Effect of physical weathering on the carbon sequestration potential of biochars and hydrochars in soil

Abstract

Physical weathering can modify the stability of biochar after field exposure. The aim of our study was to determine the potential carbon sequestration of the two chars at different timescales. We investigated the modification in composition and stability resulting from physical weathering of two different chars produced (i) at low temperature (250°C) by hydrothermal carbonization (HTC); and (ii) at high temperature (1200°C) by gasification (GS) using contrasting feedstocks. Physical weathering of HTC and GS placed on a water permeable canvas was performed through successive wetting/drying and freezing/thawing cycles. Carbon loss was assessed by mass balance. Chemical stability of the remaining material was evaluated as resistance to acid dichromate oxidation, and biological stability was assessed during laboratory incubation. Moreover, we assessed modification in potential priming effects due to physical weathering. Physical weathering induced a carbon loss ranging between 10 and 40% of the total C mass depending on the feedstock. This C loss is most probably related to leaching of small particulate and dissolved compounds. GS produced from maize silage showed the highest C loss. The chemical stability of HTC and GS was unaffected by physical weathering. In contrast, physical weathering strongly increased the biological stability of HTC and GS char produced from maize silage. After physical weathering, the half-life (t 1/2 ) of GS was doubled but only slight increase was noted for those of HTC. During the first weeks of incubation, HTC addition to soil stimulated native soil organic matter (SOM) mineralization (positive priming effect), while the GS addition led to protection of the native SOM against biologic degradation (negative priming effect). Physical weathering led to reduction in these priming effects. Model extrapolations based on our data showed that decadal C sequestration potential of GS and HTC is globally equivalent when all losses including those due to priming and physical weathering were taken into account. However, at century scale only GS may have the potential to increase soil C storage.

Towards a global assessment of pyrogenic carbon from vegetation fires

“…For example, Naisse et al . () exposed PyC (biochar) to wetting/drying and freezing/thawing cycles and noted substantial losses (10–40% C) by leaching of dissolved and small particulate PyC (<20 μ m).…”

Flux of Dissolved and Particulate Low-Temperature Pyrogenic Carbon from Two High-Latitude Rivers across the Spring Freshet Hydrograph

“…However, as physical weathering has been shown to be an important loss process for both high-and low-temperature charcoals, although on very different time scales (Naisse et al, 2015), understanding the ages of these exported carbon pools is necessary before final conclusions on their similarities can be determined. Additionally, given the fact that bulk DOC and POC in Arctic River systems have vastly different ages (e.g., Goñi et al, 2005;Amon et al, 2012), we must understand the sources and ages of dissolved and particulate PyC in order to fully understand how export of Py-DOC and Py-POC might be linked both temporally and throughout the PyC continuum.…”

Physical Disintegration of Biochar: An Overlooked Process

“…However, the converse scenario is at least as plausible: it may be essential to break BC into smaller, more easily extractable fragments to improve the opportunity for these molecular pieces to react with soil minerals, creating stable organo−mineral complexes. 27,28 These complexes are known to increase native soil organic carbon residence times. 29 Here we present data confirming the physical disintegration of biochar over short time periods (24 h), a result that has implications for this material as a soil carbon sink.…”