Vertical Distribution of Soluble Organic Nitrogen Composition in Paddy Soils: Effects of Chinese Milk Vetch Application Rates

Soluble organic nitrogen (SON) plays a critical role in soil nitrogen cycling, yet the effects of Chinese milk vetch (CMV) application on the composition of SON within paddy soil profiles remains poorly understood. This study aimed to investigate the effects of varying CMV application rates on the composition and vertical distribution of SON in paddy soils, evaluating its potential implications for soil fertility and nitrogen leaching. A three-year field experiment was conducted in a subtropical rice cultivation system, employing four CMV application rates (0, 15,000, 30,000, and 45,000 kg ha−1). Soil samples were collected from three depth layers (0–20 cm, 20–40 cm, and 40–60 cm) at the rice maturity stage, and SON components, including free amino acid nitrogen (FAA-N), amide nitrogen (A-N), and soluble protein nitrogen (SP-N), were analyzed. The results demonstrated that CMV application significantly enhanced SON content, particularly in the topsoil (0–20 cm), with a 49.87% increase under the 45,000 kg ha−1 treatment. SON constituted over 50% of the total soluble nitrogen in the 0–60 cm layer, with FAA-N, A-N, and SP-N identified as the predominant components. CMV facilitated the accumulation of small-molecular SON (e.g., FAA-N and A-N) in deeper soil layers, with FAA-N content increasing by 79.13% in the 20–40 cm layer and A-N content increasing by 64.85% in the 40–60 cm layer under the 45,000 kg ha−1 treatment. In contrast, high-molecular-weight SON (e.g., SP-N) primarily accumulated in the topsoil due to stronger adsorption, while small-molecular-weight SON exhibited greater mobility, thereby elevating the risk of nitrogen leaching. These findings highlight the need for optimized CMV application rates to balance soil fertility and environmental sustainability in rice cropping systems.