Underappreciated role of canopy nitrogen deposition for forest productivity.

Abstract

Atmospheric nitrogen (N) deposition is generally expected to stimulate plant carbon (C) sequestration and promote tree growth, thereby mitigating atmospheric CO₂ accumulation. Yet, the magnitude of N deposition contribution to forest productivity remains contentious. While correlative studies suggest substantial plant growth enhancement, controlled fertilization experiments typically demonstrate a limited impact. This discrepancy may arise from whether or not to consider canopy N uptake processes. Here, we conducted a 10-y field experiment comparing canopy addition of N (CAN) with understory addition of N (UAN) at the rate of 0, 25, and 50 kg N ha^-1 y^-1 in a temperate deciduous forest in central China. We show that CAN significantly enhanced net primary productivity by 37.0% over control, driven by enhanced leaf litterfall, wood and fine root production, whereas UAN effects were marginal (8%). ¹⁵N isotopic tracing revealed that CAN, through enhanced plant N uptake and increased ecosystem N retention, yielded a 3.5-fold higher C sequestration efficiency (∆C/∆N) of 54.5 ± 7.7 kg C kg^-1 N, than UAN (12.2 ± 3.4 kg C kg^-1 N) resulted from greater N loss through leaching. Physiological measurements indicated CAN enhanced leaf photosynthetic rates, modified leaf morphology, and extended leaf lifespan via delayed senescence. These findings provide robust empirical evidence that canopy N uptake is crucial for maximizing N-induced forest productivity, thereby holding significant implications for refining global C models and urging modelers to incorporate canopy processes for more accurate projections of future C sinks and climate change mitigation strategies.