OPPORTUNITIES FOR INCREASED NITROGEN-USE EFFICIENCY FROM IMPROVED RESOURCE-MANAGEMENT IN IRRIGATED RICE SYSTEMS

Research and extension work to improve nitrogen (N) management of irri gated rice has received considerable investment because yield levels p resently achieved by Asian farmers depend on large amounts of N fertil izer. Most work has focused on placement, form, and timing of applied N to reduce losses from volatilization and denitrification. In contras t, less emphasis has been given to development of methods to adjust N rates in relation to the amount of N supplied by indigenous soil resou rces. As a result, N fertilizer recommendations are typically made for districts or regions with the implicit assumption that soil N supply is relatively uniform within these domains. Recent studies, however, d ocument tremendous variation in soil N supply among lowland rice field s with similar soil types or in the same field over time. Despite thes e differences, rice farmers do not adjust applied N rates to account f or the wide range in soil N supply, and the resulting imbalance contri butes to low N-use efficiency. A model for calculating N-use efficienc y is proposed that explicitly accounts for contributions from both ind igenous and applied N to plant uptake and yield. We argue that increas ed N-use efficiency will depend on field-specific N management tactics that are responsive to soil N supply and plant N status. N fertilizer losses are thus considered a symptom of incongruence between N supply and crop demand rather than a driving force of N efficiency. Recent k nowledge of process controls on N cycling, microbial populations, and soil organic matter (SOM) formation and decomposition in flooded soils are discussed in relation to N-use efficiency. We conclude that the i ntrinsic capacity of wetland rice systems to conserve N and the rapid N uptake potential of the rice plant provide opportunities for signifi cant increases in N efficiency by improved management and monitoring o f indigenous N resources, straw residues, plant N status, and N fertil izer. (C) 1998 Elsevier Science B.V.