Transformation of external chemical nitrogen in soil organic nitrogen fractions and their relationship

【Objectives】 In this study, the transformation and relationship of external chemical N in soil organic nitrogen (SON) fractions were determined during one growing season of rice in order to provide a theoretical basis for reasonable fertilizer application and the effective adjustment to N fertilizer. 【Methods】 A pot experiment was carried out on a subtropical paddy soil in Jiangxi province, and a total N 180.0 kg/ha (15N labeled urea, atom 10.3%) was applied to a rice crop during one growing season under the conventional farmer practices. Distribution and dynamics of the chemical N fertilizer in different SON fractions (i.e., amino acid N, amino sugar N, hydrolysable ammonium N, hydrolyzable unknown N and acid insoluble N) were measured. Path analysis was used to evaluate the transformation process between SON derived from the fertilizer in soil-plant system.【Results】 1) The accumulation of fertilizer-derived N in different SON fractions was season-specific. The recovery contents of the fertilizer-derived N in soil amino acid N and amino sugar N at the jointing stage are significantly higher than those at the tillering stage (P0.05). The enrichments of 15N are significantly declined from the jointing stage to filling stage (P0.05). The dynamics of enrichment of 15N in soil amino acid N and amino sugar N during the growing season can be fitted as a Gauss equation, the maximum contents are 26.5 and 8.4 mg/kg, respectively, which are found between the tillering stage and jointing stage. The enrichment of 15N in hydrolysable ammonium N is declined gradually from the tillering stage to harvest stage, and the dynamics of the enrichment of 15N in soil hydrolysable ammonium N could be expressed as an exponential decline equation. The enrichment of 15N in hydrolyzable unknown N is gradually in a dynamical equilibrium from the tillering stage to harvest stage, and the dynamics of the enrichment of 15N in soil hydrolysable unknown N can be fitted as an exponential equation. The enrichment of 15N in acid insoluble N can be fitted as a symmetry equation, and the minimum of the enrichment of 15N is 7.9 mg/kg, which is found between the jointing stage and filling stage. 2) At the tillering stage and jointing stage of rice, preferential enrichment of 15N is found in soil hydrolysable ammonium N and amino acid N, accounting for 21.5% and 14.8% of the total N of the chemical N fertilizer, respectively. The accumulation of residual fertilizer-derived N in acid insoluble N is higher at the filling and harvest stages, accounting for 8.7% and 12.7% of the total N of the chemical N fertilizer, respectively. 3) The path analysis indicates that the hydrolysable ammonium N is a temporary pool for rapid chemical N fertilizer retention and is apt to release N for crop uptake simultaneously. In contrast, the amino acid N could serve as a transitional pool of available N in soil system through transferring into hydrolysable ammonium N and amino sugar N. 4) The fertilizer-derived N in hydrolysable ammonium N and amino acid N are closely related to fertilizer-derived N in crop uptake. 【Conclusions】 In a plant-soil system, the fertilizer N cycling during a growing season is closely related to temporal patterns of fertilizer N transformation into different SON fractions. The fertilizer-derived N can be derived into three pools with different availabilities. The hydrolysable ammonium N fraction can serve as a temporary pool containing readily available N to be released fast, while the amino acid N can be considered as a transitional pool for the transformation of the hydrolysable ammonium N and amino sugar N, and the acid insoluble N is tightly associated with fertilizer N stabilization. Importantly, there is an interim shift among the three substantial N pools to maintain soil N cycling and supply in a soil-plant system.