Abstract
The present study was carried out during a period of 1 y (from May 1, 1997 to April 30, 1998) in a rice cultivation area not affected by livestock farming, farmstead, and industrial complexes to investigate the balance of nutrients in rainfall, irrigation water, runoff water, and infiltration water. The soil of the experimental field belonged to the Jisan series (SiL; fine loamy, mixed mesic family of Fluventic Haplaquepts) covering on area of 5,000 m2 (100 m × 50 m). In the investigated area, the amount of rainfall was 1,095 mm during the cropping season and 415 mm during the non-cropping season (off-season). The annual rainfall was 1,510 mm. The total amount of runoff water was 1,043 mm during the cropping season and 281 mm during the off-season and the added total amount of runoff water during the two seasons was 1,324 mm. The input amounts of nitrogen and phosphorus were as follows: 217.6 kg ha-1 for nitrogen and 27.2 kg ha-1 for phosphorus from chemical fertilizer, 39.5 and 0.7 kg ha-1 y-1 from rainfall, 29.7 and 0.5 kg ha-1 from irrigation water, 1.0 and 0.2 kg ha-1 from irrigation sediments, and 34.0 and 12.3 kg ha-1 from rice straw, respectively. The amount of biological nitrogen fixation was 94.8 kg ha-1 y-1. The residual amounts of nitrogen and phosphorus in soil were 85.0 and 15.6 kg ha-1 y-1, respectively. The output amounts of nitrogen and phosphorus from different channels were as follows: 157.9 kg ha-1 y-1 for nitrogen and 4.5 kg ha-1 y-1 for phosphorus from runoff water, 3.2 and 0.9 kg ha-1 y-1 from runoff sediments, 20.3 and 0.2 kg ha-1 from infiltration water, and 111.6 and 19.2 kg ha-1 from rice plant absorption. The nitrogen losses from paddy fields were 24.0 kg ha-1 y-1 by denitrification and 44.6 kg ha-1 by ammonia volatilization. When the loss ratio was calculated based on the amount of applied chemical fertilizer, about 68.6% of nitrogen was lost by rainfall-runoff while 16.7% of phosphorus was lost. The loss ratio of nutrients by infiltration was as follows: nitrogen, 9.4% out of the amount applied; and phosphorus, 0.6%. Also, about 11% of nitrogen was lost by denitrification and about 20.5% by ammonia volatilization.
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