No 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.

Loss of carbofuran from paddy water and in flooded soil was studied in the laboratory and under field conditions. Carbofuran was rapidly hydrolyzed to carbofuran phenol in just five days after its application to paddy water. Hydrolysis of carbofuran appeared to be primarily due to chemical, but degradation of carbofuran phenol was biological.Previous applications of carbofuran to paddy water had no appreciable effect on the rate of degradation of the insecticide. However, in soil from the carbofuran-treated plot, a more rapid degradation of carbofuran occurred only after three weeks of incubation under laboratory conditions.

Estimation of the nitrogen balance for irrigated rice and the contribution of 'phototropic nitrogen fixation. Field Crops Res., 9: 17-27. The total N content of soils from long-term fertility plots in two sites in the Philippines was measured by Kjeldahl analysis. One site had grown 24 and the other 17 crops of irrigated rice (Oryza sativa L.). It appears that the total soil N at each site did not decrease during the cropping period. There was litt? evidence that N, P, o r K fertilization affected the total soil N content. Nitrogen (NH, and NO;) input by rainfall varied between 0.6 and 2.4 kg of N/ha per year. Calculations based on crop yields and known N inputs suggest that the two flooded rice crops grown each year resulted in a positive N balance equivalent to 79 and 103 kg N/ha per year. An attempt was made to measure the accumulation of N that may occur in the oxidized surface layer of the soil in the field as a result of N fixation by phototrophic microorganisms during a rice crop. No acetylene reduction activity or accumulation of N (Kjeldahl analysis) was observed in the surface soil when the light was not allowed t o penetrate to the water and soil surface. Plots open to the light accumulated the equivalent of approximately 6-8 kg N/ha in the surface soil between transplanting and heading.

Summary A short-term laboratory acetylene reduction assay using cut plant-soil samples incubated in the dark was developed for measuring relative N 2 -fixing activities associated with field-grown rice plants. The assay sample consists of rhizosphere soil, root, and cut stem and leaf sheath. The cut plant-soil assay is relatively simple, rapid, and convenient; it reduces, if not eliminates, the problems encountered in whole-plant (field, pot, and water culture) and excised roots assays. Varietal differences in N2-fixing activity were detected with the new assay technique.