Preservation of Plant Residues in Soils Differing in Unsaturated Protective Capacity

Abstract 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 µm) in grassland and uncultivated soils. To test this hypothesis, 14 C‐labeled ryegrass (grown in a phytotron and continuously labeled with 14 CO 2 ; 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‐µm fraction. After 3 d of incubation, the percentage of applied 14 C that had respired showed a significant correlation ( r = −0.85) with the saturation deficit. After 53 d of incubation, the amount of 14 C 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 14 CO 2 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.