Importance of charcoal in determining the age and chemistry of organic carbon in surface soils

Abstract

[1] Understanding the chemical character and turnover time of the oldest soil organic carbon (SOC) fraction is fundamental in deciphering soil carbon sequestration processes and the fate of soil-eroded carbon in aquatic sediments. Two main processes are thought to extend the turnover time of SOC: protection by the mineral matrix and chemical recalcitrance. Various oxidation methods have been proposed to isolate the oldest and most recalcitrant SOC fraction, which is often assumed to be black carbon (BC). However, few data have been published that confirm the chemical character of the isolated fractions. Using established and newly developed methods together with 13 C-NMR spectroscopy and AMS dating, we show that protection by the mineral matrix prolonged the turnover time of SOC by tens of years, but long-term (hundreds of years) stabilization was controlled by the inherent recalcitrance of SOC, determined by the type of ecosystems. In ecosystem without significant fire occurrences, the older SOC pool was comparably small and was represented by alkyl carbon. In ecosystems with high fire frequency charcoal constituted the oldest SOC pool, and constituted up to 35% of the total SOC. By applying methods with different oxidative strengths, it was possible to isolate different age groups of charcoal with different degrees of weathering. Further substantiation of this finding could provide a much greater resolution of paleo-fire events. Our results demonstrate that fire frequency plays a dominant role in determining the chemical nature and 14 C abundance of SOC and that the separation of age groups of charcoal provides a means to reconstruct detailed fire histories. Our results indicate that modeling SOC turnover, transport and sequestration for frequently burnt environments requires modification of existing models, specifying an input and decay function for the charcoal pool in different environments.

Storage and Turnover of Organic Matter in Soil

“…Both factors, the loss of permafrost and more severe burning, are predicted to drive these ecosystems to become net C sources . As mentioned in the section on recalcitrance, grasslands in Australia with frequent fires have relatively high proportions of chemically recalcitrant black carbon (Krull et al 2006). Changes in fire frequency linked to climate or land management may ultimately control a region's status as a C source or sink.…”

“…Black carbon, organic matter transformed by pyrolization, remains a candidate for recalcitrance. Comparing a grassland, woodland, and subtropical rainforest in Australia, Krull et al (2006) concluded that although the woodland and rainforest had greater aggregate and mineral protective capacity, respectively, the grassland supported a larger reservoir of more stable carbon due to frequent inputs of charcoal from regular fires. In a Russian…”

“…More recently, ultraviolet radiation and chemical (H 2 O 2 or NaOCl) methods have been used to oxidize some organic matter and leave behind less reactive, and radiocarbon-older, organic residues (Krull et al 2006;Mikutta et al 2006;Zimmermann et al 2007). In two comparisons, NaOCl appears to cause less mineral alteration and yield an older OM fraction than does HCl (Mikutta et al 2006;Zimmermann et al 2007).…”

Radiocarbon and Soil Carbon Dynamics

“…Intrinsic recalcitrance: While all organic carbon is ultimately thermodynamically unstable, soils contain a number of compounds that may be difficult to decompose owing to their specific chemical structure. Pyrolized carbon formed in fire can be one example (Krull et al 2006b, Preston & Schmidt 2006; some lipid compounds are also extremely long-lived in soil (Mikutta et al 2006). …”

Maize yield and nutrition during 4 years after biochar application to a Colombian savanna oxisol

“…In contrast, biomass-derived black carbon (C), or biochar, is much more stable. While biochar must eventually mineralize in soil (Goldberg 1985;Schmidt and Noack 2000), a fraction remains in a very stable form with a 14 C age greater than that of the oldest soil organic matter (SOM) fractions (Krull et al 2006;Pessenda et al 2001;Skjemstad et al 1996).…”