Effects Of Nitrogen Deposition On Ecophysiological Characteristics And Interspecific Relationship Of Quercus Acutissima And Robinia Pseudoacacia

Over the last century, human activities, such as fossil fuel combustion and agricultural fertilizer use, have caused a great increase in nitrogen emissions and elevated nitrogen depositions toward land and oceans. Increased nitrogen deposition will influence nitrogen cycle, plant growth, species interaction and species composition of the terrestrial ecosystems. Competition and facilitation exist in plant communities, and they might be affected by many biotic and abiotic factors. Nitrogen deposition can influence the balance between competition and facilitation. Therefore, the ecological effects of nitrogen deposition were concerned by ecologists. Because China is one of the three areas with highest nitrogen deposition rate, investigation about the effects of nitrogen deposition on Chinese warm temperate regions will have important implications. The effects of simulated nitrogen deposition on growth and physiological characters of two typical trees in warm temperate forests (Quercus acutissima and Robinia pseudoacacia) were studied, as well as interspecific relationship between the two species. The effects of canopy species Q. acutissima and R. pseudoacacia on understory plants in natural forests were also studied. The research aims to understand various responses of different parts in forests under the background of increased nitrogen deposition. And it may be important for understanding and predicting the effects of nitrogen deposition on temperate vegetation and provide scientific evidences for vegetation restoration and forest management.According to the observation of nitrogen deposition and the results of model prediction, four nitrogen addition levels (0,2,5, and10g N m-2·a-1) were conducted in a greenhouse experiment. Seedlings of Q. acutissima and R. pseudoacacia were grown in monoculture or mixed species cultivations at each nitrogen addition level. Ammonium nitrate (NH4NO3) solution was added to soil every15days during the period of the experiment, simulating wet nitrogen deposition falling onto seedlings understory. Growth, morphology, biomass and physiological characteristics were measured in seedlings of Q. acutissima and R. pseudoacacia. The effects of nitrogen deposition on ecophysiological characteristics and interspecific relationship between Q. acutissima and R. pseudoacacia were analyzed and discussed. Three types of forests (Q. acutissima pure stands, R. pseudoacacia pure stands and mixed stands) were selected to survey species composition and environmental factors. And how canopy species influenced understory plants by regulating environmental factors was also discussed.Results of simulated experiment indicated that the effects of nitrogen deposition on growth and physiology of seedlings of Q. acutissima and R. pseudoacacia were species-specific. Increased nitrogen deposition had no significant effects on biomass accumulation of both studied species; however, biomass allocations were significantly changed. As nitrogen deposition increased, biomass allocation to roots was reduced while biomass allocation to leaves was increased in Q. acutissima seedlings. Leaf is the main organ that acquires light, and leaves of Q. acutissima seedlings became larger as nitrogen deposition increased, which was in favor of absorbing more light. Meanwhile, photosynthesis was enhanced as nitrogen deposition increased, and actual photochemical efficiency of PSII in the light and relative electron transport rate were both significantly increased at the highest nitrogen addition level. Growth of Q. acutissima seedlings became slow when they were in the late period of growing season, however, nitrogen deposition promoted height increment of Q. acutissima seedlings during this period. Therefore, nitrogen deposition weakened the decline of Q. acutissima seedlings and extended its growth period, which also promoted the plant growth. Although nitrogen deposition did not significantly influence biomass accumulation of Q. acutissima seedlings during the three months in the experiment, it benefited the growth of Q. acutissima seedlings at the points of morphology and physiology.The effects of nitrogen deposition on R. pseudoacacia seedlings were different from that of Q. acutissima seedlings. Biomass allocation to roots for R. pseudoacacia decreased as nitrogen deposition increased, while investment to leaves had no significant change. Nitrogen deposition had adverse effects on photosynthesis of R. pseudoacacia seedlings. Photosynthetic rate and photochemical reactivity of PSII were both decreased as nitrogen deposition increased. The temperate forests are generally nitrogen-limited, and plant growth was usually limited by available nitrogen. Thus, plant growth could be promoted by increasing soil nitrogen content. Nitrogen deposition alleviated the limitation for Q. acutissima growth and enhanced its growth and photosynthetic capacity. However, R. pseudoacacia could utilize nitrogen effectively at low nitrogen level due to its capacity to nitrogen-fixation. Thereby, R. pseudoacacia might reach the status of nitrogen saturation, and