Distribution of organic matter in aggregates of eroded Ultisols, Central China

The relationship between organic matter contents and soil aggregates of particle size ranging from >4, 2–4, 1–2, 0.5–1 to 0.25–0.5 mm in three eroded (slightly, moderately and severely) Ultisols in central China was investigated using dry-sieving and wet-sieving methods, as well as chemical analyses. In different eroded Ultisols, the size distribution percentages of >4 and 2–4 mm macroaggregates obtained by dry-sieving method were all higher than that by wet-sieving method by separately between 12.3 and 45.7%, 3.0 and 24.7%, but the percentages of 0.25–0.5, 0.5–1 and 1–2 mm macroaggregates showed opposite trend. Microaggregates (<0.25 mm) averagely constituted the 30.1, 39.7 and 58.2% of aggregates after wet sieving in slightly, moderately and severely eroded soils, respectively. As the degree of erosion increased, the average content of >0.25 mm water-stable aggregates (WSA) and average of mean weight diameter (MWD) decreased from slightly eroded soil to severely eroded soil by 28.0% and 0.14 mm, respectively. The organic matter (OM) content of macroaggregate fractions from slightly and moderately eroded Ultisols rose with increasing in particle size fractions from 0.25–0.5 to >4 mm, but that of macroaggregates fractions from severely eroded Ultisols decreased with increasing in particle sizes. The percentages of >0.25 mm WSA correlated well with the OM contents of eroded Ultisols (r2 = 0.6573, n = 19). The OM contents of macroaggregates in eroded soils showed high correlation with that of the bulk soil, and the correlation coefficients decreased in the order of slightly > moderately > severely eroded Ultisols.