Opening the black box: Soil microcosm experiments reveal soot black carbon short-term oxidation and influence on soil organic carbon mineralisation.

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 C OC to soil with and without added BC (soot or biochar) to investigate whether it suppresses OC mineralisation; experiment two added C 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 C 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 C from the OC added to the soil. In experiment two, we found that the efflux C from soil with added C soot significantly differed from the control, but this efflux declined over time. There was a cumulative loss of 0.17% C 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.