Abstract
One day after application, urea-N remaining in the floodwater and determined as water-soluble N (urea-N + NH4 +-N) was used to calculate the potential N loss from lowland rice soils. Actual N loss calculated from 15N balance measurements using forced air exchange (airflow rate: 20 L min-1) in greenhouse pots. Conditions for variable potential N loss were created by manipulating the method of urea application and duration of presubmergence or by selecting soils with diverse cation exchange capacities (CEC). Potential N loss tended to be lower than actual N loss; the differences were, however, nonsignificant. The method of urea application that led to the lowest potential N loss from a Guthrie silty clay loam (Typic Fragiaquult) also led to the least 15N loss and vice-versa (r=0.99**). Duration of presubmergence did not alter the relationship between potential and actual N loss although it influenced the rate of urea hydrolysis in floodwater. The primary depencence of actual N loss on water-soluble N was maintained in soils differing in CEC (r=0.83**). The association between potential and actual N loss was closer for high-CEC soils (≥ 20 cmol [+] kg-1 soil, r=0.91**) than for low-CEC soils (<20 cmol [+] kg-1 soil, r=0.85**). Ammonia volatilization could be more closely predicted by potential N loss than could apparent denitrification.
The results of this study suggest that potential N loss calculated from one-time determination of water-soluble N in floodwater can be a good index of actual N loss from flooded, puddled rice soils. Notable exceptions are to be expected for soils in which water-soluble N gets lost from floodwater either before (soils with fast urea hydrolysis in floodwater) or after (soils with steady leaching) determination of potential N loss.
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Katyal, J.C., Gadalla, A.M. Fate of urea-N in floodwater. Plant Soil 121, 21–30 (1990). https://doi.org/10.1007/BF00013093
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DOI: https://doi.org/10.1007/BF00013093