DATA.xlsx

Forest ecosystem response to nitrogen (N) deposition depend on N retention, distribution in ecosystem. 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 young secondary forest using a 15N tracer approach where 15N was added either as NO3-15N or NH4+-15N. We found that total ecosystem 15N recovery was significantly higher under UAN than CAN during early stages (7-120 days), although this difference diminished by 365 days. Canopy 15N addition enhanced long-term N retention in woody biomass, with stem components emerging as the dominant sink, accounting for up to 34.9% of recovered 15N after one year. In contrast, UAN favored short- term N retention in understory vegetation and long-term N retention in soil (0-40 cm). Soil profile patterns further revealed divergent N redistribution: 15N recovery increased with soil depth under CAN but decreased under UAN from 120 to 365 days. Plants took up more 15NO3- than 15NH4+, especially in long-lived woody plant tissues. By contrast, the soil retained similar amounts of 15NH4+ and 15NO3-. These findings demonstrate that N addition pathway significantly alters both the magnitude and spatial distribution of N retention in forest ecosystems, with canopy-level processes enhancing the stability of N in trees and deeper soil layers.