DECOMPOSITION OF WHEAT-STRAW AND RYE RESIDUES AS AFFECTED BY PARTICLE-SIZE

Effects of contact between the soil and crop residues on the processes of residue decomposition are still poorly understood. The objective o f this study was to investigate the effects of residue particle size o n the decomposition of wheat (Triticum aestivum L.) straw (C/N=270) an d green rye (Secale cereale) residues (C/N=9). Residue particle size w as used as a means to vary the contact between crop residues and the s oil. Carbon mineralization was measured during 102 d for straw and 65 d for rye, on residues ranging in sizes from laboratory model (0.03 cm ) to field-scale (10 cm). The soil was a silt (Typic Hapludalf) and th e incubation was performed at 15 degrees C. The effects of particle si ze on C mineralization varied for the two residues. In the first two d ays of incubation, decomposition rate of rye increased with decreasing particle size but thereafter, the trend was reversed. In 65 days, 8% more C was decomposed in the 7-cm residues than in the 0.03-cm ones. F or wheat straw, early decomposition (3-17 days) was faster for the sma ll-sized particles (0.06 and 0.1 cm). Thereafter, the largest size cla sses (5 and 10 cm) decomposed faster. After 102 days, the very fine pa rticles (less than or equal to 0.1 cm) showed the greatest and the int ermediate size classes (0.5 and 1 cm), the lowest amount of C minerali zed. We hypothesized that greater availability and accessibility of N was responsible for the higher rates of decomposition observed for fin ely-ground wheat straw while a physical protection of finely ground re sidues was probably involved in the observed reverse effect for rye.