Combined use of stable nitrogen and oxygen isotopes to constrain the nitrate sources in a karst lake

Abstract

Nitrate is a highly concerned pollutant in global aquatic ecosystem, resulting in eutrophication and water quality deterioration. As a result of the dissolved inorganic carbon fertilization effect, lake ecosystem is expected to respond more sensitively to nitrogen (N) addition in karst region than in non-karst region. Identifying accurately the sources of nitrate in lake system is an important prerequisite for formulating effective strategies on reducing nitrate and restoring water ecosystem. Quantitative identification of nitrate sources to lakes in karst region is limited until now. In this study, δ15N-NO3− and δ18O-NO3− were jointly used to identify the nitrate sources in Caohai Lake, a typical karst lake. The δ15N-NO3− and δ18O-NO3− of lake water displayed significant seasonal variations. The average δ15N-NO3− values during normal, dry and wet seasons were 6.6‰, 12.7‰ and 0.9‰, respectively. Accordingly, the average δ18O-NO3− values were 11‰, 13‰ and 16‰, respectively. The average contribution percentages of nitrate from agricultural activities, precipitation and sewage were 42 %, 41 % and 17 %, respectively. Strict measures should be taken to prohibit unreasonable agricultural activities and to improve nutrient use efficiency through optimized fertilization and irrigation management. In view of the sensitivity of karst lake ecosystem to N/P addition, higher discharge standard requirement is necessary for restoring water ecosystem and maintaining good water quality. This study proved that the combined use of δ15N-NO3− and δ18O-NO3− is a promising method for identifying quantitatively the nitrate sources in karst lake system.