Soil organic matter chemistry in allophanic soils: a pyrolysis‐GC/MS study of a Costa Rican Andosol catena

Abstract

Soil organic matter (SOM) in allophanic soils is supposed to accumulate due to protection caused by binding to allophane, aluminium and iron. We investigated a catena of allophanic and non-allophanic soils in Costa Rica to determine the effect of such binding mechanisms on SOM chemistry. These soils contain no contribution of black carbon. Molecular characterization of litter, extractable and dispersed organic matter was done by Curie-point pyrolysis-GC/MS. The molecular chemistry of the organic fractions indicates a strong decomposition of plant-derived organic matter and a strong contribution of microbial sugars and N-compounds to SOM. Both the decomposition of plant-derived SOM -including that of relatively recalcitrant compounds -and the relative contribution of microbial SOM were greater in allophanic samples than in non-allophanic ones. This suggests that chemical protection does not act on primary OM, although it may influence the accumulation of secondary OM in these soils. The effect of allophane on SOM contents in such perhumid soils is probably through incorporation of decomposition products and microbial SOM in very fine aggregates that -in a perhumid environment -remain saturated with water during much of the year. Greater concentrations of aliphatics are found in allophanic residues, but there is no evidence of any specific mineral-organic binding. The results do not support the existing theory of chemical protection of plant-derived components through binding to allophane, iron and aluminium.

Nature, properties and function of aluminum–humus complexes in volcanic soils

“…The role of Al-humus complexes in enhancing the resistance of humic substances to microbial decomposition has been demonstrated by incubation experiments (Martin et al, 1966;Juste et al, 1975;Martin et al, 1982;Rasmussen et al, 2006). However, a pyrolysis-GC/MS investigation of soil organic matter in Andosols from Costa Rica did not support the theory of strong chemical protection of plant-derived components through binding to allophane, iron or aluminum (Buurman et al, 2007). Therefore, other processes may be important in SOM sequestration in Andosols.…”

“…Due to high water retention by micro-aggregates, it is plausible that anaerobic conditions may hinder aerobic decomposers in the aggregate interiors, even under otherwise well-aerated conditions at the soil pedon scale. For example, Buurman et al (2007) reported incorporation of SOM into 10 μm-diamater aggregates that remained saturated with water throughout much of the year, thereby hindering microbial decomposition. As fluid migration is reduced inside the fractal structure of the aggregates, the SOM adsorbed or trapped in these fractal pore structures is less available to microbes and enzymes (Chevallier et al, 2010).…”

“…The large accumulation of organic matter results from a combination of high detritus inputs associated with the generally high fertility and productivity of Andosols and from effective stabilization of SOM against decomposition. Research has shown that plantderived SOM is strongly degraded and that it is the microbial SOM fraction that contributes substantially to SOM pools in Andosols (Buurman et al, 2007). Stabilization of SOM in Andosols has been attributed to i) formation of the SOM in organo-mineral and/or organo-metallic (Al/Fe-humus) complexes (Inoue and Higashi, 1988;Nanzyo et al, 1993;Neculman et al, 2013;Percival et al, 2000;Rumpel et al, 2012;Torn et al, 1997), ii) low activity of soil microorganisms due to low soil pH, Al toxicity, low base cation content, and/or P deficiency (Tokashiki and Wada, 1975;Tonneijck, 2009), iii) physical protection of the SOM in stable microaggregates characteristic of variable charge soils (Huygens et al, 2005;Baldock and Broos, 2011), iv) sorption and deactivation of exoenzymes involved in the extracellular depolymerization component of SOM decomposition (Saggar et al, 1994;Miltner and Zech, 1998), v) burial of organic-rich surface horizons by repeated additions of airfall tephra deposition, and vi) the presence of microbiallyrecalcitrant charcoal (especially in melanic epipedons) (Miyazaki et al, 2009(Miyazaki et al, , 2010Nishimura et al, 2006).…”

Pyrolysis-Gas Chromatography to Evaluate the Organic Matter Quality of Different Degraded Soil Ecosystems

“…Furfural (N), is mostly derived from carbohydates, ligno-cellulosic materials, proteins and other aliphatic organic compounds (Bracewell & Robertson, 1984), indicating the presence of rapidly metabolisable organic substances. Acetic acid (K) is preferentially derived from pyrolysis of lipids, fats, and waxes, cellulose, carbohydrates (Bracewell & Robertson, 1984) and represents relatively less-degraded lignocellulose material (Sollins et al, 1996;Buurman et al, 2007). Phenol (Y) is derived from amino acids, tannins and fresh or condensed (humic) lignocellulosic structures (van Bergen et al, 1998;Lobe et al, 2002).…”

Caracterização das substâncias húmicas extraídas do solo do manguezal de Pai Matos (Cananéia, SP, BR) e de marismas da Espanha (Galícia e Valência)

“…Ultimamente, a pirólise associada à cromatografia gasosa/espectrometria de massas (Pi-CG/EM) também tem sido muito utilizada no estudo tanto da MOS quanto de suas SHs (SAIZ-JIMENEZ; DE LEEUW, 1986;SCHNITZER, 1997;LEINWBER;SCHULTEN, 1999), pois permite seu maior detalhamento químico, permitindo distinguir por exemplo, a contribuição de compostos derivados diretamente das plantas e de microorganismos (ALCÂNTARA et al, 2004;BUURMAN et al, 2006;PETERSE;MARTINS, 2007;, complementando assim, as informações obtidas pelas demais técnicas químicas e espectroscópicas citadas anteriormente (STEVENSON, 1994).…”