Long-term black carbon dynamics in cultivated soil
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Black carbon (BC) is a quantitatively important C pool in the global C cycle due to its relative recalcitrance compared with other C pools. However, mechanisms of BC oxidation and accompanying molecular changes are largely unknown. In this 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 and changes in BC stocks were obtained from soil collected in fields that were cleared from forest by fire at 8 different times in the past (2, 3, 5, 20, 30, 50, 80 and 100 years before sampling) in western Kenya. BC contents rapidly decreased from 12.7 to 3.8 mg C g−1 soil during the first 30 years following deposition, after which they slowly decreased to a steady state at 3.5 mg C g−1 soil. BC-derived C losses from the top 0.1 m over 100 years were estimated at 6,000 kg C ha−1. 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 the first 10 and 30 years by 133 and 192%, respectively, indicating oxidation was an important process controlling BC quality. Al, Si, polysaccharides, and to a lesser extent Fe were found 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.
KeywordsBiochar Black carbon Charcoal FTIR Long-term agriculture XPS
The financial support from the Vietnam Education Foundation (VEF) and the Wu Fellowship for Binh Nguyen is gratefully acknowledged. Support for sample analyses was obtained from the Coupled Natural and Human Systems Program of the Biocomplexity Initiative of the NSF under grant BCS-0215890. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of VEF or the National Science Foundation. Many thanks to Joseph Njeri, Lawrence Lanogwa and Wilson Okila for help in soil sample procurement. We also thank Brett Gleitsmann, Benjamin Amadalo, Lod Mise, Charles Mwoshi, Mr. Murgong, Hon. Daniel Otiende and Mr. Kima for maps and assistance in locating forest and agricultural conversion sites, establishing land cultivation ages and reconstruction of farmer cropping and land use histories. We are grateful for institutional support from the Kenya government ministries of agriculture, livestock and environment and natural resources, through their district offices in Nandi, Vihiga and Kakamega. A portion of this research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.
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