Natural abundance sediment δ15N as a proxy for long-term gross N-turnover processes, GHG emissions, and denitrification hotspots within fluvial ecosystems

Nitrogen-rich agricultural headwater streams are known hotspots for fluvial greenhouse gas (GHG) emissions and denitrification, yet the underlying processes driving these elevated rates are not fully understood. In this study, we examined these mechanisms by combining measurements of gross nitrogen turnover processes, open-channel GHG and N 2 saturation (%) and fluxes, and δ 15 N isotopic analysis of stream sediment and water at nine headwater stream sites with varying levels of agricultural land use. To assess seasonal patterns, data were collected across two transitional periods: spring–summer and winter– spring. Catchment land use emerged as an important environmental driver of variability, as open channel GHG emissions and denitrification rates were up to 11 times higher in fertilized grasslands and croplands compared to those in forested areas. In-vitro nitrogen turnover rates followed a similar trend and were mainly positively related to both GHG and N 2 oversaturation. This finding suggests that the excess nitrogen inputs in agricultural streams promote enhanced nitrogen turnover and gaseous carbon and nitrogen losses. We also observed a proportional increase in CO 2 , CH 4 , and N 2 saturation in the water column with sediment δ 15 N enrichment, a known indicator of long-term nitrogen turnover processes. Because the highest GHG emissions and denitrification N 2 losses occurred within streams in fertilized areas, our findings highlight the potential of using sediment δ 15 N as an indicator of long-term anthropogenic hotspots of fluvial GHG emissions and denitrification rates.