DATA.xlsx

As Earth's largest terrestrial carbon (C) sink, forests' C storage and response to nitrogen (N) deposition depend on N retention, distribution in the ecosystem, and canopy processing of atmospheric N through interception, transformation, and assimilation. However, most N deposition simulation experiments focus on understory applications, overlooking canopy interception and foliage uptake of atmospheric N. This study compares canopy (CAN) and understory N addition (UAN) methods in a secondary deciduous forest by using a ¹⁵N tracer (¹⁵NO₃^-and ¹⁵NH₄⁺) approach. 7 days after the ¹⁵N application, canopy foliage and branches under CAN retained 7.49% of ¹⁵N-1.4 times higher than UAN (5.64%). Conversely, the forest ground layer retained more ¹⁵N in UAN (46.03%) than CAN (10.08%). 365 days later, canopy trees became the primary ¹⁵N pool, with foliage and branches absorbing 7.87% (CAN) and 14.53% (UAN), and stems absorbing 31.63% and 26.94% of ¹⁵N under CAN and UAN, respectively. ¹⁵N recovery from trees accounted for 96.44% (CAN) and 74.1% (UAN) of total plant recovery, while the ground layer still retained more ¹⁵N under UAN (13.61%) than CAN (1.31%). From 7 days to 365 days, the fine roots retained more ¹⁵N under UAN than CAN. Soil ¹⁵N recovery declined steeply in both treatments: from 39.73% to 0.1% under CAN and 41.56% to 0.36% under UAN. The forest retained more ¹⁵NO₃^- than ¹⁵NH₄⁺, likely due to the higher ¹⁵NO₃^-mobility and preferential uptake bytrees. No aboveground carbon sequestration differences emerged between UAN and CAN. In conclusion, our study highlights the forest canopy's critical regulatory role in processing deposited N short-term and canopy trees' function as the primary N sink long-term, emphasizing the need to monitor long-term N deposition effects on forest ecosystems. The system remains capable of buffering increased N deposition without signs of saturation.