Analysis of the rotting process of rice straw-calcium cyanamide mixture by physical fractionation

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Abstract
The rotting process of a rice straw-calcium cyanamide mixture (rice straw compost) at 50°C, was studied by physical fractionation methods. The compost was divided into six fractions using a combination of methods which included the use of different sizes of sieves, sedimentation, and centrifugation. The six fractions obtained are Fl (1.0 mm), F2 (1.0–0.25 mm), F3 (0.25–0.043 mm), F4 (sedimented at 4°C), F5 (sedimented by centrifugation at 104 × g for 10 min), and F6 (supernatant). The results obtained are summarized as follows: 1. With the progress of rotting, rice straw became finely divided, and about 80% of dry matter consisted of fractions F4, FS, and F6 within 10 weeks. The distribution of size fractions remained nearly constant afterwards. 2. The same trends were observed for the fractional distribution of carbon and nitrogen per pot. They were distributed in the order of F6>F5>F4 and F5>F6>F4, respectively. 3. Eighty-five to 90% of the total nitrogen was concentrated in fractions F4-6. 4. C/N ratio of each fraction was almost constant after 5 weeks incubation, except for Fl, and the values decreased with a decrease in size. 5. Ash content of each size fraction was nearly unchanged after 10 weeks, Ash was concentrated in fraction F4. 6. The fractions obtained by physical fractionation of rice straw compost were tentatively divided into three major groups: I. Fractions Fl-3 consist of coarse plant tissues which became dull yellowish brown to grayish yellow brown. II. Fractions F4 and F5 consist of fine grayish yellow brown plant tissues, microbial cells, and their debris. III. Fraction F6 consists of humic substances.

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