Abstract
To understand the characteristics of soil organic nitrogen fractions under vegetation restoration in karst areas, soil samples from grassland (5-year-old), shrub-grassland (15-year-old), shrubland (20-year-old) and forestland (40-year-old) were taken as the research objects. The Bremner acid-hydrolysis method was used to determine the soil nitrogen fractions, and the correlations between them and soil physical and chemical properties were analyzed. The results showed that with the increase of soil depth, the mass fractions of soil total nitrogen (TN), non-acid-hydrolyzable nitrogen (AIN), acid-hydrolyzable nitrogen (TAN) and their various components all showed a downward trend. The mass fractions of various organic nitrogen components and their proportions in TN from large to small were as follows: AIN, acid-hydrolyzable unknown nitrogen (TUN), acid-hydrolyzable ammonia nitrogen (AMN), acid-hydrolyzable amino acid nitrogen (AAN), acid-hydrolyzable amino sugar nitrogen (ASN). TUN and AMN were likely the main sources of available nitrogen for plant uptake and utilization. The contents of both TAN and AIN increased significantly with the increase of TN content (P < 0.001). However, vegetation restoration increased the proportion of AIN and decreased the proportion of TAN. Among them, the 15-year-old shrubland had outstanding soil nitrogen-supplying capacity, while there was no significant difference in soil nitrogen-supplying potential among different restoration years. Soil organic carbon, total nitrogen and ammonium nitrogen were all extremely significantly (P < 0.001) positively correlated with soil organic nitrogen fractions. The changes in the characteristics of organic nitrogen fractions affected the soil nitrogen supply and storage capacity. The results of this study can provide a reference for nutrient management during the restoration of degraded soils.