Long-term black carbon dynamics in cultivated soil

Abstract

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.

Post-wildfire Erosion in Mountainous Terrain Leads to Rapid and Major Redistribution of Soil Organic Carbon

“…The Sierra Nevada is a highly fire-prone region (Westerling et al, 2006), and it might be that numerous fires in the Sierras after the Gondola Fire deposited ash and smoke in the vicinity (Peterson et al, 2015). The lack of significant difference in the PyC and other SOC fractions in the eroding burn and control plots also suggests input of PyC from sources other than the Gondola Fire itself, or that much of the PyC formed on the eroding hillslope was lost in the initial erosion event or via rapid decomposition (Kuzyakov et al, 2009;Nguyen et al, 2009). Reported decomposition rates for PyC range from decadal to millennial time scales (Kuzyakov et al, 2009;Lehmann et al, 2009;Bird et al, 2015), and reflect the source material for the PyC and environmental conditions.…”

“…Generally, PyC is considered to have a longer mean residence time than non-pyrogenic soil C, typically on the multi-centennial time scale (Hammes et al, 2008;Bird et al, 2015). However, in the past two decades, there has been growing evidence for PyC decomposing on shorter time scales, on the order of days to years (Cheng et al, 2006;Nguyen et al, 2009;Soucémarianadin et al, 2015). Furthermore, Bird et al (2015) suggested a multi-pool model of PyC decomposition, where physical or chemical components of PyC are considerably more susceptible to decomposition than others.…”

Modeling black carbon degradation and movement in soil

“…If BC decomposition was included, it is typically described as one pool with exponential decay. However, whilst incubation experiments and chronosequence studies indicate that BC is degraded to some extent (Hamer et al 2004;Kuzyakov et al 2009;Nguyen et al 2009;Nguyen and Lehmann 2009;Zimmerman 2010), incubations with aged BC (Cheng et al 2008;Liang et al 2008), budget analyses (Lehmann et al 2008a) and 14 C dating of BC found in soils indicate that at least part of it lasts for millennia in some systems (Passenda et al 2001;Gouveia et al 2002;Gavin et al 2003;Schmidt et al 2002). This suggests that there are different chemical fractions within BC that turn over at different rates.…”

“…Hammes et al (2008) discussed the possibility of using a two-or multi-pool model, but concluded that not enough data were available to parameterize it. However, many experiments on BC turnover have been conducted in recent years, making at least parameterization of the pool with the fastest turnover time possible (Hamer et al 2004;Kuzyakov et al 2009;Nguyen et al 2009;2010;Nguyen and Lehmann 2009;Whitman 2010;Zimmerman 2010). Nguyen et al (2009) found that BC in a chronosequence after fire could be adequately described as a two-pool model where one pool disappeared within decades whilst the other did not degrade.…”

Function of Wildfire-Deposited Pyrogenic Carbon in Terrestrial Ecosystems

“…Aged PyC particles showed slightly lower aromaticity and released larger and more aromatic clusters into solution or as colloids, which suggests oxidation over a 10-year study period (Schneider et al, 2010). Similarly, many studies showed aged PyC particles decrease in C concentration and an increase in functional groups relative to fresh PyC (Hockaday et al, 2007;Cheng et al, 2008;Nguyen et al, 2008;Ascough et al, 2011;Singh et al, 2012), which possibly resulted from environmental weathering, microbial decomposition (Hockaday et al, 2006(Hockaday et al, , 2007, or faunal bioturbation (Domene, 2016). PyC particles have been shown to decrease in specific surface area over time (Hockaday et al, 2007), although the physical fragmentation of larger particles may ultimately increase total surface areas in soil.…”