Dramatic source-sink transition of N2O in the water level fluctuation zone of the Three Gorges Reservoir during flooding-drying processes

Elucidation of the dominant factors influencing N2O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N2O emission. However, the comprehension of response patterns and mechanisms governing N2O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces significant uncertainty to N2O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-filled pore space gradients (WFPSs) to investigate the potential N2O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N2O flux pattern differed significantly between two zones, with the highest N2O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N2O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density significantly impacted N2O fluxes across all sites, while NO3-and SOC facilitated N2O emissions in T1-T2 and T4-T5, respectively. It was evident that N2O production primarily contributed to nitrification in the transition zone and was constrained by the mineralization process, whereas denitrification dominated in the lacustrine zone. Notably, the annual N2O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N2O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.

The contribution of sediment desiccation and rewetting process to eutrophication in the presence and absence of emergent macrophytes

The purpose of current study was to investigate the effects of sediment desiccation on nutrient dynamics and eutrophication in wetlands during the presence or absence of wiry and sturdy rooted emergent macrophytes, based on the hypothesis that sediment desiccation negatively correlated with plants nutrient uptake abilities and positively with nutrients fluxes at sediment-water interface. Growth of four emergent macrophytes, including two wiry rooted plants, i.e., Alocasia cucullata and Aglaonema commutatum, and two sturdy rooted plants, i.e., Cannabis indica and Acorus calamus, were grown and investigated in dried-rewetted sediments (DS) and constantly wet sediments (WS), respectively, for 6 months. The findings revealed that sediment drying and rewetting process significantly decreased the diffusion of overlying nutrient into sediment and the particle size density, porosity, and nutrients' repository ability in DS treatments, while the sediment bulk density and mineralization of organic macronutrients increased. Compared to WS treatments, the DS treatments impaired plant growth, root biomass, shoot biomass, and stimulated higher fluxes of ammonium nitrogen ([Formula: see text]-N, 0.042-0.081 mg m ^{- 2} d ^{- 1}) and phosphate (P[Formula: see text] 0.009-0.030 mg m^-2 d^-1) at sediment-water interface upon rewetting. The higher internal release of macronutrients and dissolved organic carbon (DOC) from DS led to the higher chlorophyll-a (Chl-a) concentrations (34.47-21.28 to 41.76-33.36 μg L^-1) in their water column than in the water column of WS. The wiry rooted plants with higher root biomass displayed lower internal release of [Formula: see text]-N, PO₄³-P and DOC and water column Chl-a concentrations than the sturdy rooted plants in two sediment types. Root biomass of plants correlated positively with TN (63-87%) and TP (56-78%) removal percentages from WS and DS. These results demonstrated that sediment desiccation process reduced plant growth and enhanced internal loading of nutrients and consequently accelerated eutrophication in these wetlands.

The bacterial community structure and N-cycling gene abundance in response to dam construction in a riparian zone

Dam construction has significantly altered riparian hydrological regime and environmental conditions in the reservoir region, yet knowledge concerning how bacterial community and N-cycling genes respond to these changes remains limited. In this study, we investigated the bacterial community composition, network structure and N-cycling genes in the water level fluctuation zones (WLFZs) of the Three Gorges Reservoir (TGR). Here, samples collected from five different water levels were divided into three groups: waterward sediments, interface sediments, and landward soils. Our results show that higher contents of NO₂^--N, SOC, DOC, NH₄⁺-N, and TP were characterized in waterward and interface sediments whereas higherNO₃^--N content was observed in landward soils. The α-diversity of bacterial community decreased gradually from waterward sediments to landward soils. Compared with waterward sediments and landward soils, the interface sediments showed a unique bacterial community pattern with diverse primary producers as well as N-cycling microbes. The interface sediments also had a much more complex co-occurrence network and a higher possible community stability. Among all of N-cycling genes, higher abundances of nrfA and AOA amoA genes were observed in interface sediments. The dissimilarity in bacterial community composition and N-cycling gene abundance was mainly driven by water-level. Moreover, random forest model revealed that AOA amoA and nirS genes were the most sensitive indicators in response to water level fluctuations. Overall, this study suggests distinct abundance, diversity, and network structure of microbes in riparian sediments and soils across the gradient of water levels and enhances our understanding with respect to comprehensive effects of dam construction on nitrogen cycle.

