Reversal of rice yield decline in a long-term continuous cropping experiment

In a long-term continuous cropping experiment at Los Banos, Philippines, th ree rice (Oryza sativa L,) crops were grown each year with the goals of max imum annual grain production and high N use efficiency. Our objective was t o identify the factors responsible for the restoration of yields occurring after 1991. From 1968 to 1991, grain yields declined at an annual rate of 1 .4 to 2.0%. From 1991 to 1995, dry season (DS) yields in the highest N trea tment increased to within 80 to 100% of the simulated yield potential; yiel ds in the unfertilized control did not increase. Increased solar radiation, increased N rate, and improved timing of N applications accounted for the restoration of yields in the DS, Wet season yields increased in fertilized and unfertilized plots due to greater solar radiation, improved timing of N applications, and increased soil N supply due to dry fellow periods in thr ee years. Residual benefits of soil aeration were short-term. Reducing prep lant N fertilizer and increasing the number of split applications had a gre ater effect on increasing yield than the increase in the amount of N applie d. Our results provide evidence that N deficiency caused the yield decline before 1991. However, the actual processes that caused a decline in soil N supply or plant N uptake remain to be determined. It is possible to sustain high yields and high N use efficiency if fertilizer regimes are updated re gularly to maintain the congruence between crop N demand and the N supply f rom soil and fertilizer.