Canopy nitrogen addition enhance the photosynthetic rate of canopy species by improving leaf hydraulic conductivity in a subtropical forest

Multi-ecosystem services differently affected by over-canopy and understory nitrogen additions in a typical subtropical forest

Obtaining a holistic understanding of the impacts of atmospheric nitrogen deposition on multiple ecosystem services of forest is essential for developing comprehensive and sustainable strategies, particularly in heavy N deposition regions such as subtropical China. However, such impacts remain incompletely understood, with most previous studies focus on individual ecosystem function or service via understory N addition experiments. To address this knowledge gap, we quantified the effects of over-canopy and understory N additions on multiple ecosystem services based on a 7-year large-scale field experiment in a typical subtropical forest. Our results showed continued over-canopy N addition with 50 kg ha-1  year-1 over a period of 4-7 years significantly increased plant nutrient retention, but did not affect the services of soil nutrient accumulation, water yield, C sequestration (in plants and soil), or oxygen release. There were trade-offs between the soil and plant on providing the services of nutrient accumulation/retention and C sequestration under over-canopy N addition. However, without uptake and retention of tree canopy, the trade-off between soil and plant were more weaken under the understory N addition with 50 kg ha-1  year-1 , and their relationships were even synergetic under the understory N addition with 25 kg ha-1  year-1 . The results suggest that understory N addition cannot accurately simulate the effects of atmospheric N deposition on multiple services, along with mutual relationships. Interestingly, the services of plant N, P retention, and C sequestration exhibited a synergetic increase under the over-canopy N addition but a decrease under the understory N addition. Our results also found tree layer plays a primary role in providing plant nutrient retention service and is sensitive to atmospheric N deposition. Further studies are needed to investigate the generalized effects of forest canopy processes on alleviating the threaten of global change factors in different forest ecosystems.

Alternating processes of dry and wet nitrogen deposition have different effects on the function of canopy leaves: Implications for leaf photosynthesis

Canopy leaves are sinks of dry and wet nitrogen (N) deposition, most studies have not considered the response of canopy leaves to the alternating processes of dry and wet N deposition. We manipulated a close top chamber experiment to observe the effects of simulated N deposition with the same total deposition flux but different dry to wet ratios on leaf structure and physiology by spraying NH₄Cl solution or supplying gaseous NH₃ over the canopy of seedlings of three species (Betula platyphylla, Fraxinus mandshurica, Pinus koraiensis) placed in the chamber. After 32 days of N deposition and relative to the control, the leaf morphology and mesophyll tissue structure of the three species had no significant changes under all N deposition treatments. With the increase in the ratio of dry to wet N deposition, the N concentration, N metabolizing enzyme activity and soluble protein concentration in leaves of all three species increased continuously, but for the leaf light-saturated net photosynthesis rate, B. platyphylla showed a continuous increase, F. mandshurica showed a continuous decrease, and P. koraiensis showed no significant change. We found that F. mandshurica was the only species whose foliar chlorophyll and potassium concentration decreased with the increase in the ratio of dry to wet N deposition and its leaf light-saturated net photosynthesis rate was positively correlated with foliar chlorophyll and potassium concentration, respectively. Our results indicate that dry deposition is relatively more important on leaf physiological functions in alternating deposition. B. platyphylla and P. koraiensis may better acclimate to canopy NH₃/NH₄ ⁺ deposition than F. mandshurica. Most importantly, the results indicate that a single simulated dry and wet deposition would overestimate and underestimate the response of leaf function to atmospheric N deposition, respectively. Alternating processes of dry and wet deposition should be considered for more realistic assessments of the effects of atmospheric N deposition in forests.