Kg. Cassman et al., NITROGEN-USE EFFICIENCY IN TROPICAL LOWLAND RICE SYSTEMS - CONTRIBUTIONS FROM INDIGENOUS AND APPLIED NITROGEN, Field crops research, 47(1), 1996, pp. 1-12
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.