Preservation of plant residues in soils differing in unsaturated protective capacity

This study tested the hypothesis that the decomposition of applied residue C in soil is not determined by soil texture per se but by the degree of saturation of the protective capacity of a soil. Soil protective capacity is defined as the maximum amount of C associated with clay and silt (<20 mu m) in grassland and uncultivated soils. To test this hypothesis, C-14-labeled ryegrass (grown in a phytotron and continuously labeled with (CO2)-C-14; specific activity 546 Bq mg(-1) C) was mixed with 11 soil samples differing in texture and saturation deficit, the latter being the difference between the actual and the maximum amount of C associated with the <20-mu m fraction. After 3 d of incubation, the percentage of applied C-14 that had respired showed a significant correlation (r = -0.85) with the saturation deficit. After 53 d of incubation, the amount of C-14 respired showed a significant correlation with the saturation deficit of the fine-textured soils (r = - 0.88), but not with those of coarse-textured soils. The correlation between (CO2)-C-14 production and soil texture was weak. The results confirm the hypothesis that the degree of saturation of the protective capacity of a soil predicts the decomposition rate of residue C better than does soil texture alone.