Wednesday, November 15, 2006 - 11:00 AM

Carbon and Nitrogen Cycling under Rice–Maize Cropping.

Mirasol F. Pampolino, Wenceslao M. Larazo, Ma. Carmelita R. Alberto, and Roland J. Buresh. IRRI, DAPO Box 7777, Metro Manila, Philippines

A 13-yr-old experiment in the Philippines was used to assess the effect on soil C and N of converting from a continuous irrigated rice monoculture to a rice–maize rotation. After 12 years, soil C and N were reduced in the rice–maize system but not in the rice–rice system. The reduction in soil C was 14% from an initial level of 15.4 g C kg-1 soil, and the reduction in soil N was 17% from an initial level of 1.4 g N kg-1 soil. Reductions in soil C and N were greater without fertilizer N than with fertilizer N. A decline in N mineralization during the rice cropping season was also observed in the rice–maize rotation. More soil nitrate accumulated after the maize than the rice crop, and this nitrate was lost presumably by denitrification upon flooding the soil for rice cultivation. Indigenous N supply, as determined from rice yield in the absence of fertilizer N was consistently lower following maize than rice after the first four years of the experiment. Yield of fertilized rice was equally high following rice and maize — provided fertilizer inputs were sufficient. More fertilizer N was required for rice following maize than rice. The trend in Asian lowland rice ecosystems to greater soil aeration as a result of diminishing supply of irrigation water and diversification to maize could lead to reduced soil organic matter and soil nutrient supplying capacity in one of the most important and productive agricultural production systems in the world.