How does forest fine root litter affect the agricultural soil NH₃ and N₂O losses?

In farmland shelterbelt systems, the decomposition and/or apoptosis of forest fine root litter could affect farmland soil properties at the tree-crop interface, particularly the soil nitrogen (N) cycling. However, how fine root litter affect the ammonia (NH₃) and nitrous oxide (N₂O) losses from farmland soil and the crop production is little known. A soil column experiment covering a whole rice season was conducted to evaluate the dynamics aforesaid in response to fine root litter of Populus (RP) and Metasequoia glyptostroboides (RM) with 0 and 240 kg ha^-1 N fertilizer input. Both RP and RM had minimal impact on NH₃ and N₂O emissions from soils without N input. At 240 kg N ha^-1 input, RP significantly (p < 0.05) increased total NH₃ volatilization (including yield-scaled NH₃ volatilization and emission factor) by 37.1%, while RM significantly (p < 0.05) decreased it by 18.1%. Both fine root litter significantly (p < 0.05) reduced the N₂O emissions from paddy soil receiving 240 kg N ha^-1 by 22.7-27.1%. The reduction of N₂O emission in N240 + RM was primarily attributed to higher topsoil ammonium-N but lower nitrate-N contents that indicating a reduced nitrification rate during the mid-season drainage stage. In addition, the decreases in soil AOA amoA (-39.4%) and nirS (-23.7%) gene copies explained the mitigating effect of RP on N₂O emission. Regardless of N fertilizer application or not, there was no statistically significant difference in rice grain yield between treatments with and without fine root litter, although RM reduced grain yield by 11.2-14.9% compared to treatments without fine root litter. In conclusion, the impact of fine root litter on N emissions via NH₃ and N₂O depends on both N input rates and fine root types. RM simultaneously reduce reactive farmland soil N losses via NH₃ and N₂O in the tree-crop interface soils with N input.