Density currents reduce nitrous oxide emissions in a tributary bay of Three Gorges Reservoir

Reservoirs are a significant source of the potent greenhouse gas nitrous oxide (N₂O), but there are few data on N₂O in the world's largest reservoirs and limited understanding of the factors controlling their emission rates. Here we analyzed high-resolution measurements of dissolved N₂O concentrations and fluxes in a typical tributary bay of Three Gorges Reservoir. The surface water was oversaturated in N₂O during both low and high water level (8.6 -16.4 nmol/L, 107% - 180% saturation) and N₂O fluxes varied nearly tenfold (0.2 and 1.6 μmol/(m² h)). Dissolved N₂O concentrations were characterized by pronounced vertical gradients, which were controlled by bidirectional density currents. The river water with high concentrations entered the bay as an underflow along the riverbed, the upper part of the water column was formed by intrusive backwater of Three Gorges Reservoir having significantly lower N₂O concentrations. In consequence, the N₂O emission potential of the impoundment was reduced compared to pre-impoundment conditions. These results reveal the importance of hydraulic conditions on N₂O emission from large reservoirs and suggest that flow regulation can be a potential tool for mitigating greenhouse gas emissions from manmade impoundments.

Optimized methods for diffusive greenhouse gas flux analyses in inland waters

Inland waters are considered hotspots of greenhouse gas (GHG) emissions and have been extensively researched. Static chamber (STAT) and thin boundary layer (BLE) are two commonly used methods for analyzing diffusive GHG emissions from inland waters. However, the STAT method is often disturbed by GHG bubbles; meanwhile, many kinds of headspace gas are used in the BLE method, but the differences between their diffusive GHG emission analysis results are not understood. In this study, the chamber in the STAT method was modified to combat the disturbances from GHG bubbles, and the typically used gases for the BLE method, namely, pure nitrogen, air, and filtered air, were comparatively studied. Results demonstrated that the modified chamber could effectively prevent the invasion of GHG bubbles; it increased the success rate from 67 to 90% in the field test, with no obvious impacts on the results of the GHG emission analyses. The use of air and filtered air in the BLE method yielded the lower values of GHG emissions relative to pure nitrogen, and this finding was potentially attributed to the inhibition effects of the residual GHGs and high humidity in air and filtered air on the extraction of diffusive GHGs from the surface water. This study improved the commonly used methods for diffusive GHG emission analysis, and the current findings are beneficial to the study of GHG emissions from inland waters.

Water-level alterations modified nitrogen cycling across sediment-water interface in the Three Gorges Reservoir

Water-level regime alteration-associated redox fluctuation plays a primary role in governing exchange and transformation of nitrogen (N) in water-level fluctuation zones (WLFZs), while few understanding of how hydrological regimes under reservoir operation affected N cycling across the sediment-water interface (SWI), giving rise to uncertainties in reservoir N nutrient management. Batch microcosm simulation experiments with intact sediment cores from WLFZs of the Three Gorges Reservoir (TGR) were conducted for 24 days to identify holistic flooding-drying process mechanism on N-cycling patterns. Our results showed a distinct transition of N-cycling mode across the SWI, shifting from biological denitrogen loss dominated in initial period of flooding to enhance endogenous N retention. A dramatic source-sink switch of nitrous oxide (N₂O) occurred in the first 1.5 days during the flooding period. However, combined accelerating migration of NH₄⁺-N from sediment to overlying water, and subsequently enhanced transformation of NH₄⁺-N to NO₃^--N formed from flooding to drying rotation, thereby increasing N loading to overlying water. The reason for this investigation could be attributed to intensive N loss through coupled nitrification and denitrification in oxic-anoxic microenvironments after flooding. With oxygen replenishment from atmosphere during drying phase, persistent ammonification of organic N in sediments provided sufficient source of NH₄⁺-N for the formation of NO₃^--N fraction in a more oxic overlying water. Therefore, water-level regime alteration by reservoir operation was capable of weakening N removal from water body and lengthening internal N turnover time across redox-variable SWI. These findings elucidate new understanding of holistic hydrological regime mechanisms on N cycling across SWI and provide insight to biogenic N nutrient management for improving the green credentials of hydroelectric reservoir.

Seasonal fluxes and sources apportionment of dissolved inorganic nitrogen wet deposition at different land-use sites in the Three Gorges reservoir area

