Plant And Microbial Lipids

Lipids enter the soil from the turnover of plant residues, fauna, and microorganisms. They represent a diverse class of compounds ranging from simple fatty acids to complex sterols, phospholipids, chlorophyll, waxes, and resins (cutins and suberins). Lipids are measured by sequential extraction with nonpolar solvents, such as hexane and chloroform. The average lipid content of most plants is about 5% of dry weight, with leaves containing the greatest amount. The lipid content depends highly on the plant species. The lipid content of high, cutin-containing plants, such as conifers and succulents, may reach 10% or more of the dry weight. The durability of lipids depends on their chemical complexity. Long-chain aliphatic fatty acids and phospholipids, component of membranes, are degraded relatively quickly depending on the degree of saturation or double-bond content. More complex resins can be very recalcitrant and form some of the most decomposition-resistant substances in soil. The hydrophobic character of resins allows them to sorb into hydrophobic domains of SOM, shielding them from enzymatic attack.

Microbial lipids, consisting of more than 500 different types of fatty acids, are similar in function to those of plants and animals but differ structurally. Lipids are present in appreciable amounts in fungal spores and hyphae. The lipid content of fungal mycelium averages 17% by weight and ranges from 1 to 55% depending on the species. Various phospholipids are unique and are useful for identifying microorganisms (see Chap. 3). Assays of specific fungal lipids such as ergosterol have proved useful for quantifying their biomass and for qualitative evidence of their diversity. The bacterial cell is typically 9% lipid (Table 12.2). Microbial lipids degrade readily in their unprotected state, but like plant lipids, once sorbed into the hydrophobic domains of SOM, become protected from further decomposition.

Lipids can accumulate in acidic soils where they can constitute 30% of the total SOM C (Stevenson, 1994). Soil disturbance such as tillage, erosion, or fire is needed to release these compounds into the decomposition cycle. High organic matter soils contain the greatest amount of lipids. Clay soils have higher lipid content than coarse textured or sandy soils. The decomposition of more complex lipids, such as sterols and cutins, requires multifaceted enzymatic systems or groups of enzymes. Cutin is a polymer network of oxygenated C-16 and C-18 fatty acids cross-linked by ester bonds. Gaining attention are compounds accumulating in soils with behavior similar to that of lipids termed polyaromatic hydrocarbons (PAH), released from fossil fuel combustion. Enzymes and degradative processes of the more recalcitrant lipid substances and PAH are not well understood.

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