Geoderma (Aug 2024)
Quantification of biochar in arable land: A new approach based on Rock-Eval® thermal analysis
Abstract
Pyrogenic carbon (PyC) constitutes an important pool of soil organic carbon and may remain in soil over millennia. Its accurate quantification in soil is necessary to monitor the evolution of soil carbon stocks and fluxes. This study tested new direct approaches to quantify PyC enrichment in soil, using three protocols based on the Rock-Eval® thermal method. Six industrial biochars and four cultivated soils were mixed at five different biochar/soil ratios from 0.05 % to 1 % (w/w). Biochar-C emissions could be identified in the CO2 signal from the oxidation stage (CO2oxi) between 410 °C and 610 °C, independently from carbon and carbonate contents from the original unamended soil. Based on this signal and using a reference sample, the protocol 3 allowed for a good estimation of the PyC enrichment relatively to the reference sample. This protocol is thus well adapted to monitor biochar addition in agricultural soils. However, the use of a reference sample, chosen as the sample with the lowest value of thermo-resistant organic carbon (CorgTR), may be tricky for heterogeneous surface areas and soil profiles. In the latter context, both protocols 1 and 2 are alternative methods to quantify CorgTR from biochar and soil without the need of a reference sample, although less accurate than protocol 3. Protocol 2, based on a more precise identification of the biochar-derived peak of CO2oxi, resulted in more accurate values than protocol 1. However, the use of protocol 2 is suitable when the biochar-derived peak is easily distinguishable from the peaks derived from both thermo-resistant SOM and PyC present in the original unamended soil. In conclusion, the Rock-Eval® thermal method turns out to be a good tool to quantify biochar-C in cultivated soil and is fully adapted in field experiments – in which, the initial content in biochar is controlled – aiming to decipher the potential benefits of biochar on soil functioning and related ecosystem services.
