No abstract

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 Nitrogen balance studies were made on rice (Oryza sativa) grown in flooded soil in pots. A low rate of fertilizer (5.64 mg N. kg-I soil) did not depress the N gain, but a high rate (99.72 mg N .kg -1 soil) elminated the N gain. Soil N loss was negligible since 15N applied as ammonium sulfate and thoroughly mixed with the soil was recovered from the soil-plant system after 3 crops. The observed N gain, therefore, was caused by N2-fixation, not by a reduction of soil N loss.Straw enhanced N gain at the rate of 2-4 mg per g straw. However, this gain was not observed when soil N availability was high. Dry fallow between rice crops decreased the N gain.