The fate of monsoonal atmospheric nitrate deposition in two forest catchments in Soyang lake watershed, South Korea: a mass balance and stable isotope approach

Abstract

Over 60% of South Korea is covered by forests. Nitrate deposition plays an important role as nitrogen source in these forests. Nitrate input from deposition increases nitrogen availability leading initially to increases in productivity. However, later forests may become nitrogen saturated with nitrate leaching, denitrification and forest decline. Using a mass balance approach, we compared the atmospheric nitrate input and output by stream runoff in two forest sub-catchments, a deciduous and a mixed catchment, within the Soyang lake watershed, the main drinking water reservoir for the 20-million-person metropolis Seoul. A dual stable isotope approach was used to identify the origin of nitrate in stream discharge of the two sub-catchments. Extremely different monsoon seasons in 2013 and 2014 drove the nitrate discharge. Total nitrate-N export was closely related with rainfall events and intensities. Nitrate-N discharge at the deciduous forest was lower than nitrate-N deposition, thus sink conditions prevailed. Nitrate-N discharge at the mixed forest was consistently higher. During the heavy monsoon season of 2013 the mixed forest turned from a sink for atmospheric nitrate towards a source, i.e. nitrate amounts in the discharge were higher than atmospheric inputs. Nitrate isotopic compositions of stream water from both forests revealed soil nitrification as main nitrate source. However, under conditions of heavy monsoon rainfall direct runoff of atmospheric deposition nitrate was identified in the mixed forest. Lower nitrate assimilation capacities of conifers compared to broadleaf trees are probably the driver for a lower nitrate retention capacity making coniferous forests more prone to nitrate leaching.