| Acid deposition has been a serious worldwide environmental problem that affects human well-being. Southern China has become the third largest region receiving acid deposition after Europe and North America, and acid deposition in Zhejiang province located at Yangtze River delta is even more serious. Acid deposition had negative effects on forest health, which caused crown defoliation, soil acidification, and the decline of the forest. The responses of forest to acid deposition varies due to different ecological system, soil condition, acidity, acid depositon type and exposure time, etc. Therefore, to understand the characteristics of acid deposition and its impact on the eco-physiology and growth of trees in subtropical region is very important.In this study, the characteristics of atmospheric wet deposition were monitored from March 2010 to February 2011 in Tiantong National Forest Ecosystem Observation and Research Station, Zhejiang province. We studied the dynamic change of acidity and ionic composition of precipitation. In addition,1-year-old seedlings of Schima superba and Pinus elliottii, were treated with two types of acid deposition SO42-:NO3-ratios (8:1 and 0.7:1) with two applications (foliar spraying and soil drenching) at two pH levels (pH 3.5 and pH 2.5) over a period of 18 months. Visible foliar symptom, growth and eco-physiological properties of seedlings, and soil chemical characteristics were analyzed. The objective was to determine the ecological responses of subtropical tree species to acid stress, to find out the key factors of deforestation, and to provide theoretical basis for the restoration of the forest ecosystem in acid deposition region. The main results are as follows:1) The annual average pH value of precipitation in Tiantong was 4.37, and acid rain frequency was 93.6%, which indicated that Tiantong was heavy acid deposition region. The annual Volume-Weighted Mean (VWM) anions and cations concentration were 140.7μeq·L-1 and 158.9μeq·L-1. Winter and spring had high concentrations of chemical components, while antumn and summer had relative low values. SO42- and NO3- were two dominant anions in rainwater, and N4+ and Ca2+were two dominant cations. The ratio of SO42- to NO3- had gradually decreased from 4.32-6.42 to 1.9 over the last 15 years, which indicated that the type of acid deposition converted from sulfuric acid deposition to sulfuric and nitric mixture acid deposition. Sea salt had effects on ion concentration of rainwater, but little effect on acidity.2) High-intensity acid deposition (pH 2.5) significantly increased total N and exchangeable acid in soil, and declined exchangeable base cations, especially Ca2+, and soil pH value. However, moderate acid deposition (pH 3.5) did not change soil pH value significantly. It suggested that soil had a certain buffering effect on acid deposition, and further acidification only occured when the H+ addition exceeded the threshold value. In the short term, the effects of acidification was greater in sulfuric acid deposition (SAD) than that of nitric acid deposition (NAD). In the long term, soil acidification will gradually increased with the accumulation of acid deposition. Acid deposition will lead to nutrient imbalance because of losses of soil exchangeable Ca2+ and exchangeable Mg+, and increase of Al3+.3) Responses of growth and eco-physiological characteristics of S. superba and P. elliottii seedlings varied of different acidity. The damage of leave in S. superba increased at high acidity. In the first year, acid deposition at pH 2.5 inhibited photosynthesis of S. superba, and the photosyntheric rate was reduced by the stomatal factor and non stomatal factor, such as decrease of chlorophyll. In the second year, photosyntheric rate of S. superba was slightly increased, probably due to a fertilization effect that improved foliar nitrogen and chlorophyll concentration. However, acid deposition had no significant effects on biomass accumulation and allocation after two growing seasons. Biomass accumulation of P. elliottii was notably stimulated and aboveground biomass allocation was increased by acid deposition, while photosynthetic rate was not change significantly. Moreover, rapid growth of P. elliottii reduced the accumulation of nitrogen in soil so as to alleviate soil acidification. Therefore, P. elliottii is better adapted to the acid deposition environment, and it is an important tree species for afforestation in acid deposition region.4) Responses of growth and eco-physiological properties of seedlings varied according to SAD and NAD. Compared with SAD, NAD at moderate acidity had a stronger negative effect on photosynthesis and biomass accumulation of S. superba. For high-intensity acid deposition, NAD potentially alleviated the negative effects of acidity on physiological properties and growth, due to a fertilization effect. However, high foliar nitrogen resulting from excessive nitrogen might cause the imbalance of nutrients of seedlings, which would offset the increase of photosynthetic rate due to the high concentration of nitrogen. Thus, the different responses of growth of seedlings to different acid deposition type depending on detrimental effects of H+ and nutrient fertilization effect.5) Due to the different spraying methods, tree responses to direct effects of acid deposition on leaves and indirect effects of soil acidification were different, and not synchronized. In the first year, saturated photosynthetic rate of S. superba in foliar spraying treatments was significantly lower than that of soil treatments, and relative growth rates were decreased, indicating that S. superba damaged mainly by direct acid stress in the short term. Addition, leaf nitrogen and chlorophyll content in soil drenching treatments were significantly higher than that of foliar spraying treatments, and acid deposition under soil application promoted the growth of P. elliottii. However, the release of active Al3+ in soil by acid deposition increased the Al concentration in fine root and leaves of seedings. The results of path analysis also showed that exchangeable Al3+ and exchangeable H+ in soil were the main restrict factors to tree growth. Thus, the negative effect of soil acidification on tree growth will gradually obviously.In summary, precipitation in Tiantong was severe acidification, and gradually converted from sulfuric acid deposition to sulfuric and nitric mixture acid deposition. The responses of forest trees to acid deposition varies obviously due to species, acid deposition intensity, type and spraying method. The negative effects of acid deposition on sensitive species increased with acidity. S. superba damaged mainly by direct acid stress in the short term, however, continued soil acidification and loss of nutrient may be the main factor of growth restriction under the current acidity of precipitation. The effects of different type of acid deposition on plants were the mixed results of the interaction between acidity effect and fertilizer effect. With continuous acid deposition in the future, particularly the type of acid deposition converts from SAD to NAD, effects of acid deposition to subtropical evergreen broad-leaved forest is worthy further study in the long term. |
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