Decomposition and CO₂-C evolution of okara, sewage sludge, cow and poultry manure composts in soils

Abstract

A laboratory experiment was conducted to evaluate the decomposition of various composts in soils by determining the C mineralization rate and microbial biomass level. The differences in the decomposition rate of the composts were examined. The composts used consisted of okara compost made from soymilk residues, cow manure, poultry manure, and sewage sludge composts, which were applied at rates of 50 or 150 g kg⁻¹ soil to a Chiba Light-colour Andosol. The results showed that, in general, the amount of CO₂-C released increased rapidly at the initial stage, but the pattern differed among the composts used. The production of CO₂-C depended on the amount applied and the nature of the compost materials. The CO₂-C evolution from okara-treated soil was much higher than that from the soils treated with the other three composts, presumably due to the higher level of labile C as evidenced by the larger amount of microbial biomass. The value of qCO₂ (CO₂ production per unit microbial biomass) was lower in the okara compost than in the other composts. qCO₂ was linked to the decomposability of organic materials, reflected in the CO₂-C evolution.

Key Words:

• CO₂-C evolution
• decomposition
• metabolic quotient (qCO₂)
• microbial biomass
• mineralization