Nitrogen loads alter the N2 production between denitrification and anammox in Min River Estuary, a highly impacted estuary in southeast China

Estuarine sediment denitrification and anammox in response to increased nitrogen (N) loads remain poorly understood. In this study, we used N isotope tracer approach to investigate the spatial distribution of denitrification and anammox and identified the crucial controls on the partitioning of dinitrogen gas (N-2) production along the Min River Estuary (MRE), a highly impacted estuary in southeast China. The results indicated that denitrification and anammox rates ranged from 10.5 to 70.7 nmol g(-1) h(-1) and from 0.44 to 4.31 nmol g(-1) h(-1), respectively. Relative contribution of anammox to N-2 production (R-a) was in a range of 1.04-15.1%, tending to increase toward estuary mouth. Denitrification rates were significantly higher in upper (high N loads) than in lower estuary (low N loads), while anammox rates and R-a showed inverse distributions along the MRE. Wastewater discharge caused the N point pollution triggering denitrification but inhibiting anammox. The best predictor of the variations in denitrification rates was total nitrogen, whereas pH and NH4+ could explained the variations in anammox rates across the estuary. The crucial predictors for the partitioning of N-2 production between denitrification and anammox were NH4+ and NOx. These results suggest that the increase in human activities intensity can alter the partitioning of N-2 production between denitrification and anammox, and the magnitude of this switch can be predicted by N loads in MRE and other highly impacted estuaries. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Estuarine sediment denitrification and anammox in response to increased nitrogen loads were investigated using a N isotope tracer approach. The study found that denitrification and anammox rates varied along the estuary, with different controls on the partitioning of N2 production. NH4+ and NOx were identified as crucial predictors for the partitioning between denitrification and anammox.