Atmospheric nitrogen (N) deposition has substantial effects on forest ecosystems. The effects of N deposition on understory plants have been simulated by spraying N on the forest floor. Such understory addition of N (UAN) might simulate atmospheric N deposition in a biased manner, because it bypasses the canopy.

We compared the effects of UAN and canopy addition of N (CAN) at 0, 25, and 50 kg N ha^–1 year^–1 on specific leaf area (SLA), leaf construction costs (CC), concentrations of leaf carbon ([C]), nitrogen ([N]), phosphorus ([P]), minerals ([Mineral]), nitrate ([NO₃^-]), lignin ([Lignin]), lipids ([Lipid]), organic acids ([OA]), soluble phenolics ([SP]), total non-structural carbohydrates ([TNC]), and total structural carbohydrates ([TSC]) in six dominant understory species in a subtropical evergreen forest after five years of N treatments.

We found that leaf CC, [C], [Lignin], [OA], [TNC] and [TSC] were significantly affected by N-addition approach and rate, but leaf [P] and [Lipid] were affected by N-addition approach and N-addition rate, respectively; leaf CC, [C], [P], [OA], and [TNC] were significantly lower under UAN than under CAN, but leaf [TSC] and [Lignin] were significantly higher and lower, respectively, under UAN than under CAN at 50 kg N ha^–1 year^–1; the decline of leaf [C] and [Lignin] contributed to the significantly lower leaf CC under UAN than under CAN.

Synthesis. We show that canopy and understory N addition exerted significantly different effects on leaf traits of understory plants. The results indicate that understory plants in subtropical forest respond differently to understory addition of N from those to atmospheric deposition of N. Further studies are warranted to evaluate the unbiased ecological processes and functions of forest ecosystem responding to atmospheric N deposition via both canopy and understory N addition experiments over a longer term.