Soil organic nitrogen(SON)consists of several N forms with complex chemical structure,which is the source of soil mineralizable N(SMN),it is therefore essential to elucidate the contributions of SON fractions to SMN in clearly understanding the SON availability.In this study,representative paddy soil samples were collected across the Dongting Lake region for identifying the intrinsically relations between SON fractions and SMN.The soil organic N composition and SMN were determined by Bremners acidhydrolysis method and waterlogged incubation procedure,respectively.The results showed that soil acidhydrolyzable N(AHN)varied in the range of 545.7~2 276.9 mg/kg that accounted for 58.6%~83.8% of total soil N,and thus serves as the main component of SON.It also showed the content of AHN among different paddy soils subtypes generally decreased in the order of Fluvisols,Gleysols and Cambisols.As to proportions of varying AHN fractions in total soil N ,the ratios for amino acid N(AAN),amino sugar N(ASN),ammonia N(AN)and unknown hydrolysable N(UHN)ranged from 25.6%~43.1%，2.6%~9.0%，11.9%~22.3% and 8.0%~25.3%,respectively.Some soil properties,i.e.,soil organic carbon,soil total N and clay content were all significantly(P < 0.01)correlated to SMN(R=0.569~0.753),indicating that organic matter content and texture are important soil indices for affecting the amount variation of SMN.Pearson correlation analysis showed that all organic N components in AHN were all well correlated with SMN(R=0.427~0.858,P < 0.05),however,multiple stepwise regression and path analyses showed that AAN is the only one organic N form that has important direct contribution to SMN and thus serves as the primary source of SMN.The values of decision coefficient in path analysis for AAN,ASN,AN and HUN were 0.685,0.251,0.028 and-0.050,respectively,which suggested that it is essentially beneficial to the increase of SMN supplying capacity with increasing the proportions of AAN,ASN and AN in SON.