Long-term decomposition of organic materials with different carbon/nitrogen ratios

Quantitative information on the decomposition rates and patterns of organic residues is fundamental for a better understanding of organic matter dynamics and nutrient cycling in soils. A laboratory incubation experiment was conducted under constant temperature (30degreesC) for 3 years to study changes in total carbon (C), total nitrogen (N), and C/N ratio of crop residues (rice, corn, alfalfa) and animal feces (pig, cattle) decomposing with and without soil. On average, rice, corn, and alfalfa residues lost a larger proportion of their original organic C than cattle and pig feces (82 vs. 70%). The presence of soil did not affect the total amount of organic C lost, with the exception of corn residues in which it increased it from 79 to 84%. Alfalfa residues, pig feces, and cattle feces lost a larger proportion of their total N than rice residues (38 vs. 15%). Both rice and corn residues showed an initial increase in total N, which may have been due to N immobilization from the soil and/or N fixation. Rice residues started losing total N after 15 months, whereas corn residues still showed an increase in total N after three years. The presence of soil decreased the loss of total N in alfalfa residue (32 vs. 48%) and pig feces (38 vs. 51%). The C/N ratios of all materials decreased during the decomposition and, with the exception of pig feces were similar to that of the soil by the end of the study (11: 1 to 12: 1). These results provide quantitative information that could be used to improve the management of residues and feces.