Nitrogen Removal Capacity of the River Network in a High Nitrogen Loading Region

Denitrification is the primary process that regulates the removal of bioavailable nitrogen (N) from aquatic ecosystems. Quantifying the capacity of N removal from aquatic systems can provide a scientific basis for establishing the relationship between N reduction and water quality objectives, quantifying pollution contributions from different sources, as well as recommending control measures. The Lake Taihu region in China has a dense river network and heavy N pollution; however, the capacity for permanent N removal by the river network is unknown. Here, we concurrently examined environmental factors and net N₂ flux from sediments of two rivers in the Lake Taihu region between July 2012 and May 2013, using membrane inlet mass spectrometry, and then established a regression model incorporating the highly correlated factors to predict the N removal capacity of the river network in the region. To test the applicability of the regression model, 21 additional rivers surrounding Lake Taihu were sampled between July and December 2013. The results suggested that water nitrate concentrations are still the primary controlling factor for net denitrification even in this high N loading river network, probably due to multicollinearity of other relevant factors, and thus can be used to predict N removal from aquatic systems. Our established model accounted for 78% of the variability in the measured net N₂ flux in these 21 rivers, and the total N removed through N₂ production by the river network was estimated at 4 × 10⁴ t yr^–1, accounting for about 43% of the total aquatic N load to the river system. Our results indicate that the average total N content in the river water discharged into Lake Taihu would be around 5.9 mg of N L^–1 in the current situation, far higher than the target concentration of 2 mg of N L^–1, given the total N load and the N removal capacity. Therefore, a much stronger effort is required to control the N pollution of the surface water in the region

Dissimilatory Nitrate Reduction Processes in Typical Chinese Paddy Soils: Rates, Relative Contributions, and Influencing Factors

Using soil slurry-based ¹⁵N tracer combined with N₂/Ar technique, the potential rates of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), and their respective contributions to total nitrate reduction were investigated in 11 typical paddy soils across China. The measured rates of denitrification, anammox, and DNRA varied from 2.37 to 8.31 nmol N g^–1 h^–1, 0.15 to 0.77 nmol N g^–1 h^–1 and 0.03 to 0.54 nmol N g^–1 h^–1, respectively. The denitrification and anammox rates were significantly correlated with the soil organic carbon content, nitrate concentration, and the abundance of <i>nosZ</i> genes. The DNRA rates were significantly correlated with the soil C/N, extractable organic carbon (EOC)/NO₃^– ratio, and sulfate concentration. Denitrification was the dominant pathway (76.75–92.47%), and anammox (4.48–9.23%) and DNRA (0.54–17.63%) also contributed substantially to total nitrate reduction. The N loss or N conservation attributed to anammox and DNRA was 4.06–21.24 and 0.89–15.01 g N m^–2 y^–1, respectively. This study reports the first simultaneous investigation of the dissimilatory nitrate reduction processes in paddy soils, highlighting that anammox and DNRA play important roles in removing nitrate and should be considered when evaluating N transformation processes in paddy fields