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Regulation of Invertebrate Grazers as a Means to Enhance Biomass and Nitrogen Fixation of Cyanophyceae in Wetland Rice Fields¹

ABSTRACT

The effect of invertebrate grazing on the growth and N fixation of blue-green algae (Cyanophyceae) in flooded rice soils was investigated by depressing grazer populations. Grazers were controlled with commercial pesticides and seeds of neem (Azadirachta indica). Algal N fixation and standing biomass were estimated by acetylene-reduction activity and chlorophyll a measurements. Suppression of ostracod (Ostracoda) grazing by Pertbane or neem seeds tripled blue-green algal biomass and increased N fixation rates 10-fold. In the absence of ostracods, free living blue-green algae multiplied rapidly early in the rice cultivation cycle to be succeeded by chlorophytes. Carbofuran was not an effective control measure. Suppression of molluscan grazing had little effect. The population of tubificids (Tubificidae) was higher in the plots where algal growth was stimulated than in other plots. Total rice grain N was increased up to 37% when grazing was arrested.

¹ Contribution from the Soil Microbiology Dep., International Rice Research Institute, P.O. Box 933, Manila, Philippines. This investigation was supported by a grant from the United Nations Development Programme.

² Research Associate, Boyce Thompson Institute, Cornell University, Ithaca, NY 14853; Research Assistant, IRRI, P.O. Box 933, Manila, Philippines; Microbiologist, Bangladesh Rice Research Institute, P.O. BRRI, Joydepbur, Dacca, Bangladesh; and Department Head, Soil Microbiology, IRRI, P.O. Box 933, Manila, Philippines.

Received for publication June 28, 1982. Accepted for publication January 31, 1983.