To investigate the effects of inorganic and organic fertilizer application on the nitrogen (N) status of soil, we evaluated the N content in the soil of a paddy field in Kameoka, Kyoto, that had been treated for 34 years with five types of fertilizers: chemical fertilizer without N (plot K1), chemical fertilizer (plot K2), chemical fertilizer and rice straw (plot K3), chemical fertilizer, rice straw and silicic slag (plot K4), and chemical fertilizer, rice straw, and cow dung (plot K5). The N in the surface soil from each plot was separated into four fractions: inorganic extractable N (Iex-N), fixed NH^+_4-N (Ifix-N), organic mineralizable N (Omin-N), and organic stable N (Osta-N). The total N contents for plots K1, K2, K3, K4, and K5 (1.32, 1.45, 1.71, 1.92, and 3.57 g kg^<-1>, respectively) suggest that organic matter application increased the total N content. Iex-N ranged from 3.4 to 5.4 mg kg^<-1> (〜0.2% of the total), irrespective of fertilizer application. The Omin-N contents (55.7, 68.0, 87.4, 83.8, and 163.3 mg kg^<-1>, respectively; 4.2-5.1% of the total) suggest that application of inorganic and organic fertilizer increased the Omin-N content. Omin-N content was positively correlated with organic N content in a solution obtained by extracting with a 1/15 M phosphate buffer. The Ifix-N contents (96.6, 93.8, 111.2, 105.0, and 111.2 mg kg^<-1>, respectively) suggest that application of organic matter increased fixation of NH_4-N. In conclusion, fractionation of soil N clearly demonstrates the effects of fertilizer management on the N status of the paddy field soil, which is important both agriculturally and environmentally.ntent was 96.6, 93.8, 111.2, 105.0 and 111.2 mg kg^<-1>, suggesting increased fixation of NH_4-N due to organic matter application. In conclusion, fractionation of soil N clearly demonstrated the effect of fertilizer management on the N status of the paddy field, which plays a very important role from both agricultural and environmental points of view.
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