Sequential extraction methods for soil organic nitrogen

Abstract

(pp. 833–841) In order to learn the status of available soil nitrogen (N), two sequential extraction methods (A and B) were tested. Proposed methods are as follows. (A) Extraction with water, KCl, acetic acid, 1/15 M phosphate buffer, sulfuric acid, and NaOH. (B) Extraction with water, KCl, 0.01 M sulfuric acid, 0.1 M sulfuric, 0.2 M sulfuric acid and 0.4 M sulfuric acid. 1) NO3-N was only detected in a solution extracted with water. Major N in an extract with KCl was in ammonium form. Extracts with phosphate buffer and sulfuric acid contained organic-N only. When these two fractions were examined by size exclusion chromatography (HPLC-SEC), only one major peak in each extract appeared. 2) Minerals, such as Al and Fe in each extract were determined using ICP (Varian Co., Ltd.). With the increase in concentration of sulfuric acid, the high Fe concentration in each extract increased. On the other hand, much higher Al was found in extracts with lower concentrations of sulfuric acid compared to Fe. 3) Though some crop species such as spinach and carrot show the potential to take up organic-N in a soil applied with organic matter, this sequential soil-nitrogen extraction method may be useful to evaluate availability of soil nitrogen fertility with more accuracy compared to conventional methods in the case of these superior crop species.