| With the development of global industrialization process,the atmospheric nitrogen compounds emissions from the anthropogenic activities such as the fossil fuel combustion,the production and use of nitrogen fertilizer and livestock husbandry have significantly increased.The transfer and deposition of the atmospheric active nitrogen have been the important process of N distribution in regional and global scales.A few amount of nitrogen deposition will improve plant availability of nitrogen level and enhance productivity of the terrestrial ecosystem with nitrogen shortage in the short term.However,the N saturation would occur with the long-term and sustained N input in the ecological system.The negative effect of N saturation could seriously disturb and destruct the material flowing and circulation of terrestrial ecosystem.The Yangtze River Delta(YRD),which is one of the rapid developments of economy in China,has become the representative region of atmospheric nitrogen deposition increased significantly caused by anthropogenic activities.It will provide fundamental insights for studies of regional carbon cycle to explore the spatial and temporal patterns of nitrogen deposition in YRD,and to assess the impacts of enhanced nitrogen input on subtropical terrestrial ecosystem carbon cycling.This study estimated and analyzed the spatial and temporal of the different forms inorganic nitrogen dry and wet deposition fluxes using the satellite date of tropospheric NO2 column density,the monitoring data on precipitation and atmosphere chemistry and the output data of the atmospheric chemical transport model(MOZART-4).In addition,this study used leaf-scale hyperspectral reflectance measurements to detect the seedling growth response and assess the nitrogen saturation process of the typical subtropical forest tree species under simulated N additional treatment during a period of two years.Besides,based on the estimated atmospheric nitrogen deposition in YRD and the NDVI vegetation parameter of forest,the nitrogen saturation process of forest ecosystem was analysed in the YRD from regional scale.The major results are as follows:(1)The fluxes of atmospheric inorganic nitrogen deposition(DIN)were about 63.88-97.02kg N ha-1 yr-1(with the mean value was 78.07kg N ha-1 yr-1)during the period of 1996 to 2011,which was much higher than that of the critical load of N deposition(40kg N ha-1 yr-1)in the southern of China.The characteristic of spatial pattern of N deposition in YRD was that the value of N deposition fluxes gradually decreased from northwest to southeast.The highest value appeared in the north of Xuanzhou in Anhui province,Huzhou plain in Zhejiang province and Shanghai,where the annual amount of N deposition was over 100 kg N ha-1 yr-1.While the lowest value appeared in the southern of coastal region located in the Zhoushan islands in Zhejiang province,with the annual amount of N deposition was less than 40 kg N ha-1 yr-1.For the temporal changing,the fluxes of N deposition increased by 1.67 kg N ha-1 yr-1 in YRD which mainly due to the increasing of nitrate nitrogen deposition.The maximum increasing rate appeared in Nanjing,Yanchen-Taizhou and the cities around Taihu Lake,where the increasing rates were over 4.0 kg N ha-1 yr-1,while the minimum rate appeared in the forest region in the southern Zhejiang province,which was less than 1 kg N ha-1 yr-1.For the different ways of deposition,the fluxes of N dry deposition are the main component which account for 61.81%±10%of DIN in YRD.For the components of N deposition,the fluxes of ammonium(NHx)deposition are the major component which account for 73.35%±5%of DIN in YRD.(2)The results of simulated N deposition experiment indicated that the chlorophyll concentrations of Schima superba,Cyclobalanopsis glauca,Cinnamomum camphora,Masson pine and Chinese fir have the most highest value in LN(60 kg N ha-1 yr-1)and MN(120 kg N ha-1 yr-1)treatment while the leaf nitrogen contents of Schima superba,Cyclobalanopsis glauca,Cinnamomum camphora have the highest value in MN treatment.As an important indicator of the growth of plants,chlorophyll content could be indicated by the spectral characteristics.The leaf and canopy spectral reflectance characteristics of five typical subtropical forest tree seedlings were similarity with N treatment experiment.Compared with the CK(control)treatment,the reflectance of five trees reduced in visible wavelength under the LN,MN,and HN(240 kg N ha-1 yr-1)treatments.It has the positive effect on the growth of Masson Pine and Chinese fir with short-term LN and MN treatments while it has no significant influence on Schima superba,Cyclobalanopsis glauca,Cinnamomum camphora.On the contrary,except for Chinese fir,it has the positive effect on the growth of the other four typical subtropical forest tree seedlings with long-term LN and MN treatment.While it has inhibited the growth of five tree species seedlings in HN treatment.(3)The correlation analysis between the fluxes of atmospheric N deposition and NDVI of subtropical forest in YRD using the satellite data indicates that both the fluxes of N deposition and NDVI have the increasing trend during 1996 to 2011.The obviously spatial correlation was observed between N deposition and forest NDVI.The singular value decomposition analysis also indicated that the strong response of NDVI in forest ecosystem of YRD was observed due to N deposition,manifested chiefly by the temporal correlation coefficient of 0.77 and the similar spatial pattern account for 80%of YRD between them.Based on the current fluxes of nitrogen deposition,the critical load of N deposition which could produce nitrogen saturation effect for forest ecosystem is 120±5 kg N ha-1 yr-1 in YRD.According to this critical load,nitrogen saturation region of forest ecosystem are mainly distributed in the Taihu Lake which are in the north and around with cities and farms in YRD.The other forest areas located in the west and south of YRD will reach nitrogen saturation in the next 5-40 years,according to the current n deposition rate of 1-4 kg N ha-1 yr-1. |
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