Promotion of N_2O emission from a forested watershed by nitrogen saturation

日本富山Kureha山的许多溪流含有高浓度的硝酸盐。这被认为是氮饱和的结果，因为氮收支表明，在森林流域的氮沉积上，硝酸盐过量浸出到溪流中。高浓度的硝酸盐被认为是在浅层土壤中产生的，那里有大量的电子供体，即有机物存在。硝态氮和电子给体的存在都促进了温室气体N_2O的反硝化过程。为了确定氮饱和度对N_2O排放的影响，我们测量了氮饱和森林地表和非饱和森林地表向大气的N_2O通量。作为氮饱和流域的代表，选取了Kureha hill上硝酸盐浓度为180μmol/l的酸化最严重的溪流Hyakumakidani。三野隈流位于与黑叶山相邻的Imizu山上，硝酸盐含量低于20μmol/l，被选为非氮饱和流域。这两个森林流域都主要被硬木覆盖。植被和土壤类型相似，氮沉降也相似。在每个森林地面上都安装了一个开放的室来收集N_2O气体。采用非放射性活性ECD检测器气相色谱仪测定N_2O浓度。2004年10月至2005年6月，氮饱和流域的平均N_2O通量为2.42 μg-N/m^2/h，而仅为0.21 μg-N/m^2/h。在非氮饱和流域观测到。这一结果清楚地表明，高氮沉降引起的氮饱和会促进N_2O向大气的排放。

Many streams of Kureha Hill, Toyama, Japan, contain high concentration of nitrate. This is believed to be the result of nitrogen saturation, because the nitrogen budget shows the excess leaching of nitrate to the stream water over the nitrogen deposition on the forested watersheds.The high concentration of nitrate leached to the stream water was thought to be produced in the shallow soil layers where plenty of electron donors, i.e.organic matters, exist. The existence of both nitrate and electron donors may promote the denitrification process with which N_2O as a green house gas is produced. We measured the N_2O flux both from a nitrogen-saturated forest floor and from a non-saturated forest floor to the atmosphere in order to determine the effect of nitrogen saturation on the emission of N_2O. As a representative of nitrogen-saturated watershed, Hyakumakidani, one of the most acidified streams with nitrate concentration of 180μmol/l on Kureha hill, was selected. Sannokuma stream, located on Imizu hill adjacent to Kureha hill, which contains less than 20μmol/l of nitrate, was selected as a non-nitrogen saturated watershed. Both of the forested watersheds are mainly covered by hardwood. The vegetations and the soil types are similar to each other as well as the nitrogen deposition.An open chamber was installed on each forest floor to collect N_2O gas. A gaschromatograph with a non-radio active ECD detector was used for determining the N_2O concentration.The averaged N_2O flux from the nitrogen-saturated watershed was 2.42 μg-N/m^2/h from Oct.2004 to Jun.2005, while only 0.21 μg-N/m^2/h. was observed from the non-nitrogen-saturated watershed. This result clearly showed that nitrogen saturation caused by high nitrogen deposition could promote N_2O emission to the atmosphere.