Classical Fractions Of Soil Organic Matter

Chemical analyses have been used to probe the structure of SOM and define fractions that stabilize C and N and are important for controlling nutrient cycling. The classical extraction technique involves dispersion of soil in a strong base such as NaOH or Na4P2O7 followed by acidification to fractionate different types of humic substances. The nonextractable fraction is termed humin. The dispersible fraction precipitated at low pH (<2) is known as humic acid. The dissolved fraction not affected by pH is termed fulvic acid. The extraction of humic substances depends on soil type or parent material. Soils formed from volcanic parent material tend to have greater humin fractions. Highly leached forested soils tend to have a higher proportion of fulvic acids. Grassland soils will usually have equal concentrations of all fractions. Much of the humin can be dispersed using a reductive agent such as sodium thiosulfate (Na2S2O3) to counteract bridging cations such as Fe(III) to Fe(II) to remove mineral-associated humic substances. Pretreatment with HF to degrade the clay minerals will result in as much as 90-95% recovery, with the remaining C being referred to as black C (including charcoal) and acid-resistant waxy materials.

Fulvic acids are oxidized substances characterized by aromatic structures with extensive side chains and smaller N content compared to humic acids (Table 12.3). Fulvic acids of lower soil horizons, such as from a spodic B horizon, contain little N.

Fulvic acids of mollisols contain larger concentrations of N. Anoxic soil fulvic acids, such as those found in rice paddies, contain approximately 20 to 30% as much N as humic acids (Bird et al., 2002). Humic acids are larger compounds of molecular weight 10,000 to 100,000 containing greater aromatic structure, cyclic N forms, and aliphatic and peptide residues. Humic acids contain 56% C, 5.5% H, 4% N, and 33% (Table 12.3). Compared to fulvic acids, humic acids contain more S and P. The functional groups are primarily COOH groups, phenolic OH groups, alcoholic groups, and a small amount of ketone oxygen. The high oxygen content of the fulvic acids relative to the humic acids is a result of much higher COOH (carboxyl) and phenolic OH content. Both fulvic and humic acids are adsorbed to clay minerals and hydrous oxides by polyvalent cations such as Ca2+ and Fe3+. The humin fraction is more strongly bound to minerals and has a C content in excess of 60%. It is enriched in fungal melanins and paraffinic substances compared to fulvic and humic substances.

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