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 N2
saturation (%) and fluxes, and δ15N 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 N2
oversaturation. This finding suggests that the excess nitrogen inputs in agricultural streams
promote enhanced nitrogen turnover and gaseous carbon and nitrogen losses. ... mehrWe also
observed a proportional increase in CO2, CH4, and N2 saturation in the water column with
sediment δ15N enrichment, a known indicator of long-term nitrogen turnover processes.
Because the highest GHG emissions and denitrification N2 losses occurred within streams in
fertilized areas, our findings highlight the potential of using sediment δ15N as an indicator of
long-term anthropogenic hotspots of fluvial GHG emissions and denitrification rates.