Effects of N-fertilizers, straw, and dry fallow on the nitrogen balance of a flooded soil planted with rice

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−1 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 since15N 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.