To identify seasonal fluxes and sources of dissolved inorganic nitrogen (DIN) wet deposition, concentrations and δ¹⁵N signatures of nitrate (NO₃^-) and ammonium (NH₄⁺) in wet precipitation were measured at four typical land-use types in the Three Gorges reservoir (TGR) area of southwest China for a one-year period. Higher DIN fluxes were recorded in spring and summer and their total fluxes (averaged 7.58 kg N ha^-1) were similar to the critical loads in aquatic ecosystems. Significant differences of precipitation δ¹⁵N were observed for NH₄⁺-N between town and wetland sites in spring and between urban and rural sites in summer. For NO₃^--N, significant differences of precipitation δ¹⁵N were observed between town and rural sites in spring and between urban and town sites in autumn, respectively. Quantitative results of NO₃^--N sources showed that both biomass burning and coal combustion had higher fluxes at the urban site especially in winter (0.18 ± 0.09 and 0.19 ± 0.08 kg N ha^-1), which were about three times higher than those at the town site. A similar finding was observed for soil emission and vehicle exhausts in winter. On the whole, DIN wet deposition averaged at 12.13 kg N ha^-1 yr^-1 with the urban site as the hotspot (17.50 kg N ha^-1 yr^-1) and regional NO₃^--N fluxes had a seasonal pattern with minimum values in winter. The contribution to NO₃^--N wet deposition from biomass burning was 26.1 ± 14.1%, which is the second dominant factor lower than coal combustion (26.5 ± 12.6%) in the TGR area during spring and summer. Hence N emission reduction from biomass burning, coal combustion and vehicle exhausts should be strengthened especially in spring and summer to effectively manage DIN pollution for the sustainable development in TGR area.

Nutrient inputs from the leaf decay of Cynodon dactylon (L.) Pers in the water level fluctuation zone of a Three Gorges tributary

Cynodon dactylon (L.) Pers (C. dactylon) is one of the dominant plants in the water level fluctuation (WLF) zone of the Three Gorges Reservoir (TGR) tributaries. However, the leaves of C. dactylon can decay to increase the inputs of nutrients under flood inundation, increasing the risk of eutrophication in the TGR tributaries. Nutrient inputs from the leaf decay of C. dactylon in three interfaces, namely, water-sediment (WS), water-C. dactylon (WC) and water-sediment-C. dactylon (W-S-C), were estimated in a 180 d inundation experiment. The results showed that the kinetic processes of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) input accorded with the power function equation: y = ax^b for the WS, WC and W-S-C interfaces (R²s > 0.72, p < 0.001). The cumulative TDN input from leaf decay of C. dactylon in the WC interface was 506.44 mg N kg^-1 of biomass, which was significantly higher than that in the W-S-C interface with 422.24 mg N kg^-1 of biomass (p < 0.05). However, no significant differences in TDP input were found between the WC and W-S-C interfaces (p > 0.05). The total amounts of TDN and TDP inputs at the 165-175 m altitude were 21,688.81 and 13,121.68 kg year^-1, respectively, which were approximately 3.17 times those from the 145-155 m altitude of the WLF zone. The amounts of TDN and TDP inputs from the leaves of C. dactylon for the whole WLF zone were 49,261.65 and 29,803.17 kg year^-1, respectively, which were 0.1 and 2.7 times the annual permissible discharge amount of pollutants calculated from a municipal wastewater treatment plant with the peak flow of 60,000 m³/d according to Class I (A) of the Wastewater Discharge Standard (GB18918-2002) in China. Thus, the aboveground part of this perennial herb should be harvested in a timely manner before reflooding, especially at the higher altitudes of the WLF zone to decrease eutrophication risk.

The isotopomer ratios of N2O in the Shaying River, the upper Huai River network, Eastern China: The significances of mechanisms and productions of N2O in the heavy ammonia polluted rivers

In order to figure out the effects of nitrogen pollution and dams on N₂O production paths in the river systems, the isotopomer ratios in N₂O, NH₄⁺ and NO₃^-, as well as N₂ concentrations and physico-chemical characteristics of water and sediments from the Shaying River system, which is the biggest tributary and major NH₄⁺ contributor of the Huai River, were analyzed in the years 2015 and 2016. The results showed that the net productions of N₂O (△N₂O) in the river were pretty high, ranging from 12.9 to 440 nmol/L. N₂O exhibited a narrow range in δ¹⁵N^bulk (-0.04 to 13.51‰), nevertheless a wide range in δ¹⁸O (22.54 to 59.90‰). Isotopocule diagram and Pearson correlation analysis indicated that isotopomer ratios of N₂O were significantly affected by the mixing of N₂O from difference production paths, not by N₂O reduction. Relative contributions of nitrification and denitrification to N₂O in the Shaying river system were deduced from the two end-members model. The contribution of nitrification to gross N₂O was 58.5% on average, almost equal to the contribution of denitrification in summer, although denitrification was the dominant N₂O source with average contribution of 75.6% in winter. No significant relationship was found either between △N₂O and NH₄⁺ or between △N₂O and NO₃^- in the Shaying River. Heavy NO₃^- and COD loading reduced nitrification and increased the relative contribution of denitrification to N₂O in winter. Heavy ammonia pollution caused pH values to decrease apparently, from 7.5 ± 0.3 in July to 6.3 ± 0.1 in December, resulting in denitrification being the dominant source to N₂O in winter. Assimilation enhanced by the construction of dams had weakened the contribution of nitrification to N₂O in summer in the Shaying River.