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 grain yield to the applied N rate. It is a parameter that includes contributions to N-use efficiency from both indigenous N of the soil-floodwater system and applied N. Experiments were conducted to quantify P-fp and the contributions of indigenous and applied N to the N efficiency of lowland rice systems. Enormous variation was found in the indigenous N supply among farmers' fields in two rice-growing domains of Central Luzon, Philippines. Fertilizer-N rates farmers applied to these fields also varied greatly, but there was no relationship between applied N rate and indigenous N supply estimated by rice N uptake. Likewise, in the same treatment plots of a long-term experiment, season-to-season variation in the contributions of indigenous and applied N were large and reflected differences in yield and N uptake in plots without applied N. These results indicate that the ability to adjust the quantity 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 rice systems is highly variable among fields with similar soil types and in the same field over time, that field-specific N management is required to respond to this variability, and that P-fp is a useful parameter for identifying constraints to improved fertilizer-N-use efficiency in farmers' fields.