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

<p>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. It is deemed to represent a large stable pool in soils turning over on geological timescales, but research suggests it can alter soil biogeochemical cycling including that of ecosystem-derived organic carbon. Here, we established two soil microcosm chamber experiments: experiment one added <sup>13</sup>C organic carbon to soil with and without added BC (soot and biochar) to investigate whether it suppressed organic carbon mineralisation; experiment two added <sup>13</sup>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 <sup>13</sup>C organic carbon from the soil decreased over time, but the addition of soot to soil significantly reduced the mineralisation of organic carbon 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 δ <sup>13</sup>CO<sub>2 </sub>from the organic carbon added to the soil. In experiment two, we found that the efflux <sup>13</sup>C from soil with added <sup>13</sup>C labelled soot significantly differed from the control, but this efflux declined over time. There was a cumulative loss of 0.17% <sup>13</sup>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 measure, monitor and manage soils for carbon storage and sequestration in the future.</p>

Soil copper contamination effect on carbon mineralisation: evidence of a soil CO2 emission decrease from literature review

<p>Among all pollutants, copper (Cu) is of major environmental and toxicological concern with contamination from various origins. Moreover as a cation, Cu is easily complexed by the negatively charged soil organic matter (OM) inducing high concentrations in upper layers of soils where OM dominates. Due to its biotic and abiotic interactions with soil constituents Cu is expected to affect several soil processes among them the soil respiration, but studies provided contrasting results as soil respiration have been shown to decrease or increase with soil contamination depending on the studies.</p><p>In this study, we aimed at assessing how soil respiration is affected by Cu contamination in order to quantifying as a first approach the GHG emissions for a contaminated soil. We performed a quantitative review of literature focusing on soil heterotrophic respiration (thus excluding autotrophic respiration from plants) which aimed at 1) assessing the impact of a copper contamination on soil carbon (C) mineralisation and thus CO<sub>2</sub> emissions, and 2) hierarchizing the determinants of such an impact on C mineralisation compare to the influence of pedo-climatic soil parameters such as pH, clay percentage or the type of climate.</p><p>On the basis of a selection of roughly 390 literature data, global main results showed a decrease in soil CO<sub>2</sub> emission with an increase in soil Cu contamination. Data from ex situ spiking experiments could be easily differentiated from the ones originated from in situ natural contamination due to their sharper decrease in soil organic carbon mineralisation. Interestingly, ex situ spikes data on the short term provided a threshold: an increase in soil CO<sub>2</sub> emissions was noticed for data below total soil Cu content of 180 mg kg<sup>-1</sup> while a decrease was observed above this concentration. On the contrary, long-term in situ contamination due to anthropogenic activities (urbanisation, agriculture …) did not significantly impact soil carbon mineralisation except when we focused on the high inputs of industrial contamination (smelter, composted plant…). Soil pH was found as a variable of interest as acidic soils were more sensitive to Cu contamination for C mineralisation than neutral or alkaline soils, while the % of clay and the type of climate did not add explanation to the variation in C mineralisation. These results are discussed and the collected data allowed us to propose a general equation quantifying how soil respiration can be affected by a Cu contamination.</p>