Effects Of Simulated Acid Rain With Lower S/N Ratio On Five Dominant Species In Subtropical Forests

The acid rain pollution is worsing increasingly with the development of industry in recent half century, and the impact of acid rain on plant especially forest vegetation have also become more severe. Based on field investigations and experiment results with simulated acid rain, the potential effects of acid rain with higher S/N ratio (molar ration>6) on plants have been shown intensively in China. However, few reports have focus on the effect of acid rain with lower S/N ratio on plants, which has important practical significance with the decreasing of S/N ratio in future.The purpose of this study was to investigate the possible effects of simulated acid rain with lower S/N ratio on five dominant species in subtropical forests. One-year-old seedlings of Cinnamomum camphora, Schima superb, Liquidambar formosana, Pinus elliottii and Metasequoia glyptostroboides were transplanted into pots with the local soil (yellow-red soil) at Tiantong National Station of Forest Ecosystem, Zhejiang Province. The molar ratio of sulfate to nitrate of the simulated acid rain used in our study was 1.58:1, similar to the ratio of NO3- and SO42- in ambient rainfall in Zhejiang. Seedlings were treated with simulated acid rain of pH 2.5 or 3.5, respectively, every other day from April 15 to August 24 in 2009, and local reservoir water of pH 5.0-5.4 as the control. The growth parameters of stem, gas exchange parameters, Photosynthetic pigment contents, membrane penetration, malondialdehyde(MDA) and nutrient elements content in leaves were investigated at the end of the experiment. The results showed that:1,The morphological and physiological changes of seedlings were caused by the simulated acid rain with lower S/N ratio(1) No visible foliar injury was observed, while the growth of stem was inhibited in some degree with the treatment of pH 3.5 acid rain for all five species. The pH2.5 threatment resulted in typical visible foliar injury symptoms, such as etiolation, shrinkages and brown dots, except P. elliottii. The stem growth of C. camphora, S. superb, L. formosana and P. elliottii increased largely.(2) Except M. glyptostroboides, no significant difference was observed in these parameters with the treatment of pH 3.5 acid rain for the species. The pH2.5 treatment altered gas exchange parameters, including net photosynthetic rate(Pn), stomata conductance (Gs), transpiration rate (Tr) and water use efficiency (WUE). Membrane penetration and MDA contents in the seedling leaves of seedlings were increased significantly, suggesting that the decrease of photosynthetic activity was probably caused by non-stomatal factors in combination with stomatal closure. Treated with nitrogen fertilizer, total chlorophyll content of the leaves were increased, and net photosynthetic rate on the basis of chlorophyll content declined significantly under pH 2.5 acid rain treatment. It can be concluded that acid rain treatments can induce partial closing of stomata, damage membrane system and decrease photosynthetic activity of plants(3) Treated with pH3.5 acid rain, the nitrogen (N) and phosphorus (P) content in leaves of five species were not affected significantly. The content of Potassium (K), sodium (Na), Calcium (Ca), Magnesium (Mg), Aluminum(Al), Manganese (Mn), Iron (Fe), Zinc(Zn) in leaves of seedlings did not change significantly at pH3.5, except the K content of L. formosana and Al,Na content of P. elliottii which increased significantly. The N content in leaves of seedlings grew under the pH2.5 treatment owing to the nitrogen fertilizer, except C. camphora. The Ca,Mg,Mn content of L. formosana, the Ca,Mn,Na content of P. elliottii and the Al,Mg,Na content of M. glyptostroboides rised obviously, whereas the Al content of C. camphora and K content of L. formosana reduced significantly.(4) Based on the results, we can conclude that lower S/N ratio increase the supply of nitrogen to plant and improve the N content in leaves, which may promote the stem growth compared with the treatment of higher S/N ratio(molar ration>6). Furthermore, lower S/N ratio may mitigate negative effects of acid rain on photosynthesis and membrane.2,The adaptability to acid rain were various with the speciesWith the parameters measured, it can be found that P. elliottii has the best capacity to acclimate to acid rain stress followed by C. camphora, L. formosana, S. superb and M. glyptostroboides.This study improves our understandings in effects of acid rain on plant growth under lower S/N ratio.