At the 14th International Congress of Soil Science held in Kyoto, Japan (August 12-18, 1990), the International Rice Research Institute presented recent findings on the role of biological nitrogen fixation(BNF) in rice fields. These findings included recent improvements in nitrogen fixing systems that function in rice cultures, and the use of atmospheric nitrogen through aquatic legumes in rice cropping systems.
Two of the N2 fixing systems that can function in flooded soils are:
Technical improvement and the selection of suitable strains has been attempted. (In China the technology of harvesting and seeding Azolla sporocarps, a structure in or on which spores are produced was developed, allowing Azolla to be maintained during winter months.) This was limited by slow growth in the seedbed and sporulation by only a limited number of strains. A sampling of Azolla plants grown in the Philippine suggested a higher P requirement by A. microphylla than by A. pinnata.
A. filiculoides is the least heat tolerant, water temperatures>40oC greatly reduce growth. Heat and pest damage are associated, and there are a number of major insect pests in the humid tropics. Moist soil culture effectively avoids damage by insects and high temperatures.
Applied N is absorbed by Azolla and used by rice. Loss of urea decreased from 50% to 25% when Azolla was allowed to grow for two weeks after applying urea, and then incorporated into the soil.
At the end of a flooded rice crop, NH4-N dominates in the soil and NO3-N is normally negligible. Subsequent drying of the soil favors an aerobic N transformation process. As a result NO3-N accumulates in rice fields during the dry season. When soils in the Philippines were flooded during the subsequent wet season, NO3-N rapidly disappeared. Denitrification or leaching appears to be the major loss mechanism.
Factors influencing NO3-N accumulation in the soil are; soil water availability, tillage, and the presence of weeds or green manure crops.
This supports the hypotheses that plant residues can recycle soil NO3-N that might otherwise be lost upon soil flooding for wet season rice. Growth of legumes during the dry season may influence the dynamics of soil N.
Two niches for legumes in lowland rice-based cropping system are, the post rice dry season for grain legumes, and pre-rice transition for grain or green manure.
Low soil N availability results in high amount and proportion of BNF. In greenhouse experiments, growth and incorporation of legume green manure residues in lowland rice culture have been shown to result in positive N balance and increased rice N. Increased rice yields have also been reported, due to the incorporation of legume residues.
To date, research on N dynamics in lowland rice sequences has primarily been on legumes grown just prior to wet season rice.
When rice is grown for grain, the amount of N removed from the land will vary with the N content of the grain and the harvest index(grain yield/total yield) of the crop. Nitrogen removal through grain legumes can potentially reduce the N contribution to wet season rice. Conversely the amount of N that can be incorporated by some legumes is substantially higher than what is required by rice.
Nitrogen is available to rice only after mineralization. The recovery of N from green manure was 33%, similar to that of fertilizer, however, up to 75% recovery for Sesbania rostrata has been reported. The difference may be due to the amounts of green manure applied. Studies show that N lost from green manure was lower than that from equivalent amounts of fertilizer N.
In sum, conserving this dry season NO3-N can become a strategy in the management of N for maximizing rice production. Inclusion of legumes as grain and green manure in a dry season management program may enhance the soil N with BNF. Increasing the legume component in the weed flora may be a suitable approach in conserving both soil nitrate and atmospheric N where rice growing soils are left fallow during the dry season due to production constraints.
Improvement of Nitrogen-Fixing Systems and their Integration into Sustainable Rice Farming, Iwoa Watanabe and Liu Chung-chu.
Conservation and Use of Soil and Atmospheric Nitrogen Through Legumes In Lowland Rice-Based Cropping System, T, George, J.K. Ladha, D.P. Garrity and R.J. Buresh
For more information:
International Rice Research Institute
P.O. Box 933, Manila, Philippines