Twelve paired pedons were sampled to investigate the effects of vegetation and ∼120 yr of cultivation on Iowa soils. The study focused on organic matter in unfractionated and fractionated soil materials. Climate, parent material, time, and slope were held as constant as possible. Prairie‐derived mollic epipedons had higher organic C and total N contents than did forest‐derived ochric epipedons. Cultivated epipedons generally had lower organic C and total N contents than their uncultivated counterparts, but the C/N ratio was lowered markedly by cultivation only in the mollic epipedons. Although most of the organic C and total N was associated with the fine silt and coarse clay fractions of the uncultivated pedons, cultivation was associated with a relative shift of organic matter from the sand and silt fractions toward the fine clay fractions. Humic acid/fulvic acid ratios of organic matter in particle‐size fractions were always lower in the cultivated horizons than in the uncultivated horizons. The E4/E6 ratios of humic materials in the particle‐size fractions suggested that humic acids in mollic epipedons consisted of larger organic particles than those in ochric epipedons. Interpretations of the effects of cultivation on the soil organic matter must be made cautiously because of other agricultural influences on the uncultivated sites.
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Zhang et al. (1988) studied this question.