BayCEER Workshop 2016

Isotope signatures to trace the origin and fate of nitrate in the Soyang Lake Watershed, South Korea

P 4.1 in Land-use effects on ecosystems

The monsoon season in South Korea has a great influence on the biogeochemical and hydrological processes in the entire country, but is specifically of concern in the Soyang Lake Watershed which is composed of sub-catchments dominated by intensive agricultural management, and by pristine (semi-) natural broadleaf and coniferous forests. Therefore, nitrate leaching into surface waters may have different origins.

Stable isotopes are a useful tool to quantify and determinate the origin of nitrate inputs into the Soyang Lake. The δ¹⁵N values of nitrate from different sources often show overlapping ranges but the additional measurement of the δ¹⁸O values allows a more precise classification (Durka et al. 1994; Mayer et al. 2002; Deutsch et al. 2006).

With a sampling design made to determinate the influence of the precipitation regime and the land use in the nitrate discharge into the Soyang Lake Watershed, river water samples, soil water samples and rain samples were taken before and during the monsoon season in 2013 and 2014 to analyse the nitrate concentration, and ¹⁸O and ¹⁵N abundance in each phase of the nitrogen cycle.

Preliminary results suggest the heavy nitrogen fertilization in the agriculture-dominated Haean basin as the major contributor to the nitrate output into the ground water systems and into the Mandae River and therefore, nitrate input into the Soyang Lake. Nitrate from atmospheric deposition and a surplus of microbial nitrification in the forest-dominated sub-catchments also contributes in smaller amount to nitrate output to the rivers and also suggest differences in nitrate assimilation capacities between the broadleaf and coniferous forest. The monsoon is the most important driver for the nitrate release between the different types of crops and also plays an important role in the nitrate delivery from forested sites being responsible as well for more than the 80% of input from the total nitrate transport within both seasons.

Keywords: Monsoon, Isotope abundance, nitrification, denitrification