Dynamics of Available Nitrogen in Paddy Soils

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Abstract
Rice plants were grown without N fertilizer in concrete pots (1.5 m x 1.5 m), which were previously planted to rice or kept fallow after applying 15N-labelled fertilizer. The balance of N and 15N during two consecutive rice cropping seasons was calculated to analyze the dynamics of N transformation among various soil N pools, rice plant, and aquatic weeds. At harvest of the first crop, 79–83% of N in the rice plant was derived from soil N. The 15N atom % excess absorbed by rice in the first crop showed an almost exponential decrease. In the early period, NH4 +-N in soil had a 15N abundance similar to N absorbed by rice. In the middle and later periods, NH4 +-N content became null, and its 15N abundance reached approximately the natural 15N level. In the early period, however, the 15N abundance of MinNacf (mineralized N of chloroform-fumigated soil during anaerobic incubation) in the planted plots was lower than that of N absorbed by rice, but it became close to the 15N content of the N absorbed by the plant in the middle and later periods. These findings suggest that MinNacf was a direct source of N for rice after NH4 +-N in the soil was exhausted, and could thus be called “available N.” Recovery of 15N in plant and soil in the first crop harvest was high (80–99%) except in the Luisiana and Maahas fallow plots where a large amount of NH4 +-N remained in the soil till harvest. The 15N in MinNacf was 11–18% of the value of applied 15N at this time. The 15N abundance in the aquatic flora was close to that of MinNacf in the 0–1 cm layer of soil. The 15N abundance in blue-green algae was significantly lower than that in other aquatic weeds. Through the second cropping season,little NH4 +-N was found in all the soils. Growth of rice and aquatic weeds was affected by previous crop conditions. The decrease of NH4 +-N and MinNacf during crop cultivation accounted for 18–35% of 15N taken up by the plant. The 15N abundance of MinNacf was almost constant during the second cropping season. The amount of MinNacf and N taken up by rice under N-deficient conditions in the two cropping was highly correlated (r=0.885*), supporting the hypothesis that MinNacf is a major source of N for rice under these conditions.