Kg. Cassman et al., OPPORTUNITIES FOR INCREASED NITROGEN-USE EFFICIENCY FROM IMPROVED RESOURCE-MANAGEMENT IN IRRIGATED RICE SYSTEMS, Field crops research, 56(1-2), 1998, pp. 7-39
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.