Mineralization of physically fractionated rotten plant residues under upland conditions

Abstract

A well-rotten mixture of rice straw and calcium cyanamide (rice straw compost prepared indoors) was separated physically into four fractions using a combination of methods which involved sieving, sedimentation and centrifugation. The four fractions obtained were Fl-3 (>0.043 mm), F4 (sedimented at 4°C), F5 (sedimented by centrifugation at 10⁴ × g for 10 min) and F6 (supernatant). The air-dried and pulverized fractions were mixed with upland soils and incubated for 4 weeks at 30°C under upland conditions. The amount of nitrogen mineralized in each fraction was determined, In a red yellow soil, 11.2% of the organic nitrogen present in the unfractionated, air-dried sample was mineralized, compared with 6.8% in volcanic ash soil. The contribution of fraction F5 to the total amount of mineralized nitrogen in wet compost was the highest, followed by F6. Fraction Fl-3 showed immobilization of inorganic nitrogen in both soil types. On the other hand, fractionated samples obtained after air-drying the wet compost showed no immobilization for Fl-3, although the values obtained for other fractions were similar to those obtained for the corresponding fractions of wet compost. Well-rotten plant residues such as rice straw (compost prepared outdoors), Timothy and Ladino clover were air-dried, then separated into fractions and analyzed for elementary composition as well as inorganic nitrogen content using the same procedures. Although the amounts of mineralized nitrogen were higher in fractions F5 and F6 compared with other fractions, the values were much lower compared with those of undecomposed plant residues. It was found that the amount of organic nitrogen mineralized in soil was affected not only by the C/N ratio of the plant residues but also by the differences in characteristics or properties of the plant materials and soils.