increased nitrogen may have adverse effects on the growth of R. pseudoacacia. In this study, nitrogen deposition had negative effects on the physiology of R. pseudoacacia seedlings, but had no significant effects on biomass accumulation.With rapid growth, R. pseudoacacia seedlings had absolute dominance over Q. acutissima seedlings, and the plant size of R. pseudoacacia seedlings was larger than Q. acutissima seedlings. Q. acutissima seedlings were to some extent shaded by R. pseudoacacia seedlings when they were grown in mixture. Therefore, light received by Q. acutissima was much less and not enough, causing photosynthetic rate of Q. acutissima decreased. This was significant when nitrogen deposition was high. Q. acutissima seedlings regulated their morphology and physiology to acclimate themselves to less light environment by increasing specific leaf area and chlorophyll content and decreasing ratio of chlorophyll a and b. All these ways could help plants acquire more light to compensate lack of light and adapt to the shaded environments. Although instantaneous net photosynthetic rate of Q. acutissima was lower in mixture than in monoculture, biomass accumulation of Q. acutissima was not decreased by R. pseudoacacia. R. pseudoacacia provided Q. acutissima with nitrogen when they grew in mixture, and leaf nitrogen content of Q. acutissima was higher in mixture than in monoculture, especially at the two high nitrogen addition levels. In addition, photochemical reactivity of PSII for Q. acutissima was not different in mixture and monoculture, which indicated that the intrinsic photosynthetical system of Q. acutissima was not damaged by the shade from R. pseudoacacia. On the other hand, for R. pseudoacacia, increased nitrogen deposition had adverse effects on the photosynthesis; however, the adverse effects weakened when R. pseudoacacia were grown with Q. acutissima together, and actual photochemical efficiency of PSII in the light and relative electron transport rate of R. pseudoacacia were not decreased when nitrogen deposition increased. The existence of Q. acutissima relieved the adverse effects of increased nitrogen on R. pseudoacacia. Moreover, biomass of R. pseudoacacia in mixture was more than that in monoculture. There was a positive interaction (facilitation) between Q. acutissima seedlings and R. pseudoacacia seedlings according to the biomass characteristics, and the positive interaction was not changed at different nitrogen deposition levels. According to the physiological characteristics, under the background of nitrogen deposition, Q. acutissima seedlings had positive effects on photosynthesis of R. pseudoacacia seedlings. However, R. pseudoacacia seedlings had negative effects on photosynthesis of Q. acutissima seedlings.Results of field study indicated that biodiversity of shrub layer was not significantly different among three types of forests, but biomass and biodiversity of herb layer was significantly different. Highest biomass and lowest biodiversity of herb layer were found in R. pseudoacacia stands, lowest biomass and highest biodiversity in Q. acutissima stands, and middle in the mixed stands. Soil organic content and nitrogen content, which were positively correlated with herb biomass, were both highest in R. pseudoacacia stands. And biomass and litter mass were negatively correlated with biodiversity. R. pseudoacacia could enhance the nitrogen availability in soil by multiple ways, such as nitrogen-fixing, accelerating nitrogen cycle and increasing leaf litter with more leaf nitrogen content, et al. Increased available nitrogen content could improve plant growth and enhance community production. Therefore, in R. pseudoacacia stands, R. pseudoacacia increased soil nitrogen content resulting in high herb biomass. High biomass could promote plant competition with light, and high litter mass could inhibit seeds germination and seedlings survival. These may cause species loss and decrease biodiversity. Contrary to R. pseudoacacia stands, soil nitrogen content was low in Q. acutissima stands, and low herb layer biomass and high biodiversity were in accordance. Results of the field study, a supplement to the simulated experiment, will be foundation and preparation for the further study about the effects of nitrogen deposition on temperate forests.In conclusion, nitrogen deposition had different effects on Q. acutissima and R. pseudoacacia seedlings. Increased nitrogen deposition benefited growth of Q. acutissima seedlings, but had adverse effects on physiology of R. pseudoacacia seedlings. Facilitation occurred between the seedlings of Q. acutissima and R. pseudoacacia in biomass. The coexistence of Q. acutissima and R. pseudoacacia could alleviate the adverse effects of nitrogen deposition on R. pseudoacacia seedlings, which might help R. pseudoacacia to adapt to high nitrogen environments. However, R. pseudoacacia had adverse effects on physiology of Q. acutissima. Because nitrogen deposition has differential effects on different plant species and influence soil nitrogen cycle and environmental factors, the effects of atmospheric nitrogen deposition on different types of forests and different species in forest communities need further investigations.