Suspended solids induce increasing microbial ammonium recycling along the river-estuary continuum of the Yangtze River

Many large rivers worldwide are enriched with high levels of suspended solids (SS), which are known to be hotspots of many nitrogen (N) transformation processes (e.g., denitrification, nitrification). However, the influence of SS on microbial ammonium (NH4+) recycling remains unclear. Water column NH4+ regeneration rates (REGs) and potential uptake rates (U(pot)s) as well as community biological NH4+ demand (CBAD) was measured in the river-estuary continuum of the third longest river in the world-Yangtze River, which has dramatic SS gradients. We found that REGs, U(pot)s, and CBAD all increased downriver, with higher REGs, U(pot)s, and CBAD in the estuary than in the river sections. The regeneration and uptake of NH4+ were nearly balanced in the river sections, while the positive CBAD in the estuary indicated obvious NH4+ demand of microbes. Concentrations of SS, which also control the content of chemical oxygen demand and particulate N, were the main factor influencing NH4+ recycling rates and CBAD. SS-induced regenerated NH4+ in the river-estuary continuum of Yangtze River was estimated to be 11.02 x 10(8) kg N yr(-1) and accounted for about 14% of total N inputs, suggesting that regenerated NH4+ is an important N source for microbes and may influence nutrient dynamics in lower coasts. To our knowledge, this is the first study to report NH4+ recycling in Yangtze River with an emphasis on its influencing factors and contribution to N budgets.

Automated summary

This study found that NH4+ regeneration rates and community biological NH4+ demand increased downstream in the Yangtze River continuum, with higher rates in the estuary. Suspended solids were the main factor influencing NH4+ recycling rates. The regenerated NH4+ in the river-estuary continuum accounted for about 14% of total N inputs, suggesting its importance as an N source for microbes and in nutrient dynamics.