The nitrogen supplying capacities transformation of fixed ammonium and chemical nature of decomposable organic nitrogen through the successively air drying or oven drying rewetting and incubation of residual soils amended with nitrogen 15 labeled rice straw
Kawaguchi, S.; Kai, H.; Aibe, T.
Journal of the Faculty of Agriculture Kyushu University 30(4): 253-266
On the repeated either air-drying or over-drying, rewetting and incubation of residual soils amended with 15N-labelled rice straw, the nitrogen supplying capacities of soils, transformation of fixed ammonium and chemical nature of decomposable organic nitrogen were investigated. The results were summarized as follows: (1). The mineralization of nitrogen derived from rice straw of residual soils was remarkably accelerated at the early stage of incubation due to both air-drying and over-drying effects. The magnitude of a flush of mineral nitrogen was much greater in the latter than in the former. On the contrary, the mineralization of the native soil-N was continued at a fairly steady rate by the later stage of incubation. These results would account for the necessity of successively long-term application of rice straw to increase the nitrogen supplying capacities of soil. (2) A small portion of rice straw-N was fixed by soils during the mineralization process of rice straw. The application of rice straw to soils resulted in the promotion of the release of the native soil fixed ammonium, and the magnitudes of the priming effect were approximately in inverse proportion to the amounts of native fixed ammonium of soils. Ammonium mineralized due to the air-drying of residual soils was slightly fixed, while the repeated over-drying, rewetting and incubation led to a marked increase of fixed ammonium derived from both rice straw-N and soil-N. The percentages of newly fixed ammonium of mineralized nitrogen of rice straw-N and soil-N due to the over-drying effect ranged from 2.3 to 9.1% and from 1.6 to 9.7%. respectively. (3) As compared with the native soil-N, rice straw-N of the residual soils was found more in amino acid-N and unidentified-N, and comparable percentages were present in hexosamine-N and ammonium-N of the acid hydrolyzable-N. The principal fractions of organic nitrogen becoming decomposable through either air-drying or oven-drying of soils existed in unidentified-N and amino acid-N. Their susceptibilities to mineralization were much greater in the nitrogen fractions derived from rice straw-N than those from the soil-N. The susceptibility of hexosamine-N to the mineralization due to the air-drying effect was considerably lower than those of unidentified-N and amino acid-N, but enhanced significantly through the oven-drying of soils followed by rewetting and incubation.