Human activity intensity controls the relative importance of denitrification and anaerobic ammonium oxidation across subtropical estuaries

Human activities are dramatically increasing estuarine nitrogen (N) loads, further altering N processes. However, relative importance of denitrification and anaerobic ammonium oxidation (anammox) in response to human activities intensity gradient remains poorly understood for estuaries. In this study, we used a ¹⁵N isotope tracer approach to characterize the variations in sediment denitrification and anammox rates and determined the crucial factors controlling the partitioning of N₂ production and regulating N₂O production across five subtropical estuaries in southeast China. Denitrification rates increased significantly from 8.82 ± 3.89 nmol N g⁻¹ h⁻¹ (low human activity intensity) to 41.2 ± 11.5 nmol N g⁻¹ h⁻¹ (high human activity intensity) across the studied estuaries. Anammox rates not varied significantly between low (2.37 ± 0.66 nmol N g⁻¹ h⁻¹), moderate (3.96 ± 0.91 nmol N g⁻¹ h⁻¹), and high (4.04 ± 1.09 nmol N g⁻¹ h⁻¹) human activity intensity estuaries. Relative contribution of anammox to total N₂ production (R_a) decreased toward estuary mouth within each estuary. The R_a was also significantly lower in the estuaries characterized by high N loads and low DO. N₂O production rates were in a range of 0.192–1.92 nmol N₂O g⁻¹ h⁻¹ across the estuaries and controlled by water NO₂^–, salinity and TOC. DO, NH₄⁺, and NO₃^– were the best predictors of the partitioning of N₂ production between denitrification and anammox. The high human activities intensity increased NH₄⁺ and NO₃^– loads and further enhanced denitrification, leading to the decrease in R_a and increase in N₂O production. These findings suggest that low DO and high N loads estuaries can increase denitrification and N₂O emissions, whereas not affect anammox substantially under the higher intensity of human activities.