Effects of Long-Term Organic Substitution on Soil Nitrous Oxide Emissions in a Tea (Camellia sinensis L.) Plantation in China

Nitrous oxide (N₂O) is a major greenhouse gas (GHG) responsible for global warming. Improper fertilization in agricultural fields, particularly excessive nitrogen (N) application, accelerates soil N₂O emissions. Though partial substitution with organic fertilizer has been implemented to mitigate these emissions, the effect on perennial systems, such as tea plantations, remains largely unexplored. Therefore, the present study monitored soil N₂O emissions for a year in a tea plantation in South China under the following treatments: no N fertilizer (control, CK), chemical fertilizer alone (CF), replacing 40% of chemical fertilizer with organic fertilizer (CF + OF), and organic fertilizer alone (OF). Our results showed that the annual cumulative N₂O emissions from the plantation soil ranged from 1.03 to 3.43 kg N₂O-N ha⁻¹. The cumulative N₂O emissions, the yield-scaled N₂O emissions (YSNE), and the N₂O-N emission factor (EF) from the soil were the highest under the CF + OF treatment but the lowest under the OF treatment. Further analysis revealed that fertilization, mainly chemical fertilization, increased the soil ammonium (NH₄⁺-N) and nitrate (NO₃⁻-N) levels by 182–387% and 195–258%, respectively, and tea yields by 120–170%. However, tea yield decreased gradually with increasing organic substitution. These results prove that complete organic substitution reduces soil N₂O emissions and tea yield and suggest adopting an appropriate substitution rate for optimal effect. Further random forest (RF) modeling identified water-filled pore space (WFPS; 20.27% of total variation), soil temperature (Tsoil; 19.29%), and NH₄⁺-N (18.27%) as the key factors significantly contributing to the changes in soil N₂O flux. These findings provide a theoretical foundation for optimizing fertilization regimes for sustainable tea production and soil N₂O mitigation.