PNNL

Abstract

Black carbon (BC) is a quantitatively important C pool in the global carbon cycle due to its relative recalcitrance against decay compared with other C pools. However, how rapidly BC is oxidized and in what way the molecular structure changes during decomposition over decadal time scales, is largely unknown. In the present study, the long-term dynamics in quality and quantity of BC were investigated in cultivated soil using X-ray Photoelectron Spectroscopy (XPS), Fourier-Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR) techniques. BC particles, obtained from soil samples at 8 conversion ages stretching over 100 years and from a forest soil sample from Kenya, were manually picked under a light microscope for characterization and quantification. BC contents rapidly decreased from 12.7 to 3.8 mg C g¯¹ soil during the first 30 years since conversion, after which they slowly decreased to a steady state at 3.51 mg C g ¯¹soil. BC-derived C losses over 100 years were estimated at 6000 kg C ha¯¹ to a depth of 0.1 m. The initial rapid changes in BC stocks resulted in a mean residence time of only around 8.3 years, which was likely a function of both decomposition as well as transport processes. The molecular properties of BC changed more rapidly on surfaces than in the interior of BC particles and more rapidly during the first 30 years than during the following 70 years. The Oc/C ratios (Oc is O bound to C) and carbonyl groups (C=O) increased over time by 133 and 192 %, respectively, indicating oxidation was an important degradation process controlling BC quality. Al, Si, polysaccharides, and to a lesser extent Fe were rapidly adsorbed on BC particle surfaces within the first few years after BC deposition to soil. The protection by physical and chemical stabilization was apparently sufficient to not only minimize decomposition below detection between 30 and 100 years after deposition, but also physical export by erosion and vertical transport below 0.1 m.