Productivity and net greenhouse gas emissions of rubber-Ficus hirta Vahl. agroforestry vs. monoculture under nitrogen substitution

Rubber– Ficus hirta Vahl. agroforestry system presents a promising alternative to conventional rubber monoculture. However, whether transitioning rubber planting arrangement and substituting chemical nitrogen (N) fertilizer can enhance productivity while reducing net greenhouse gas emissions remains unclear. In this study, conventional single-row rubber monoculture (SR) was used as control, and three N management regimes were applied to the Ficus hirta Vahl. intercropped in a double-row rubber plantation (DR): no N fertilizer (DR-PK), full chemical N fertilizer (DR-NPK), and partial substitution of chemical N with farmyard manure (40 % substitution; DR-FYM). From May 2023 to May 2025, the total biomass increment across treatments did not differ significantly ( p < 0.05). Ficus hirta Vahl. yields were 3696, 5013, and 4378 kg ha⁻¹ in DR-PK, DR-NPK, and DR-FYM, respectively, translating to additional economic returns of 22,449, 41,851, and 25,601 CNY ha⁻¹ . Compared with SR, DR treatments slightly increased soil CO₂ emissions (70.71–78.37 t ha⁻¹) but reduced N₂O emissions (19.73–13.18 kg ha⁻¹), likely due to elevated soil temperature and lower ammonium-N in DR systems, respectively. Soil organic carbon (SOC) increased in all DR treatments, with DR-FYM showing the highest SOC gain (1.66 C ha⁻¹; p < 0.05). Furthermore, DR-FYM exhibited the highest net greenhouse gas sequestration (19.17 t CO₂-eq ha⁻¹) and the lowest carbon footprint (CF) (1.22 t CO₂-eq ha⁻¹). In conclusion, the rubber– Ficus hirta Vahl. agroforestry system enhances economic returns, while partially substituting chemical N fertilizer with farmyard manure significantly increases soil carbon storage and reduces the CF, thus promoting the sustainable development of rubber-based agroforestry systems.