Abstract
Soils hold three quarters of the total organic carbon (OC) stock in terrestrial ecosystems and yet we fundamentally lack detailed mechanistic understanding of the turnover of major soil OC pools. Black carbon (BC), the product of the incomplete combustion of fossil fuels and biomass, is ubiquitous in soils globally. Although BC is a major soil carbon pool, its effects on the global carbon cycle have not yet been resolved. Soil BC represents a large stable carbon pool turning over on geological timescales, but research suggests it can alter soil biogeochemical cycling including that of soil OC. Here, we established two soil microcosm experiments: experiment one added 13C OC to soil with and without added BC (soot or biochar) to investigate whether it suppresses OC mineralisation; experiment two added 13C BC (soot) to soil to establish whether it is mineralised in soil over a short timescale. Gases were sampled over six-months and analysed using isotope ratio mass spectrometry. In experiment one we found that the efflux of 13C OC from soil decreased over time, but the addition of soot to soil significantly reduced the mineralisation of OC from 32% of the total supplied without soot to 14% of the total supplied with soot. In contrast, there was not a significant difference after the addition of biochar in the flux of 13C from the OC added to the soil. In experiment two, we found that the efflux 13C from soil with added 13C soot significantly differed from the control, but this efflux declined over time. There was a cumulative loss of 0.17% 13C from soot over the experiment. These experimental results represent a step-change in understanding the influence of BC continuum on carbon dynamics, which has major consequences for the way we monitor and manage soils for carbon sequestration in future.
Highlights
There is approximately three times more carbon found in soils than is held in the atmosphere as CO2 (Fischlin et al, 2007; Lal, 2004; IPCC, 2019)
Charred particles are generally dominated by small polycyclic aromatic hydrocarbons (PAHs) (2-7 rings) and labile carbon forms and, whereas soot particles are mainly comprised of gas phase re-condensed highly aromatic molecules (PAHs >7 rings) and stable carbon forms (Bird et al, 2015; Koelmans et al, 2006; Meredith et al, 2012)
For the first time, that black carbon (BC) in the form of soot supresses the mineralisation of labile organic carbon in soils, with 18% less 13CO2 produced when soot is added to the soil
Summary
There is approximately three times more carbon found in soils than is held in the atmosphere as CO2 (Fischlin et al, 2007; Lal, 2004; IPCC, 2019). In spite of the critical role soils play in the global carbon cycle, we fundamentally lack detailed mechanistic understanding of the turnover of major soil organic carbon pools, so-called black carbon (BC). This limits our ability to integrate soils into policies for a net zero future. Black carbon occurs ubiquitously in the environment, playing an important role in a wide range of biogeochemical processes (Talukdar et al, 2019; Bond et al, 2013; Flanner, 2013; Masiello, 2004), and it has been suggested that it may influence the turnover of more labile ecosystem-derived SOC, defined as decaying plant residues, soil biota and exudates (Liu et al, 2018; Edmondson et al, 2015; Liang et al, 2010; Major et al, 2010). Across Continental Europe and Northern Ireland BC is quantified as part of the TOC pool via elemental analysis (de Brogniez et al, 2015; Xu et al, 2011), while BC is not accounted for in England, Wales (Bradley et al, 2006) and the Republic of Ireland (Cruickshank et al, 1998) where soil carbon measure are derived from dichromate oxidation
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