There is great research interest in carbon © bound by pyrolysis (carbonisation), due primarily to the potential role of biochar (charred biomass) as a long-term carbon sink in soils and sediments. This is because biochar resists microbial decomposition and chemical transformations in the soil for a comparatively long time. In their ground-breaking study, the authors Kuzyakov, Bogomolova and Glaser dealt with this stability of biochar in soils. For this purpose, 14C-labelled (radioactively labelled) biochar was used to monitor its decomposition to CO2 over 8.5 years and the conversion of its chemical compounds: neutral lipids, glycolipids, phospholipids, polysaccharides and benzene-polycarboxylic acids (BPCA, aromatic compound).
Extremely slow decomposition of biochar
1414C-labelled biochar was produced by charring 14C-labelled ryegrass (Lolium) residues. The resulting biochar was then mixed with soil or loess. After 3.2 years of incubation, the first measurements showed an extremely slow 14CO2 release of about 0.5% C per year. The authors therefore estimate the average residence time of biochar to be around 2,000 years, and even 4,000 years for soils in temperate climates. According to Kuzyakov, Bogomolova and Glaser, this was the first experimental proof of very slow degradation rates of biochar.
In view of the very slow degradation rate of biochar and its continuous decline, incubation was extended by a further 5 years. In total, only about 6% of the originally added biochar was mineralized (released) to CO2 in the 8.5 years. Most of this was released in the first 2 years. According to the authors of the study, this is probably the slowest decomposition experimentally obtained for any natural organic compound. The average degradation rate for biochar was thus less than 0.3% per year and is thus about 2.5 times slower, as reported in a shorter study (max. 3.5 years).
The highest proportion of 14C in the original biochar was bound in the BPCA (87%), which decreased by only 7% over 3.5 years. Condensed aromatic units were thus the most stable fraction in comparison to all other plant carbon compounds. According to the authors, the high proportion of BPCA in plant coal explains its very high stability and thus its important contribution to long-term C sequestration in soil.
Economic benefit from stable CO2-binding
The use of 14C-labelled biochar opens up a new way to track not only the plant charcoal itself, but also its transformation products. The C derived from biochar can be detected in microorganisms, dissolved organic substances and organic substance fractions – even after very slow conversion and degradation processes, since 14C analysis is highly sensitive and specific.
The results also showed a low microbial availability of plant coal and confirmed the assumption that plant coal is of negligible importance as a C source for microorganisms. The researchers were able to demonstrate that carbon from biochar is used less intensively by microorganisms than carbon from organic soil substances (dead plant and animal substances). According to the authors, this is also an indirect confirmation of a very low microbial availability of biochar. They assume that mainly lipids and polysaccharides from biochar were incorporated into microorganisms. More than 80% of glycolipids and phospholipids were decomposed within the first 3.5 years.
Original article: Biochar stability in soil. Decomposition during eight years and transformation as assessed by compound-specific 14C analysis
Author: Yakov Kuzyakov, Irina Bogomolova, Bruno Glaser
Published in: Soil Biology & Biochemistry 70 (2014), Elsevier, p. 229–236