Dynamics of Available Nitrogen in Paddy Soils

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Abstract
A field experiment was conducted to evaluate 1) the pool size of mineralized N, determined by flooded incubation of chloroform-fumigated soil (available N or MinNacf); 2) effects of N changes on the rice plant during cultivation; and 3) N turnover, determined by 15N tracer technique. Three surface soils and one subsoil from Philippine paddy fields, placed in concrete tanks, were treated with 15N labeled ammonium sulfate, flooded, and either planted to rice or kept fallow. The amount of ammonium N formed during incubation of unfumigated soil (MinN) was about 60–70% that of the ammonium N formed during incubation of fumigated soil (MinNacf). At the soil surface (0–1 cm), MinNacf contents increased in both the planted and unplanted plots. This increase was not observed when the penetration of light through the floodwater surface of concrete tanks kept in the greenhouse was inhibited by layers of black cloth. Chlorophyll-type compounds and mineralized N after air-drying and re-flooded incubation also accumulated at the soil surface in proportion to the amount of MinNacf. Below the surface soil (1–15 cm deep) MinNacf contents in both planted and unplanted plots decreased gradually, but the differences in contents ‘were slight. The 15N abundance in ammonium- N found in concrete tanks and in MinNacf decreased with time. The 15N applied was quickly incorporated into the MinNacf pool and its atom percent abundance first increased, and then decreased gradually during incubation, particularly in planted soils.

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