Incorporation of Nitrogen from Urea Fertilizer into Soil Organic Matter in Rice Paddy and Cassava Upland Fields in Indonesia

Tomonori Abe, Sambudhi S. Suwandhi, Wayan Sabe Ardjasa, Ho Ando, and Akira Watanabe

Soil Sci. Plant Nutri., 48, 825-832, 2002

Incorporation of newly-immobilized N into major soil organic matter fractions during a cropping period under paddy and upland cropping systems in the tropics was investigated in Jawa paddy fields with and without fish cultivation and a Sumatra cassava field in Indonesia. 15N-labelled urea (15N urea) was applied as basal fertilizer, and the soil samples were collected after harvest. The percentage of distribution of the residual N in soil from 15N urea into the humic acids, fulvic acid fraction, and humin were 13.1-13.9, 19.0-20.5, and 53.4_54.3%, respectively, for the Jawa paddy soils, and 14.9, 27.4, and 52.4%, respectively, for the Sumatra cassava soil. These values were comparable to the reported ones for other climatic zones. The percentage of distribution of 15N urea-derived N into humic acids was larger than that of total N into the same fraction in all the soils. The distribution into the fulvic acid fraction was also larger for 15N urea-derived N than for total N in the Jawa soils. Humic and non-humic substances in the fulvic acid fraction were separated using insoluble polyvinylpyrrolidone (PVP) into the adsorbed and non-adsorbed fractions, respectively. Less than 5% of the 15N urea-derived N in fulvic acid fraction was detected in the PVP- adsorbed fraction (generic fulvic acids). The proportion of non-hydrolyzable N remained after boiling with 6 M HC1 in the 15N urea-derived N was 9.4-13.5%, 17.3-26.7%, and 8.4- 16.6% for the humic acids, generic fulvic acids, and humin, respectively. The significantly low resistance to acid hydrolysis suggested that the 15N urea-derived N was less stable than the total N in soil regardless of the fractions of humus.


Key Words: fulvic acids, humic acids, 15N fertilizer, rice paddy, tropics.