Abstract: 【Objective】Nitrogen is an important limiting factor in sticky and thin medium-low-yield paddy soil in southern China. In order to provide the basis for reasonable fertilization and soil nitrogen pool management, the effects of long-term fertilizations on nitrogen accumulations and organic nitrogen components in soil aggregates in a yellow-mud paddy soil were investigated.【Method】Soil samples from the plough layer in the 36^th year under different fertilization treatments in the long-term experiment of yellow-mud paddy soil were collected to analyze the nitrogen accumulation and the contents of organic nitrogen components as well as their distributions in aggregates by wet screening and Bremner organic nitrogen classification method. The treatments included: no fertilization (CK), application of chemical fertilizer (NPK), chemical fertilizer plus cow dung (NPKM) and chemical fertilizer plus total rice straw returning (NPKS). 【Result】 The contents of total nitrogen in >2 mm aggregate were significantly increased by 12.7%-51.9% in fertilization treatments compared with that in CK (P<0.05). The cumulative contribution ratios of TN in >2 mm aggregate to total nitrogen in bulk soil in NPKM and NPKS treatments were 24.7 and 20.0 percentage points significantly higher than that in CK (P<0.05), respectively. The contents of acid-hydrolyzable nitrogen (AHN) and non-hydrolyzable nitrogen (NHN) in >2 mm aggregate in fertilization treatments were increased by 10.1%-36.3% and 20.7%-100.5% compared with those in CK, respectively, and the cumulative contributions of the two components to total nitrogen in bulk soil were increased as well, especially for NPKM and NPKS treatments.  In >2 mm aggregate, the contents of acid-hydrolyzable ammonia nitrogen (AMMN) in fertilization treatments were significantly increased by 17.2%-40.4% compared with that in CK (P<0.05), and the largest increasement was found in NPKM treatment. The contents of acid-hydrolyzable amino acid nitrogen (AAN) and acid-hydrolyzable unknown nitrogen (HUN) were increased most significantly in NPKS and NPKM treatments, which were 24.0% and 52.1% higher than those in CK (P<0.05), respectively. The contents of NHN and AMMN in >2mm and 0.25-2 mm aggregates were all significantly positively correlated with the contents of alkaline nitrogen in corresponding aggregates (P<0.05).  Redundant analysis (RDA) showed that the nitrogen uptake of rice plant was mainly affected by NHN components in >2 mm and 0.25-2 mm aggregates. The nitrogen uptake of rice plant was more affected by soil organic nitrogen components under NPKM and NPKS treatments than that under NPK and CK treatment.【Conclusion】The contents of total nitrogen in >2 mm aggregate and their cumulative contributions to total nitrogen in bulk soil were increased in plough layer of yellow-mud paddy soil under long-term fertilizations, especially for the combined application of organic and inorganic fertilizers. Compared with combined application of cow dung, combined application of  rice straw returning was more conducive to the accumulation of NHN in >2 mm aggregate. In >2 mm aggregate, combined application of cow dung had the biggest contribution to the increase of AMMN and HUN contents, and their cumulative contribution ratios to total nitrogen in bulk soil, while combined application of rice straw returning had the largest contribution to the increase of AAN content and their cumulative contribution ratio to total nitrogen in bulk soil. The contents of NHN, AHN and AMMN in >2 mm aggregate were closely related to alkaline nitrogen and the nitrogen uptake of rice plant in yellow-mud paddy soil, which were important available nitrogen pool. 

Key words: organic nitrogen component, aggregate, yellow-mud paddy soil, long-term fertilization