Gross nitrogen transformations in intertidal sediments of the Yangtze Estuary: Distribution patterns and environmental controls

Nitrogen (N) transformations in estuarine and coastal ecosystems play a crucial role in the global biogeochemical cycles. However, simultaneous estimation of gross N transformations and the underlying drivers across estuarine and intertidal wetlands remain poorly understood. Here, the spatial changes in gross N transformation rates and related functional gene abundances were explored in varying habitats of the intertidal zone in the Yangtze Estuary via an optimized ¹⁵N tracing model and quantitative PCR. The results indicate that the gross N transformation rates changed remarkably in estuarine and intertidal wetlands, with comparatively higher gross N transformation rates at the upper freshwater and low-salinity sites relative to high-salinity sites. Meanwhile, the functional gene abundances (amoB, UreC, hzo, nirS and nrfA) exhibited a similar spatial distribution pattern to corresponding N transformation processes. Sedimentary total organic carbon (TOC), nitrite (NO₂^–), ferrous iron (Fe(II)), and microbial functional gene abundances jointly mediated the gross N transformations. This study carried out the first simultaneous estimation of the gross N transformation pattern and driving factors in estuarine and intertidal ecosystems based on the optimized ¹⁵N tracing model, providing a better understanding of internal mechanisms on the N cycling and strategy for estuarine and coastal N management.