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.

Long term change in chemical properties of preindustrial charcoal particles aged in forest and agricultural temperate soil

“…As oxidation of charcoal particles is a preliminary step for microbial decomposition (Hammes and Schmidt, 2009), a smaller O/C value supports a greater stability of charcoal produced by pyrolysis than for fire-derived pyrogenic OM. Nevertheless, after > 150 yr, preindustrial charcoal has O/C values comparable with that of fire-derived pyrogenic OM (Nguyen et al, 2008). This suggests that the oxidation degree of charcoal increases over time until it reaches a constant value.…”

“…Even though XPS measures the binding energy of core electrons from the surface (< 10 nm) of a sample, we assumed that atomic composition established from the method is representative of the bulk properties of a sample, as charcoal samples finely ground to powder were analyzed (Nguyen et al, 2008). Charcoal contained from 45.1% to 77.2% C, 17.4% to 42.2% O and 1.34% to 1.93% N. It also contained 1.46% to 6.18% Si, 0.86% to 3.33% Al, 0.23% to 1.75% Ca and 0.16% to 1.22% Fe.…”

“…This may result from hydroxylation and carboxylation at edges of the graphene sheets in charcoal (Lehmann et al, 2005), but also from contamination by organic acids from soil solution (Pignatello et al, 2006) or microbial films in the porosity of charcoal (Lehmann et al, 2011). Nguyen et al (2008) estimated from XPS analysis an organic O/ C ratio of ca. 0.35 for fire-derived pyrogenic carbon residue in cultivated soils cleared by slash and burn.…”

“…The presence of charcoal kiln sites in agricultural soils provides the opportunity to investigate how cultivation affects the dynamics of charcoal ageing, by comparison with sites from the same episode of charcoal production that remained forested since the time of charcoal production. Nguyen et al (2008) studied the evolution of properties of BC particles over time in agricultural soils in Kenya cleared from forest by fire. They highlighted that association of charcoal with soil minerals increases over time.…”

“…As the decrease in BC loss correlated with the increase in association with soil minerals, they suggested that organo-mineral association might be an important mechanism in the stabilization of BC. Nevertheless, Nguyen et al (2008) studied BC dynamics over time in agricultural soil, which does not allow distinguishing between the effect of time and of cultivation on BC properties. By selecting kiln sites abandoned at the same time and converted to agricultural land at different times, we aimed to discriminate between the effect of time and of cultivation on the organic and inorganic composition and stability of charcoal particles aged in soil.…”