Stability of biomass-derived black carbon in soils

Abstract

Black carbon (BC) may play an important role in the global C budget, due to its potential to act as a significant sink of atmospheric CO 2. In order to fully evaluate the influence of BC on the global C cycle, an understanding of the stability of BC is required. The biochemical stability of BC was assessed in a chronosequence of high-BC-containing Anthrosols from the central Amazon, Brazil, using a range of spectroscopic and biological methods. Results revealed that the Anthrosols had 61–80% lower ( P < 0.05) CO 2 evolution per unit C over 532 days compared to their respective adjacent soils with low BC contents. No significant ( P > 0.05) difference in CO 2 respiration per unit C was observed between Anthrosols with contrasting ages of BC (600–8700 years BP) and soil textures (0.3–36% clay). Similarly, the molecular composition of the core regions of micrometer-sized BC particles quantified by synchrotron-based Near-Edge X-ray Fine Structure (NEXAFS) spectroscopy coupled to Scanning Transmission X-ray Microscopy (STXM) remained similar regardless of their ages and closely resembled the spectral characteristics of fresh BC. BC decomposed extremely slowly to an extent that it was not possible to detect chemical changes between youngest and oldest samples, as also confirmed by X-ray Photoelectron Spectroscopy (XPS). Deconvolution of NEXAFS spectra revealed greater oxidation on the surfaces of BC particles with little penetration into the core of the particles. The similar C mineralization between different BC-rich soils regardless of soil texture underpins the importance of chemical recalcitrance for the stability of BC, in contrast to adjacent soils which showed the highest mineralization in the sandiest soil. However, the BC-rich Anthrosols had higher proportions (72–90%) of C in the more stable organo-mineral fraction than BC-poor adjacent soils (2–70%), suggesting some degree of physical stabilization.