Nitrate dynamics in a reed belt of a shallow sand dune lake in Japan: Analysis of nitrate retention using stable nitrogen isotope ratios

Abstract

A stable nitrogen isotope analysis was used to clarify the relative importance of denitrification and nitrate uptake by plants in the nitrate reduction in a reed belt of L. Kamisagata (N 37°49′, E 138°53′, alt. 4.5 m, depth 30–80 cm, area 0.025 km 2 ), one of about 20 sand dune lakes in Japan. A very high concentration of NO 3 − -N with 19.0 ±5.9 mg N l −1 in spring sources decreased during passage through the reed belt along two set transect lines about 120 m long in any season, whereas progressive enrichment in 15 N-NO 3 − in flowing water was detected. Loss rate of nitrate ranged from 38.4 to 73.1% with an average of 56.7 ±11.6%. Enrichment factors calculated using a Rayleigh curve method ranged from - 1.03 to -5.12%.. The contribution of denitrification to nitrate loss ranged from 6 to 28%, with a mean of 19.5% (±7.0), whereas that of plant uptake was from 72 to 94%, with a mean of 80.5% (±7.0), indicating the importance of vegetation in a sand dune riparian zone. A technique using the variation of natural abundance of 15 N may provide useful information on the nitrate dynamics in artificial or natural wetlands under a nondestructive condition.