Isotopic composition of nitrate (NO3-) and nitrous oxide (N2O) at natural abundance have proven to be useful tools in constraining the sources and fate of NO3- and N2O in aquatic environment. Here, we provide three case studies on the application of stable isotopes to comprehensively investigate the provenance and transformations of NO3- and N2O across different environmental settings from land to coastal continuum including groundwater, streams and estuarine waters (Werribee Estuary) in south eastern Australia.
The δ15N-NO3- values of groundwater underlying horticulture areas ranged between +13 and +21‰; much heavier than the δ15N-NO3- of the agricultural-derived sources (<+8‰), suggesting active transformations of NO3-; most likely through denitrification within the vadose zone. The importance of denitrification in controlling the fate of NO3- on a catchment scale was substantiated by the isotopically heavy NO3- in the streams (up to +33‰) and the simultaneous progression of δ18O-NO3- with δ15N-NO3-. The δ15N-NO3- of the streams were significantly correlated with the percentage fertilization area, with an estimated initial δ15N of ~16‰, at the highest agricultural intensity, suggesting that the extent of denitrification could be closely related to land use.
A combination of δ15N and δ18O values in the Werribee Estuary also revealed that NO3- discharged to the estuary was dominated by NO3- laden groundwater through two hotspots, one with NO3- primarily derived from agricultural sources (13‰) and one with sewage-derived NO3- (+30‰). The isotopic composition of NO3- in the estuary showed that agricultural sources dominated over sewage inputs to the estuary, contributing up to 60% of the total NO3- in the estuary. Based on the isotopomeric signatures of N2O, groundwater discharge and chemodenitrification were two dominant N2O pathways to the estuary by contributing 20 and 80%; respectively, to the oversaturation of N2O in the estuary. This highlights the need to carefully evaluate the role of these sources in driving N2O emissions from estuarine and groundwater environments.