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Dissolved organic nitrogen (DON) is a significant nitrogen (N) pool in most soils and isconsidered to be important for N cycling. Increasing evidence suggests that not only aminoacids but also small peptides are potential direct nutrient sources for both soil microorganismsand plants. However, current understanding of turnover and underlying mechanismsof those low molecular weight DON (LMW DON) compounds in soils remainsincomplete. In this study, we aimed to investigate mineralization of LMW DONcompounds in a paddy soil from different depths under different N fertilisationpractices; and to test the linkages between soil characteristics and LMW DONturnover. To accomplish this, we collected soils from different depths (i.e., 0-10, 1020, 20-30 and 30-40 cm) in a paddy field under long-term different fertilisationpractices (i.e., control without fertiliser, chemical fertiliser and organic fertiliseralone and their combination) in southern China. We measured soil physiochemicalproperties and abundance and structure of soil microbial communities after long-termfertilisation management. In 14C mineralization assay (7 days at 22 oC), we used theamino acid L-alanine and its peptide tri-L-alanine as model L-enantiomer substratesto investigate the rates of LMW DON mineralisation in paddy soils. Overall, adouble first-order kinetic model conformed very well to the experimental data ofamino acid and peptide mineralization. For both form of LMW DON, the poolsizes of both catabolic and anabolic processes were significantly affected by Nfertiliser, soil depth, and their interaction (P <0.0001), thus contributing to pronounceddifferences in microbial C use efficiency among treatments. However, microbial uptakerates of peptide but not amino acid were significantly affected by N fertiliser, soildepth, and their interaction (all P <0.05). Results of canonical correspondenceanalysis suggest that microbial C use efficiency of both LMW DON was positivelycorrelated with soil pH, while microbial uptake rates were predominately associatedwith other soil biochemistry properties (e.g. soil and microbial C and N indices). [ABSTRACT FROM AUTHOR]