Estimation of the nitrogen balance for irrigated rice and the contribution of phototrophic nitrogen fixation

Abstract 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 little evidence that N, P, or K fertilization affected the total soil N content. Nitrogen (NH 4 + and NO 3 − ) 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 oil when the light was not allowed to 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.