NITROGEN-USE EFFICIENCY IN TROPICAL LOWLAND RICE SYSTEMS - CONTRIBUTIONS FROM INDIGENOUS AND APPLIED NITROGEN

Partial factor productivity (P-fp) from N fertilizer is the ratio of g rain yield to the applied N rate. It is a parameter that includes cont ributions to N-use efficiency from both indigenous N of the soil-flood water system and applied N. Experiments were conducted to quantify P-f p and the contributions of indigenous and applied N to the N efficienc y of lowland rice systems. Enormous variation was found in the indigen ous N supply among farmers' fields in two rice-growing domains of Cent ral Luzon, Philippines. Fertilizer-N rates farmers applied to these fi elds also varied greatly, but there was no relationship between applie d N rate and indigenous N supply estimated by rice N uptake. Likewise, in the same treatment plots of a long-term experiment, season-to-seas on variation in the contributions of indigenous and applied N were lar ge and reflected differences in yield and N uptake in plots without ap plied N. These results indicate that the ability to adjust the quantit y of applied N in relation to variation in the indigenous N supply is as important to increased P-fp as are the timing, placement and source of applied N. We conclude that the indigenous N supply of lowland ric e systems is highly variable among fields with similar soil types and in the same field over time, that field-specific N management is requi red to respond to this variability, and that P-fp is a useful paramete r for identifying constraints to improved fertilizer-N-use efficiency in farmers' fields.