| Atmospheric bulk deposition of nitrogen is the important way to obtain nitrogen in forest ecosystems, but the excess nitrogen deposition can lead to forest soil acidification, tree nutrient imbalance, the changes in species diversity. Urban forest has important regulatory effect on light, heat, gas, water, soil and other ecological factors, so it plays an irreplaceable role on the city's ecological and environmental protection. Effects of nitrogen content in atmospheric deposition on nitrogen content in urban forest ecosystem has been discussed by locating observed nitrogen content of atmospheric precipitation, leaves of chief species and soil in ChangSha Camphor forest ecosystem, meanwhile, the functions and environmental regulation mechanism in urban forest ecosystem which has been revealed. The results showed:NH4+-N content in precipitation has an obvious characteristic of monthly dynamics, a highest NH4+-N content appears in March at 6.7mg.L-1, By contrast, it reaches the lowest content at 2.7mg.L-1 in August, the monthly changes of NO3--N in precipitation is more stable, content in March, August, October,November are all above 1.2 mg.L-1. The highest content appears in November which is up to 1.9mg.L-1, while the lowest content is 0.4mg.L-1 in July. NH4+-N content in precipitation and rainfall were positively related(R2=0.3569), But NO3--N content doesn't got a obvious correlation with rainfall (R2=0.1603)Annual average precipitation pH value is lower than 5.6. Acid rain rate reaches the lowest in August at 38%,By contrast, it reaches the highest rate in November at 81%. Acid rain pollution is more severely in winter and spring than in summer and autumn.The monthly dynamic trend of average leaf Total Nitrogen content in Cinnamomun camphora, Helicia cochinchinens and Manglietia fordiana present an obvious monthly dynamic change. The average leaf total nitrogen content of which appears highest content in April at 17.48g.kg-1, and it reaches its lowest content 10.78g.kg-1 in October.The Available Nitrogen content and total Nitrogen in soil remain essentially the same in soil layer (0-15cm). The Total Nitrogen content and Available Nitrogen content in soil layer (0-15cm) reaches their higher level in March, August, September and October reaches the highest level. In June the Total Nitrogen content presents the lowest content, while Available Nitrogen appears in April. The Available Nitrogen content and total Nitrogen in soil remain essentially the same in soil layer (15-30cm).There are Certain correlation between Rhizosphere soil Nitrogen content and non-rhizosphere soil Nitrogen content, and Correlation coefficient of total N content is 0.7078,meanwhile, Correlation coefficient of Available Nitrogen content is 0.3401. It shows that the rhizosphere soil content connectes with non-rhizosphere soil Nitrogen content, and the total Nigtrogen content significantly influenced each other.Total N content of litter, it is the average monthly at 9.37 g.kg-1 in spring, it significantly increased and reached the highest among the year on average for 9.63 g.kg-1 in summer, autumn the average content is 9.34 g.kg-1, the lowest average level for the whole year, only 8.71 g.kg-1 in winter.The NH4+-N content and NO3--N content in precipitation are correlative with the Total Nitrogen content in plants, the Total Nitrogen content and Available Nitrogen content in soil layer in some degree.The related coefficients of the NH4-N content and the NO3-N content in precipitation with the Total Nitrogen content in plants are 0.4143 and 0.5313 respectively.Plant total N average uptake rate reaches the highest in summer at 20.81, and the lowest appears in winter which is only 8.38. Biological return rate is 10.51 in spring, there is a rising trend in summer, and it reaches the peak for whole years at 11.57, it decreases gradually in autumn and winter, winter return rate is the lowest for whole year, which is only 7.01. |
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