Effects of physical, chemical, and biological ageing on the mineralization of pine wood biochar by a Streptomyces isolate

Abstract

If biochar is to be used for carbon (C) management, we must understand how weathering or ageing affects biochar C mineralization. Here, we incubated aged and unaged eastern white pine wood biochar produced at 350 and 550°C with a Streptomyces isolate, a putative biochar-decomposing microbe. Ageing was accelerated via three different processes, namely, (a) physical ageing-subjecting biochar to alternating freeze-thaw and wet-dry cycles, (b) chemical ageing-treating biochar with concentrated hydrogen peroxide and (c) biological ageing-incubating biochar in the presence of nutrients and microorganisms. Elemental composition and surface chemistry (Fourier Transform Infrared spectroscopy) of biochar samples were compared before and after ageing. Biochar C mineralization between ageing treatments was significantly different in the case of 350°C biochar (p value = 0.03). Among the 350°C biochars, physical ageing resulted in the greatest increase (by 103%) in biochar C mineralization (p value = 0.05). However, in the case of 550°C biochar, ageing did not result in a significant change in biochar C mineralization (p value = 0.40). Biochar C mineralization was positively correlated with an increase in O/C ratio post-ageing (rs = 0.86, p value = 0.01). In the case of 350°C biochar, surface oxidation during ageing enhanced biochar degradation by the isolate. For 550°C biochar, however, ageing did not significantly increase biochar C mineralization, likely due to high condensed aromatic C content and lower surface oxidation during ageing. The results from our study suggest that low temperature aged biochar is more susceptible to biological degradation by soil microbes. These findings have implications for the use of biochar for long term C storage in soils.

Fig 1. Cumulative biochar C mineralization over time.

Data represent mean cumulative C mineralized from unaged and physically, chemically and biologically aged biochar samples over time, with uninoculated blanks subtracted and normalized with mean biochar-C. N = 3 for physical, chemical and unaged, N = 5 for biological. Error bars represent 95% confidence intervals. The left panel shows biochar produced at 350°C and the right panel shows biochar produced at 550°C.

Fig 2

Changes in surface chemistry during ageing inferred using FTIR spectroscopy (a) FT-IR spectra of unaged and physically, chemically and biologically aged biochar samples produced at 350°C (left panel) and 550°C (right panel). Labels on top indicate the peak names assigned to different functional groups as described in detail in supplementary information (O-H: O–H stretching in carboxylic acids, phenols, alcohols at 3370 cm^-1; al CH: aliphatic C-H stretch in CH₃ and CH₂ at ~2932 cm^-1 and C-H bending of CH₃ and CH₂ at 1413 cm^-1; CO₂: CO₂ asymmetric stretching at 2350 cm^-1; C = O: C = O stretch in carboxylic acids and ketones at ~1701 cm^-1; ar C = C: aromatic C = C vibrations and stretching of quinones at ~1593 cm^-1; CO*: C–O stretching and O–H bending of COOH and/or C–OH stretching of polysaccharides at ~1200 cm^-1; ar C-H: aromatic C-H out of plane deformation at 810 cm^-1. (b) The clustering of biochar FT-IR spectra based on Ward’s hierarchical clustering method represented as a dendrogram. The distance of the link between any two clusters (or samples) is a measure of the relative dissimilarity between them.

Fig 3. Relationship between mean cumulative biochar-C mineralized and molar O/C ratio.

N = 3 for physical, chemical and unaged, N = 5 for biological treatments. Error bars represent standard error of the mean. Shapes indicate unaged, physically, chemically and biologically aged biochar samples produced at 350°C (black) and 550°C (gray).