Abstract: The transformation of soil nitrogen (N) in forests is closely linked to plant uptake and utilization and its environmental effects. The input of soluble carbohydrates released from litter decomposition is one of the factors affecting soil N immobilization. In this study, with soil and litter samples from a subtropical Cunninghamia lanceolata forest, we conducted an incubation experiment with four treatments, including control (CK, soil alone), adding fresh C. lanceolata litter extract (Le), glucose addition (G), and combined glucose and ammonium nitrate (Gn). The samples were incubated under controlled temperature and aerobic conditions for 60 days. We investigated the effects of different carbon sources on soil N immobilization by analyzing the changes in soil soluble carbon and nitrogen contents, greenhouse gas emission rates and their cumulative emissions. The results showed that: 1) At 30 minutes after incubation, the content of dissolved organic carbon (DOC) in all treatments decreased to different degrees (reduction less than 138.48 mg·kg^-1), with the strongest reduction being observed in the Le treatment. On the 7^th and 14^th day after incubation, the content of DOC in all carbon addition treatments significantly decreased (reduction more than 915.24 mg·kg^-1), which was higher in the G and Gn treatments than in the Le treatment. On the 60^th day after incubation, the reductions of DOC in all carbon addition treatments exceeded 1000 mg·kg^-1. Compared with CK, all carbon addition treatments significantly reduced the content of soluble nitrogen in various forms of the soil. On the 7^th day after incubation, all forms of soil soluble N showed the strongest decline. The Le treatment mainly reduced dissolved organic nitrogen (DON) (contributing 54.7% to total soluble nitrogen (TSN) reduction), while G and Gn treatments primarily reduced NH₄⁺-N (contributing 66.8%-73.9% to TSN reduction). The effect of carbon addition on soil inorganic N (NH₄⁺-N and NO₃^--N) immobilization persisted at least until the 14^th day after incubation. On the 60^th day after incubation, carbon addition still decreased soil NH₄⁺-N, but increased the NO₃^--N. 2) The CO₂ emission rates in all carbon addition treatments were significantly higher than CK during the first 3 days of incubation. CO₂ emissions stabilized after 5 days. The cumulative CO₂ emissions in both G and Gn treatments were significantly higher than those in the Le treatment. During the incubation period, CH₄ emissions were negative under all treatments, with low N₂O emissions. The N₂O emission rates and cumulative emissions in the Gn treatment were significantly higher than those of the other treatments. 3) There were significant positive correlations between net DOC changes and net TSN, NH₄⁺-N, and NO₃^--N changes during incubation. In conclusion, carbon addition promoted microbial activities, which consumed DOC for N immobilization, and soil NH₄⁺-N more effectively immobilized than NO₃^--N. With sufficient supply of labile carbon, the potential for NH₄⁺-N immobilization could persist for up to 60 days.

Key words: subtropical forest, nitrogen immobilization, nitrogen form, carbon availability, dissolved organic nitrogen