Changes in soil organic compound composition associated with heat-induced increases in soil water repellency

Abstract

Soil heating, as for example experienced during vegetation fires, often increases soil water repellency; however, no detailed analysis of the soil chemical changes associated with this increase has been conducted to date. Here we characterize the changes in organic compound composition associated with heat-induced increases in water repellency for three Australian eucalypt-forest soils (one sandy loam, two sands). Laboratory heating (300 • C) strongly increased water drop penetration times (WDPTs) in all soils. Soils were extracted by accelerated solvent extraction (ASE) with an iso-propanol/ammonia mixture (IPA/NH 3 95:5) and pure iso-propanol (IPA). Extracts were fractionated into less and more polar fractions and analysed by GC-MS. Water repellency was eliminated in unheated and heated soils by IPA/NH 3 , but not by pure IPA. Before heating, total solvent extracts were dominated by n-alkanols, terpenoids, C 16 acid, C 29 alkane, β-sitosterol and polar compounds. After heating, dominant compounds were aromatic acids, aldehydes, levoglucosan, simple sugars and glycosides. Heating resulted in a sharp absolute decrease of homologous aliphatic series of alkanols and alkanes, a shift of fatty acid signature to members <C 20 and an increase in total content of aromatic compounds. Heating also caused the formation of complex high-molecular-weight compounds detected in the more polar fractionated extracts and low-molecular-weight oxo-and hydroxyacids and aromatics in the IPA/NH 3 solvent. We speculate that these compounds in conjunction with fatty acids of <C 12 interact with organic and mineral soil surfaces and cause the observed strong increases in soil water repellency following heating.

Short-term changes in soil Munsell colour value, organic matter content and soil water repellency after a spring grassland fire in Lithuania

“…The authors found that at a pH between 3 and 6 the hydrophilic acids dominate the dissolved organic fraction, while at a pH between 7 and 9, humic acids were the dominant fraction. Humic and fulvic acids are recognized to be potential sources of SWR (Atanassova and Doerr, 2011;Badía-Villas et al, 2013;DeBano, 2000). Humic acids increase in percentage in the humin fraction after laboratory heating and real fires (González-Peréz et al, 2004).…”

“…Heat changes the SOM composition through thermal alteration and chemical transformation. Heating also induces an increase in the content of aromatic compounds, the formation of complex high-molecularweight compounds and low-molecular-weight oxo-and hydroxyacids (Atanassova and Doerr, 2011). Soil moisture controls SWR.…”

Fire effects in the molecular structure of soil organic matter fractions under Quercus suber cover

“…However, polysaccharide-derived compounds, i.e. levoglucosan, glycosides and mono-saccharides, have been previously found to be abundant in the soluble SOM fractions of heated soils (Atanassova and Doerr, 2011), that could have resulted from their de novo formation, volatilisation from particulate organic matter, or from the breakdown of polysaccharides. The fact that we find a reduction of these compounds may also respond to a loss by lixiviation and/or to the fast turn-over after the fire of the easily available C source, represented by the soluble saccharides from the activity of heterotrophic organisms.…”

“…These changes include the transformation of biogenic chemical structures, accumulation of newly-formed structures, as well as a complex balance between the input and diminution of materials with differential resistance to thermal degradation, i.e. fresh or charred biomass (González-Pérez et al, 2004Knicker, 2007;Atanassova and Doerr, 2011;Faria et al, 2015). The pyrogenic signature in SOM is in the focus of research, not only in the Mediterranean area, but also in other parts of the world (Dyrness et al, 1989;Fernández et al, 1997;Neff et al, 2005;Dymov and Gabov, 2015).…”

Management options for water-repellent soils in Australian dryland agriculture

“…The waxy components coat soil particles (Ma'shum et al 1988;Franco et al 1995) and cause water repellency, by diffusion of hydrophobic substances onto sand surfaces during heating and especially during wetting-heating-drying cycles (Franco et al 1995). Researchers have extracted and characterised hydrophobic compounds from repellent soils and found several different waxy molecules including unbranched and branched C 16 -C 32 fatty acids and esters, alkanes, phytanols, phytanes and sterols (Spadek et al 1994;Franco et al 2000a;Horne and McIntosh 2000;Morley et al 2005;Atanassova and Doerr 2011). Because these compounds are common components of soil organic matter, water repellency is generally confined to the topsoils where organic matter accumulates.…”