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:

  • Symbiotic(Azolla-Anabaena)

  • Flood tolerant legumes

  • Azolla-Anabaena Symbiosis

    Since the 1980s the aquatic fern Azolla has been used in the phosphorus- rich fields of Southern Mindanao, Philippines. The Euazolla species was recently introduced because of it's potential biomass production, which is higher than that of A. pinnata. Factors that limit the use of Azolla are the availability of chemical N fertilizer, and the cultivation of more economical crops. Technical constraints include; difficulty in obtaining inoculum, P deficiency, low heat tolerance, damage by insects and fungi, and poor water control.

    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.

    Genetic Enhancement

    Recent attempts to improve Azolla-Anabaena through sexual hybridization and algal inoculation were successful in increasing N2-fixing ability. Hybridization has been reported and confirmed between A. filiculoides and A. microphylla in which N content of hybrids was higher than the parent A. microphylla. Crosses between A. mexicana and A. filiculoides show improved tolerance to heat and snail attack. A. microphylla and A. mexicana were also hybridized.

    Rice-Azolla-Fish culture

    A combined rice-Azolla-fish culture protects the environment and increases the farmer's income through fish production and reduced consumption of fertilizers and pesticides. A system developed by the Fujian Academy of Agricultural Sciences, useing three kinds of fish and several strains of Azolla,, could potentially reduce the use of inorganic fertilizer by 75%, and reduce the incidence of rice disease and insect pests. A polyculture of fish accelerates N recycling among Azolla, fish, fish feces and rice. Azolla N is incorporated into fish protein and the rice plant. Topsoil in rice fields and fish pits was enriched with N and P.

    Azolla and Applied Inorganic Fertilizer

    Azolla may decrease floodwater volatilization of NH3 through the interruption of sunlight. Light increases floodwater pH, which in turn stimulates NH3 volatilization from surface area applied urea, therefore a reduction of light from shading would decrease this volatilization.

    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.

    Flood-Tolerant Legumes

    Used as a green manure for rice, the major N fixing legumes for rice are Astragalus in the temperate region and Sesbania cannabina in the tropics. Green manures can be inserted into the cropping calendar when the growth interval is short, or they can be intercropped. Some species of aqautic Sesbania, Aeschynomene and Neptunia, form nodules on aerial stems that enable them to fix N2 under submerged conditions, and their N2 fixation is tolerant to combined N. Within 45 days S. rostrata, possibly the fastest nitrogen-fixing plant known, can accumulate 110 kg N/ha, while Aeschynomene afraspera accumulates up to 90 kg N/ha. S. Rostrada and Setaria Sphacelata can be harvested for forage several times during the dry season, and then be encorporated into the soil before rice is planted.The estimated fertilizer substitute values range from 60 to 120 kg N/ha.

    Soil-Nitrogen Dynamics

    The focus of a second presentation at the Soil Science Congress in Japan was soil-N dynamics in relation to the dry/wet seasons in rice-based cropping systems. More than 50% of N used by lowland rice is normally derived from mineralization of soil organic N by free living rice plant-associated bacteria. The remaining N is provided through fertilizer input sources. Effectively managing native soil N and BNF for use in rice-based cropping systems requires an understanding of the effects of the dry-wet-flooded moisture sequences on retention, gain and loss of N.

    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.

  • In a greenhouse test conducted in the Philippines, continuous flooding of the soil during a dry season fallow prevented NO3 accumulation. Drying and drying/wetting treatments resulted in accumulation of NO3

  • A tilled fallow accumulated three times more NO3-N than a non tilled fallow.
  • Plant N uptake decreased soil NO3-N and therefore may conserve NO3-N from subsequent loss upon flooding by recycling the native soil N through plant residues that are incorporated prior to rice planting.

  • Higher rice yields following a weedy rather than a weed-free fallow, which may be due in part to N contribution from weed biomass.
  • 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.

    BNF and Nitrogen Balance

    Legumes in a rice- based cropping system can:

  • decrease the amount of native soil NO3-N by utilizing it in plant biomass, and

  • enhance the native soil N through BNF.
  • 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.

    Presented in the Symposium "role of biological nitrogen fixation in sustainable agriculture", by IRRI, Kyoto, Japan, 1990.

    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:

    J.K. Ladha
    International Rice Research Institute
    P.O. Box 933, Manila, Philippines