Nitrogen Dynamics And Greenhouse Gas Emissions At Shaoshan Forest Catchment Under The Influence Of Acid Deposition

Global nitrogen (N) cycling has been altered by fuel combustion and industrial development. Large amounts of active nitrogen (NH3and NOx) enter into the atmosphere, rapidly mobile and deposit to terrestrial ecosystem. It is lead to many environmental problems, such as soil acidification and greenhouse effect. Elevated N deposition was loaded to south china, and yet the N dynamics and its environmental effects is indistinct for the forest ecosystem. This issue should be pay more attention to the hot and humid forest areas in mid-subtropical China.The observations were performed at the Shaoshan forest catchment, which mainly covered by needle broadleaf trees in the mid-subtropical of south china. Four plots (hillslope top hillslope bottom; upper groundwater discharge zone and bottom groundwater discharge zone) were set in the catchment. To understand the input-output dynamics of active nitrogen and greenhouse gas emission in this catchment, weekly throughfall and soil water were collected at four plots, meanwihle wet only precipitation and runoff water were sampled as the same schedule from January2013to December2013. The results show that:(1)the value of rainfall pH ranged from3.20to3.50with the average of4.29. The pH of throughfall and runoff pH ranged from4.02to5.06and from6.31to6.67respectively. These were decreased comparison of those from2001to2004at the same site, suggesting that precipitation of Shaoshan area is continuous acidification and the acidity of water chemistry in this watershed also gradually increased. These is due to the rapid development of tourism and industrial in this decade at Shaoshan area, especially the motor vehicle dramatically increasion.(2)The order of annual mean pH value of soil water in four plots was D> C> B>A. And the maximum concentration of mean NH4+-N and NO3--N in four plots was the A point. Furthermore, The minium value of mean NO3--N and NH4+-N in four plots are the D point and C point, respectively. This may be related to the soil moisture and the strength of denitrification processing on the production of OH-differences.(3)The input of active nitrogen in Shaoshan forest was2.469g·m-2a-1in whole2013year, with the NO3--N and NH4+-N input of1.384g·m-2·a-1and1.085g·m-2·a-1, respectively. The NO3--N input flux was7%of active N input, which is lower than those (60%) from the period of2001to2004indicating that NO3--N input play important role in this area currently. (4)The estimated flux of dry-deposition was0.563g·m-2·a-1,only as23%of active N input, with the highest in spring and summer and the lowest in autumn and winter, showing that active N input was major in frequent and abundant precipitation in spring and summer.(5)The output flux of active nitrogen in Shaoshan forest was0.366g·m-2·a-1, which is mainly composed of NO3--N(99.2%of output of active N). The output of active N was14.8%of input, which is greater than those from10years ago (8%-9%),but the output of activeN does not increase with the increasing of input, showing Shaoshan forest watershed system have not reached nitrogen saturation state.We observed the greenhouse gas emission from soil in this catchment by static box for a year on a regular basis (every two weeks) in Shaoshan forest.The results show that:(1)The annual mean flux of N2Owas0.2g C m-2a-1.The order of soil N2O emission flux at different locations was hillslope Bottom(50.8g N m-2a-1)>Top of the water infiltration(40.3g N m-2a-1)>hillslope Top(7.2g N m-2a-1)>Bottom of the water infiltration(6.1g N m-2a-1.The Soil N2Oemission flux of hillslopeTop(A)and lower of the water infiltration(D)was relatively stable, and High N2O emission rate was found from hillslope Bottom(B)and upper of the water infiltration(C) in the April to June and changes with rainfall.These indicating that the soil of Shaoshan forest was net source of N2O,and emissions intensity affected by geographical environment. Thealternating of wet and dry environment is beneficial to denitrification process of N2O production and emission, and long-term watered-out state is beneficial to denitrification process with the NO3-direct reduction of N2and released.(2)The annual mean flux of CO2was367.0g C m-2a-1.The order of soil CO2emission flux at different locations washillslope Bottom>hillslope Top>upper of the water infiltration> lower of the water infiltration.The annual mean flux of CH4was6.8g C m-2a-1.The soil of hillslope top(A) and bottom(B) is a sink of CH4, whereas the soil of upper(C) and bottom(D)groundwater discharge zone in shaoshan forest is a source of CH4.(3)By fitting emission flux and the relationship between soil temperature and humidity during the observation found that there was a significantly negative linear correlation between the flux of CO2、N2O and soil temperature, and there was a significantly negative linear correlation between the flux of CO2and soil moisture.