Abstract
To estimate the amount of available nitrogen released under waterlogged conditions, the extracting condition of soil nitrogen with phosphate-buffered solution was investigated. 1) The amount of soil nitrogen extracted with neutral 1/15 mol L^<-1> phosphate-buffered solution were divided into the first component of extractable soil nitrogen within 15 min (N1) and the second component of extractable soil nitrogen over 15 min (N2). The increase of N2 component up to 12 h is showed by first-order reaction. The amount of extracted soil nitrogen from 30 min to 2 h was denoted by N1+extracting hour × k ×N2, with k representing the constant rate. Therefore, the extracting time was optional from 30 min to 2 h. But, we proposes 1-h extraction. 2) Arrhenius's law can be applied to the amount of soil nitrogen extracted with neutral 1/15 mol L^<-1> phosphate-buffered solution within the extracting temperature-range of 10-30℃, and it is possible to convert the amount of extracted soil nitrogen at any extracting temperature within 10-30℃ to the amount at standard extracting temperature (i. e., 20℃) with the value of the activating energy (24.3 kJ mol^<-1>). So the amount of extracted soil nitrogen should be represented by the converted amount at 20℃ (standard temperature) using Arrhenius's law. 3) The correlation between amounts of extracted soil nitrogen and amounts of available nitrogen incubated under waterlogged conditions became lower when phosphate concentration of the buffer solution increased to 2/15 or 4/15 mol L^<-1> and potassium chloride was added to the neutral 2/15 mol L^<-1> phosphate-buffered solution; when the pH of the buffer solution decreased to 5.0 from 7.0 in case of Andosol soil samples; and when the soil: solution ratio changed to 1 : 20 from 1 : 5 except for Andosol and Yellow soil samples. 4) From the above results, to estimate the amount of available nitrogen released under waterlogged conditions from that of extracted soil nitrogen, 1-h continuous shaking, a 1 : 5 ratio of soil : solution, and a pH 7.0 buffer solution with 1/15 mol L^<-1> phosphate concentration and without potassium chloride are suitable. 5) Under the above method, in the case of Andosol, Gray Lowland and Gley soil samples, it was possible to estimate the amount of available nitrogen released under waterlogged conditions from that of extracted soil nitrogen. However, in the case of Wet Andosol and Yellow soil samples, it was difficult to estimate the amount of available nitrogen released under waterlogged conditions. Detailed research is necessary for these two kinds of soil groups.
