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.