The underestimated role of manganese in modulating the nutrient structure in a eutrophic seasonally-stratified reservoir.

Accumulation and subsequent release of nutrients have great potential to trigger algal blooms in lakes and reservoirs. We conducted high vertical resolution (2 m interval) monitoring at ∼monthly intervals over a year for hydrological parameters, Chl-a, ammonium (NH4+), nitrate (NO3-) and different species of phosphorus (P) and manganese (Mn) in a 40-meter-deep subtropical reservoir (Shanmei Reservoir) in Fujian, southern China. In this seasonally stratified reservoir featured with high nutrient loading, the consistent trend in the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) between the euphotic zone and the hypolimnion, coupled with its mirrored correlation with Chl-a concentration indicates that upward flux from the hypolimnion affects phytoplankton growth in the euphotic zone. The monthly variation of the depth-integrated multiple species of N and P indicates that during the stratification period in the hypoxic hypolimnion, approximately 80% of the DIP is removed, leading to a remarkable decoupling phenomenon between NH4+ and DIP. This process effectively increases the ratio of DIN to DIP in the hypolimnion, thereby significantly reducing the potential of algal blooms caused by the upward flux. A robust positive linear correlation between iron-manganese bound phosphorus (CBD-P) and particulate Mn was observed during stratification period implying that DIP was scavenged by sediment-released Mn throughout the water column. Vertical profiles during stratification showed that upward diffusion of Mn2+ facilitated the formation of Mn oxide zones near the oxycline. The most significant decrease in DIP inventory occurs when Mn oxide zones migrate either upwards from the bottom or downwards from the oxycline, indicating that the migration of Mn oxides on the vertical profile is a key factor in the decoupling of NH4+and DIP. Our findings underscore the importance of Mn cycling as an underappreciated DIP self-immobilization process in the water column of reservoirs characterized by high nutrient loading. Furthermore, we propose that denitrification and Mn cycling establish a consecutive feedback mechanism, preventing excessive nutrient accumulation in low oxygen bottom water. In the context of global changes, we anticipate a heightened prominence of this feedback mechanism.

Permeability decides the effect of antibiotics on sedimentary nitrogen removal in Jiulong River Estuary.

Sedimentary denitrification takes place beneath the oxic layer at the sediment-water interface, where nitrate and antibiotics need to diffuse through the overlying water. However, the antibiotics' effect on sedimentary N removal and associated N2O production has not been adequately investigated under in situ conditions. Here, isotope pairing techniques, including slurry incubations (potential) and intact core incubations (in situ), combined with metagenomic analysis were applied to investigate the impacts of two protein-inhibiting antibiotics (oxytetracycline and thiamphenicol) on sediment nitrogen removal in a subtropical estuary. Slurry incubations showed that the two antibiotics significantly inhibited denitrification (67-98%) and anammox (49-99%), while intact core incubations presented no antibiotic effect at upstream but significant inhibition (23%-52%) at downstream. Meanwhile, N2O yields were stimulated up to 20 folds in slurry incubations yet showing insignificant response in intact cores. Such contrasting results between up- and down-stream and between slurry and intact core incubations strongly indicated that permeability, which determines diffusion of antibiotics to microbes, is the key to exert the effect of antibiotics on in situ sedimentary nitrogen removal processes regardless the existence of antibiotics resistance genes. This diffusive obstruction may mitigate the toxic effect of antibiotics on nitrogen removal related microbes in natural environments.

Transition of source/sink processes and fate of ammonium in groundwater along with redox gradients.

Ammonium (NH4+) retention/removal processes in groundwater are of great interest because of the continuous increase in nitrogenous compound loading due to anthropogenic activities. However, the transition of multiple co-occurring transformation processes that determine the fate of NH4+ in groundwater along a redox gradient remains underexplored. We selected a high nitrogen (N) groundwater system in the western Hetao Basin, China, to identify and quantify NH4+ source and sink processes, including mineralization, dissimilatory nitrate reduction to ammonium (DNRA), nitrification, and anammox, to better understand the dynamics of NH4+. Based on redox-sensitive parameters, that is, the oxidation-reduction potential (ORP) and NH4+ and nitrate (NO3-) contents, etc., the groundwater system was classified into three zones from upstream to downstream: zone I (oxidizing), zone II (moderately reducing), and zone III (strongly reducing). Using the 15N tracing technique, we found that NH4+ was mainly produced by mineralization while < 2% was produced by DNRA throughout the study area. Mineralization increased downstream because the supply of biodegradable N-containing compounds was augmented, which created a strong redox gradient to host a serial reaction chain. In zone I, NH4+ was mainly transferred to NO3- via nitrification, whereas in zones II and III, NH4+ was mainly transferred to N2 via anammox. The average NH4+ production/consumption ratios (P/C) in zones I, II, and III were 0.7, 6.9, and 51.1, respectively. Obviously, the NH4+ purification ability can only exceed the supply under aerobic conditions, thus suggesting that NH4+ will accumulate without limitation and be retained in strongly reducing groundwater. The situation of NH4+ accumulation would deteriorate over space and time in groundwater as human activities increase without an additional artificial supply of oxidants. The results provide mechanistic insights for quantitatively comprehending the dynamics and fate of NH4+ in groundwater, shedding light on groundwater NH4+ mitigation techniques.