DECOMPOSITION OF DIFFERENT ORGANIC MATERIALS IN SOILS

Laboratory experiments were conducted to evaluate organic C mineraliza tion of various organic materials added to soils. A soil sample was mi xed with organic material to approximate a field application of 9 g or ganic C kg(-1) soil (0.9% or 50 Mg ha(-1)). The organic materials used were four crop residues [corn (Zea mays L.), soybean (Glycine max L. Merr.), sorghum (Sorghum vulgare Pers.), and alfalfa (Medicago sativa L.)], four animal manures [chicken (Gallus domesticus), pig (Sus scrof a), horse (Equus caballus), and cow (Bes taurus)] and four sewage slud ges [Correctionville (Imhoff tank), Charles City (holding tank), Daven port (secondary digester), and Keokuk (primary digester)]. The soil-or ganic material mixture was incubated under aerobic conditions at room temperature (20+/-2 degrees C) for 30 days. The CO2 evolved was collec ted in standard KOH solution by continuously passing CO2-free air over the soil. Results showed that, in general, the amounts of CO2-C relea sed increased rapidly initially, but the pattern differed among the or ganic materials used. More than 50% of the total CO2 produced in 30 da ys of incubation was evolved in the first 6 days. Expressed as percent ages of organic C added, the amounts of CO2 evolved ranged from 27% wi th corn to 58% with alfalfa. The corresponding percentages for animal manures ranged from 21 to 62% with horse and pig manures, respectively , and for sewage sludges they ranged from 10 to 39% for Charles City a nd Keokuk sludges. All CO2 evolution data conformed well to a first-or der kinetic model. Potentially, readily mineralizable organic C values and first-order rate constants (k) of the organic matter-treated soil s ranged from 1.422 g C kg(-1) soil with a k value of 0.0784 day(-1) t o 6.253 g C kg(-1) soil with a k value of 0.0300 day(-1). The half-liv es of the C remaining in soils ranged from 39 to 54 days for plant mat erials. The corresponding half-lives for the C remaining from animal m anures and sewage sludges ranged from 37 to 169 days and from 39 to 33 0 days, respectively.