Incorporation of nitrogen from urea fertilizer into soil organic matter in rice paddy and cassava upland fields in Indonesia |
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Authors: | Tomonori Abe Sambudhi S Suwandhi Wayan Sabe Ardjasa Ho Ando Akira Watanabe |
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Institution: | 1. Graduate School of Bioagricultural Sciences , Nagoya University , Nagoya, 464-8601, Japan;2. Faculty of Agriculture , Gadja Mada University , Yogyakarta, 55581, Indonesia;3. Taman Bogo Experiment Station, Department of Agriculture , Purbolinggo Village, East Lampung, 34192, Indonesia;4. Faculty of Agriculture , Yamagata University , Tsuruoka, 997-8555, Japan |
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Abstract: | 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 HCl 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. |
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Keywords: | fulvic acids humic acids 15N fertilizer rice paddy tropics |
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