Effects of organic and plant residue quality and orchard management practices on decomposition rates of residues

In grassed-down fruit orchards, understorey plant residues have been shown to constitute an important source of soil organic matter and plant nutrients in biological and integrated fruit production systems. Under these systems, contrasting understorey plant residue types and different orchard treeline management practices may affect the decomposition rates and nutrient release from these residues. Although a recent field study has found these factors to be important, their detailed comparisons have not been conducted. In the present study, two incubation experiments were conducted concurrently in the laboratory at 25 degrees C for 110 days. Experiment I examined the relationships between residue quality characteristics and decomposition rates of a range of six organic residues and three understorey plant residues from biological and integrated fruit orchard systems. A control treatment without added residue was also studied. All residues were analyzed for important biochemical quality factors. Experiment II evaluated the effects of simulated orchard treeline management practices, involving adding the above three orchard understorey plant residues to each of three different surfaces consisting of bare soil, pea straw mulch and pine bark mulch. Plots of cumulative CO2-C production of residues fitted an exponential relationship of C=C0(1 - e(-kt)) where k is the cumulative CO2-C decomposition rate constant and C0 the asymptotic cumulative maximum C mineralized. This relationship accounted for 97 to 99% of the variations in CO2-C production in all residues except sawdust (78%) and pine bark (89%). No significant differences occurred in k and C0 values of the three orchard understorey plant residues. These values were similar to those of the clover residue but were significantly higher than those of pea straw, barley straw, spent mushroom compost, pine bark and sawdust. Results of all residues were significantly correlated with cellulose, lignin, lignin + polyphenol contents, cellulose:N, lignin:N, (lignin + polyphenol):N, and C:N but not polyphenol content. Decomposition rates of understorey residues were most rapid on bare soil compared to mulched surfaces and were significantly lowest on pea straw surface.