Distribution and Mineralization of Organic Nitrogen in Fluvo-Aquic Soils with Varying Textures

The purpose of this study was to determine the characteristics and influencing factors of organic N mineralization in fluvo-aquic soils with varying textures. Based on a 27-year long-term experiment, a 48-week aerobic incubation experiment was carried out to ascertain the features of organic N mineralization in fluvo-aquic soils with varying textures. SON fractions and soil particle-size fraction compositions were analyzed to determine the distribution of SON and the factors impacting soil organic N mineralization. The results showed that the concentrations of the SON fractions increased to different extents with increasing sand content in fluvo-aquic soils with varying textures. Conversely, the proportions of hydrolyzable ammonium N (HAN), amino acid N (AAN), and hydrolyzable unidentifiable N (UHN), which account for the total SON decreased. However, the proportion of acid-insoluble N (NHN) to overall SON increased from 26.2% in sandy soil (SS) to 49.3% in loamy clay soil (LC). Most of the organic N was in the clay-sized fractions, whereas the proportion of SON in the sand-sized fractions decreased as the sand content decreased. The fluvo-aquic soils with varying textures presented a decrease in the total amount of mineralizable N (N₀ + Pool I) in the following order: loamy clay (LC, 134.5 mg·kg^− 1) > sandy clay loam (SCL, 106.4 mg·kg^− 1) > sandy soil (SS, 88 mg·kg^− 1). In contrast, the mineralization rate constant (k) and the labile mineralizable N pool (Pool I) displayed the opposite trends. The changes in organic N in the clay-sized fractions were positively linked to the soil mineralizable N pools (except for Pool I). In terms of the composition of organic N, HAN, and AAN in fluvo-aquic soils with varying textures were the main contributors to mineralizable N. In conclusion, the potential mineralization N (N₀) and N₀ + Pool I of the SS were the lowest because the SS had lower SON fractions than the other soils did, and the contribution of the clay-sized fraction to the mineralized organic N in the SS was also smaller. In contrast, the percentages of hydrolyzable organic N fractions, especially HAN and AAN, to SON were greater in SS, resulting in a greater value of k and Pool I. Thus, the organic N in fluvo-aquic soils with high sand content was characterized by a small reservoir with high turnover rates and poor sustained N supply capacity.