Changes of Nitrogen Transformation Rates and Related Functional Genes Abundance under Different Dissolved Oxygen Levels in sediments form an Urban River

Abstract:

In the nitrogen rich urban river sediments, we investigated the nitrogen transformation rates and nitrogen-cycling genes in response to different dissolved oxygen (DO) supply levels (saturation, DO > 8.00 mg L⁻¹; aerobic, 2.50 mg L⁻¹ <DO < 4.00 mg L⁻¹; anoxic, DO < 0.5 mg L⁻¹; no control, 0.34 mg·L⁻¹ < DO < 5.76 mg·L⁻¹).

The five nitrogen transformation rates (ammonium release, ammonia oxidation, nitrite oxidation, denitrification, and anammox) and their corresponding integrated amounts were solved by the least square analysis. Results showed that the total amount of ammonium oxidation, nitrite oxidation, denitrification, and anammox increased with the elevated dissolved oxygen levels, but the amount of ammonium release decreased inversely. The increasing DO level also raised the total amount of nitrogen loss (from 6.12 mg N to 35.44 mg N) and its proportion to ammonium liberated (from 12.96% to 99.84%), but the contributions of anammox to nitrogen loss in each incubation showed no significant difference (83.36% to 89.19%).

The dissolved oxygen facilitated an exponential increasing of the anammox oxidizing archaea (AOA) and bacteria (AOB), and raised the denitrifiers (nirK and nirS gene) abundance by an order, but its influence on anammox (hzsB) was insignificant. Four quantitative response relationships between nitrogen transformation rates, nitrogen functional genes abundances, and nitrogen concentrations were established by stepwise linear regression analysis. These relationships confirmed that different nitrogen transformation processes were coupled at the molecular level (functional genes), especially for the coupling of ammonium oxidation and anammox.