Distribution of Organic forms of Nitrogen in Paddy Soils in Tropical and Temperate Regions

Abstract

Soil organic nitrogen distribution was analyzed for 85 soils from South East Asian countries, 24 Japanese soils and 15 soils from Mediterranean countries. The forms of organic nitrogen were determined as amide-N, α-amino-N, hexosamine-N, unknown-N, and nonhydrolyzable-N according to the improved method previously reported. South East Asian soils showed low percentages of hydrolyzable-N and α-amino-N in total-N compared with the values in the Japanese and Mediterranean countries’ soils. To characterize the nitrogen distribution of Tropical Asian soils in more detail, several multivariate statistical methods were applied. After factor analysis was performed, three factors were obtained which corresponded to the α-amino-N, hexosamine-N, and exchangeable-N forms independently. Numerical taxonomic method for cluster analysis was used on the basis of the similarity of organic nitrogen distribution. Based on the cluster analysis, the soils were assigned to 13 classes and finally divided into two groups. The two groups were confirmed with a nonlinear mapping algorithm and designated as A and B groups. Hydrolyzable organic N content and the percentages of α-amino-N in the hydrolyzable-N of group B were lower than those of group A. Each soil was coded on the projection of nonlinear mapping according to the climatic and physiographic regions. The soils which had good water regimes were assigned to group A. Group B comprised the soils which had dried markedly. Thus, even under field conditions, repeated mineralization associated with the natural air-drying effect would result in the lowering of the α-amino-N content in soils. © Taylor & Francis Group, LLC.