Addition of nitrogen to canopy versus understorey has different effects on leaf traits of understorey plants in a subtropical evergreen broad-leaved forest

Atmospheric nitrogen (N) deposition has substantial effects on forest ecosystems. The effects of N deposition on understorey plants have been simulated by spraying N on the forest floor. Such understorey 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 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 understorey species in a subtropical evergreen forest after 5 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 understorey N addition exerted significantly different effects on leaf traits of understorey plants. The results indicate that understorey plants in subtropical forest respond differently to UAN 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 CAN and UAN experiments over a longer term.

Automated summary

The study compared the effects of understorey nitrogen (UAN) and canopy nitrogen (CAN) addition on leaf traits of understorey plants in a subtropical evergreen forest. The results indicated significant differences in the effects of different addition methods on leaf traits, suggesting that understorey plants respond differently to UAN compared to atmospheric nitrogen deposition.