A correlation of 15N isotope ratios in wetland plants with NO3-N in groundwater that receives nitrogen loading from fertilizer at a golf course in northern lower Michigan

Abstract

The correlation between nitrogen in groundwater and nitrogen in vegetation in a wetland receiving groundwater inputs was analyzed by measuring the stable isotope δ15N in vegetation and the NO3-N in groundwater on a golf course in northern lower Michigan. Through fertilization of a golf course green, nitrogen became enriched in the groundwater of an adjacent wetland. A grid with 52 data points was established on the wetland and groundwater and vegetative samples were collected from various points within the grid. Levels of NO3-N in groundwater and δ15N in shallow and deep-rooted plants were measured and compared to determine the correlation between groundwater nitrogen and the nitrogen that is taken up in plants. The groundwater nitrogen plume and enriched vs. unenriched plants were mapped in the wetland. δ15N values in shallow-rooted plants varied widely and did not show a relationship between enriched δ15N values in the plants and elevated NO3-N in groundwater. Enriched δ15N values in deep-rooted plants corresponded with elevated NO3-N in groundwater, indicating that they take up nitrogen from groundwater. With further research it may be possible to use elevated levels of δ15N in deep-rooted plants as an aid in mapping groundwater nitrogen plumes.