The status and origin of available nitrogen in soils

The availability of soil nitrogen (N) is often evaluated based on the amount of released inorganic N during incubation of a soil at 30 degrees C under field moisture conditions (referred to as the incubation method). Phosphate buffer extractable organic N (PEON, referred to as the phosphate buffer extraction method), appeared to be strongly correlated (r=0.91***) with the inorganic N by the incubation method, based on an analysis of 20 soil samples collected from various soil types. The substances in the soil extracts with phosphate buffer were analyzed by size-exclusion and ion-exchange-resin HPLC with a UV detector (280 nm). One major peak was detected in both types of HPLC chromatograms irrespective of the soil types examined. Furthermore, the peak area in the size-exclusion and ion-exchange-resin HPLC showed a high correlation with both the amount of PEON and the protein content in the soil extracts with phosphate buffer. In order to determine the origin of this homogeneous N compound that appears as one major peak in the two types of HPLC, three kinds of organic materials, i.e. glucose and ammonium sulfate, 4:1 mixture of rice bran and rice straw (RBS), and egg albumin were added to soil, and their soil extracts with phosphate buffer were traced using size-exclusion HPLC. The initial peaks of the organic materials disappeared rapidly on the HPLC chromatogram, and the area of one major peak that was present in the original soil increased, even though different organic materials were applied. However, when chloramphenicol and RES were added simultaneously, the area of this major peak was small and various peaks were appeared, but when cycloheximide and RES were added, the area of the major peak became larger than in the case of RES without addition of antibiotics. These findings suggest that a protein-like substance (about 8,000-9,000 Da) extracted with phosphate buffer may be homogeneously produced through the interaction of microorganisms, and it could be an important constituent of the available N in soil.