonment of waste application about 50 years ago. The total amount of BC in the Witzwil profiles ranged from 3.2 to 7.5 kg BC m −2 , with 21 to 69 percent of it stemming from below the former ploughing depth. Under the premise of negligible rates of BC consumption since abandonment of waste application, minimum BC transport rates in these peats are 0.6 to 1.2 cm a −1 . The high mobility of BC might be explained by high macropore volumes in combination with occasional water saturation. By means of DSC peak temperatures, different types of BC could be distinguished, with deeper horizons containing BC of higher thermal stability. Application of combustion residues likely involved a mixture of various BC types, of which thermally more stable ones, most likely soots, were preferentially transported downwards.

Long-term black carbon dynamics in cultivated soil

“…A preferential leaching loss of less oxidized BC as shown by Leifeld et al (2007) for soot in comparison to larger char particles is unlikely able to explain the observed increase in oxidation, as only char was sampled here with particle sizes that are much larger than condensation products from combustion. However, both small and large BC particles had similar molecular chemical properties.…”

Nitrogen and Carbon Leaching in Repacked Sandy Soil with Added Fine Particulate Biochar

“…Smaller particles have greater surface area/volume ratios than larger particles and thus, in general, a larger capacity to hold nutrients. However, fine biochar particles or components may also be transported downward in soil with the water movement or horizontally by surface water runoff (Leifeld et al, 2007;Major et al, 2010). Very fine biochar particles (<1 (xm) will most likely be present in the biochar material afi:er pyrolysis, especially if small-sized feedstock particles are used, as they are in for example, fast pyrolysis processes (Laird et al, 2009).…”

Impact of a Historical Fire Event on Pyrogenic Carbon Stocks and Dissolved Pyrogenic Carbon in Spodosols in Northern Michigan

“…Limited data on PyC stocks and dynamics in many ecosystems and subsoil horizons, however, still hinder a better understanding of PyC stabilization processes and accurate estimations of the global PyC budget. Despite the apparent stability of PyC in soils, there is growing evidence that a fraction of PyC is subjected to redistribution in the landscape and lost from soils via erosion (Rumpel et al, 2006(Rumpel et al, , 2009Boot et al, 2015;Güereña et al, 2015;Cotrufo et al, 2016) and can be vertically transported to lower soil depths via translocation (Dai et al, 2005;Leifeld et al, 2007) and leaching (Hockaday et al, 2006(Hockaday et al, , 2007Major et al, 2010). This suggests that PyC is much more dynamic in soils than previously thought.…”

“…PyC stocks measured for B horizon in our study were 3.1-to 3.5-fold greater than those reported by Soucémarianadin et al (2014) 3-5 years after the fire. It is possible that (1) these stocks in the B horizons result from great amounts of PyC generated immediately after fire in the 100-year burned site, and that (2) that particulate PyC vertically moved to lower soil dephs via translocation over time (Dai et al, 2005;Leifeld et al, 2007;Koele et al, 2017). Further studies are needed to determine the major factors driving the stocks and transport rates of PyC to lower soil depths.…”