system, crop management, and climatic
season. The N input from biological N 2 fixation
is smaller where soil N status is high (e.g., due
to mineral fertilizer N use) and soil P status is
low.
Sediments (W) are a major nutrient input in
traditional lowland rice systems, particularly in
irrigated rice systems located in river deltas
that are regularly affected by natural flooding.
The flood prevention structures and dams that
are installed to improve irrigation and drainage,
however, have decreased the addition of
nutrients in sediment inflow.
In the past, organic nutrient sources such as
farmyard manure, legume green manure, and
azolla were a major source of nutrient inputs,
but their use has declined in many regions
since the introduction of the Green Revolution
technology.
Values