Possible role of aluminum in stabilizing organic matter in particle size fractions of Chernozemic and Solonetizic soils

Curtin, D. 2002. Possible role of aluminum in stabilizing organic matter in particle size fractions of Chernozemic and Solonetzic soils. Can. J. Soil Sci. 82: 265-268. Although phyllosilicate clays, with their large surface areas, are often considered to play the leading role in stabilizing soil organic matter against microbial attack, several studies have suggested recently that oxides of Al and Fe may stabilize organic matter in some soils. The distribution of organic C and oxides in clay (<0.2 and 0.2-2 mum) and silt fractions (2-5, 5-20, and 20-50 mum) of four Saskatchewan soils (organic C ranged from 21 to 46 g kg(-1)) was examined to differentiate the contributions of oxides and specific surface to organic matter retention. Carbon concentrations in the particle size fractions (separated following ultrasonic dispersion of the soils) tended to be highest in the fine silt and coarse clay fractions, not in the fine clay as would be expected if specific surface was the sole factor governing organic matter content. When data for the four soils were pooled there was a strong relationship between organic C (y) in the size fractions and Al (x) extracted by dithionite-citrate-bicarbonate [y = 33.9 x(0,5) - 7.3; R-2 = 0.90***], suggesting a role for Al in determining the C storage capacity of the size fractions. The C: Al ratio increased from an average of 12:1 in clay-sized material to 28:1 in coarse silt. Because it had less Al per unit mass of C, organic matter in the silt separates may be more weakly bonded to mineral material than is clay-associated organic matter. This may imply that organic matter bound to silt is less stable, and thus susceptible to mineralization, than is organic matter residing in the clay fraction.