Efficient management of soil and biologically fixed N2 in intensively-cultivated rice fields

A decline in productivity in wetland rice has been detected in some intensively-cultivated experimental farms in Asia since the early 1980s. Increased doses of fertilizer N are being used in both experimental and farmersʹ fields to maintain the original yield levels. Little attention has been paid to judicious management of native soil N, which is the principal N source for rice, and to biological N2 fixation (BNF), which largely replenishes the soil N concentration. We review here various effects of long-term flooding and puddling associated with intensive cultivation of wetland rice on soil N availability and BNF. Some strategies are suggested to efficiently manage these two N sources to sustain high productivity of the ricelands.