Promotion of N_2O emission from a forested watershed by nitrogen saturation

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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.