PHYSICAL FACTORS INFLUENCING DECOMPOSITION OF ORGANIC MATERIALS IN SOIL AGGREGATES

Glucose or starch labeled with 14C was mixed thoroughly into slurried soils. Aggregates of different sizes were obtained from the soils as they dried. The labeled substrates were considered to be distributed in both micro- and macropores in the aggregates. Control samples (labeled substrates in macropores only) were prepared by adding the labeled carbohydrates after the formation of the aggregates. The various samples were sterilized by .gamma.-irradiation and stored at -15.degree. C. Samples were wetted to about -20 pKa, inoculated with soil organisms and incubated for 4 wk at 28.degree. C in closed systems, which enabled regular measurement of 14CO2 released. Based on the 14CO2 released, it was concluded that starch was protected from microbial attack when present in micropores in aggregates made from fine sandy loam. After incubation samples were dried and rewetted. The flush of 14CO2 released was twice as big for samples containing labeled starch compared with glucose, showing that disruption of aggregates, containing residual starch and rearrangements of soil components are as important as chemical and biological factors in causing the flush of CO2 resulting from wetting a soil. Mechanical disruption of the aggregates resulted in a similar flush of 14CO2.