Response of ammonia oxidation activities to water-level fluctuations in riparian zones in a column experiment

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.

Seasonal changes in N-cycling functional genes in sediments and their influencing factors in a typical eutrophic shallow lake, China

N-cycling processes mediated by microorganisms are directly linked to the eutrophication of lakes and ecosystem health. Exploring the variation and influencing factors of N-cycling-related genes is of great significance for controlling the eutrophication of lakes. However, seasonal dynamics of genomic information encoding nitrogen (N) cycling in sediments of eutrophic lakes have not yet been clearly addressed. We collected sediments in the Baiyangdian (BYD) Lake in four seasons to explore the dynamic variation of N-cycling functional genes based on a shotgun metagenome sequencing approach and to reveal their key influencing factors. Our results showed that dissimilatory nitrate reduction (DNRA), assimilatory nitrate reduction (ANRA), and denitrification were the dominant N-cycling processes, and the abundance of nirS and amoC were higher than other functional genes by at least one order of magnitude. Functional genes, such as nirS, nirK and amoC, generally showed a consistent decreasing trend from the warming season (i.e., spring, summer, fall) to the cold season (i.e., winter). Furthermore, a significantly higher abundance of nitrification functional genes (e.g., amoB, amoC and hao) in spring and denitrification functional genes (e.g., nirS, norC and nosZ) in fall were observed. N-cycling processes in four seasons were influenced by different dominant environmental factors. Generally, dissolved organic carbon (DOC) or sediment organic matter (SOM), water temperature (T) and antibiotics (e.g., Norfloxacin and ofloxacin) were significantly correlated with N-cycling processes. The findings imply that sediment organic carbon and antibiotics may be potentially key factors influencing N-cycling processes in lake ecosystems, which will provide a reference for nitrogen management in eutrophic lakes.

Mechanisms Driving the Distribution and Activity of Mineralization and Nitrification in the Reservoir Riparian Zone

The riparian zone ecosystems have greater energy flow and elemental cycling than adjacent terrestrial and aquatic ecosystems. Mineralization and nitrification are important initiating processes in the nitrogen cycle, but their distribution and activity under different environmental conditions in the riparian zone and the driving mechanisms are still not clear. We investigated the effects of environmental and microbial factors on mineralization and nitrification activities by analyzing the community of alkaline (apr) and neutral (npr) metallopeptidase, ammonia-oxidizing archaea (AOA), and bacteria (AOB) in soils and sediments under different land-use types in the riparian zone of Miyun Reservoir, as well as measuring potential nitrogen mineralization and ammonia oxidation rates (AOR). The results showed that the mineralization and nitrification activities of soils were greater than those of sediments. AOA and AOB dominate the ammonia oxidation activity of soil and sediment, respectively. NH4+ content was a key factor influencing the ecological niche differentiation between AOA and AOB. The high carbon and nitrogen content of the woodland significantly increased mineralization and nitrification activity. Microbial communities were significantly clustered in the woodland. The land-use type, not the flooding condition, determined the distribution of microbial community structure. The diversity of npr was significantly correlated with potential N mineralization rates, while the transcript abundance of AOA was significantly correlated with ammonia oxidation rates. Our study suggests that environmental changes regulate the distribution and activity of mineralization and nitrification processes in the reservoir riparian zone by affecting the transcript abundance, diversity and community structure of the microbial functional genes.

Shifts of soil archaeal nitrification and methanogenesis with elevation in water level fluctuation zone of the three Gorges Reservoir, China

The water level fluctuation zone is a unique ecological zone exposed to long-term drying and flooding and plays a critical role in the transport and transformation of carbon and nitrogen materials in reservoir-river systems. Archaea are a vital component of soil ecosystems in the water level fluctuation zones, however, the distribution and function of archaeal communities in responde to long-term wet and dry alternations are still unclear. The community structure of archaea in the drawdown areas at various elevations of the Three Gorges Reservoir was investigated by selecting surface soils (0-5 cm) of different inundation durations at three sites from upstream to downstream according to the flooding pattern. The results revealed that prolonged flooding and drying increased the community diversity of soil archaea, with ammonia-oxidizing archaea being the dominant species in non-flooded regions, while methanogenic archaea were abundant in soils that had been flooded for an extended period of time. Long-term alternation of wetting and drying increases methanogenesis but decreases nitrification. It was determined that soil pH, NO3--N, TOC and TN are significant environmental factors affecting the composition of soil archaeal communities (P = 0.02). Long-term flooding and drying changed the community composition of soil archaea by altering environmental factors, which in turn influenced nitrification and methanogenesis in soils at different elevations. These findings contribute to our understanding of soil carbon and nitrogen transport transformation processes in the water level fluctuation zone as well as the effects of long-term wet and dry alternation on soil carbon and nitrogen cycles. The results of this study can provide a basis for ecological management, environmental management, and long-term operation of reservoirs in water level fluctuation zones.

Effects of water level on nitrous oxide emissions from vegetated ditches

Nitrous oxide (N₂O) is considered a powerful greenhouse gas. Vegetated ditches are an important source of N₂O emissions in the agricultural systems. However, few studies have examined on the relationship between N₂O emissions and the water level in vegetated ditches. To investigate the effect of water level on the N₂O emissions, three pilot-scale ditches vegetated with Myriophyllum aquaticum were constructed with low (LW), medium (MW), and high (HW) water levels. The examined results indicated that the M. aquaticum ditches decreased N₂O emissions by 38.4% and 67.9% in MW and HW, respectively, as compared to the LW ditch. In addition, the N₂O emission factor decreased with increasing water level in the order of: LW (0.18%) > MW (0.11%) > HW (0.06%). The MW and HW ditches reduced the N₂O emissions by controlling the sediment nitrogen contents, in which the ammonia nitrogen increased with increasing the level of water, while nitrate nitrogen decreased with increasing the level of water. The increase in the level of water significantly reduced the gene abundance of ammonia-oxidizing archaea (AOA) (p < 0.05), thereby reducing the N₂O emissions in the MW and HW conditions due to the positive correlation between N₂O emissions and AOA gene abundances. The unclassified_k_norank_d_Bacteria was the dominant denitrifying bacterial genus observed in the M. aquaticum ditches, and its highly relative abundance yielded low N₂O emissions in the HW ditch. These findings indicate that reducing N₂O emissions may be achieved by controlling the water level in vegetated ditches.

Effects of Land-Use Type and Flooding on the Soil Microbial Community and Functional Genes in Reservoir Riparian Zones

Ecological processes (e.g., nutrient cycling) in riparian zones are often affected by land-use type and flooding. The extent to which land-use types and flooding conditions affect soil microorganisms and their ecological functions in riparian zones is not well known. By using high-throughput sequencing and quantitative PCR (q-PCR), we tested the effects of three land-use types (i.e., forest, wetland, and grassland) and two flooding conditions (i.e., landward locations and waterward locations within the land-use types) on soil microbial communities and microbial functional genes in the riparian zones of a reservoir. Land-use type but not flooding significantly affected soil microbial community composition at the phylum level, while both land-use type and flooding significantly affected the orders Nitrosotaleales and Nitrososphaerales. Alpha diversity was higher in the wetland and forest regardless of flooding conditions. Functional gene abundance differed among the three land-use types. Archaeal amoA (AOA) and nirS genes were more abundant in the wetland than in the grassland or forest. Bacterial amoA (AOB), nirK, nirS, and nosZ genes were more abundant in the waterward location than in the landward location but only in the wetland. Soil pH, moisture, and concentrations of soil organic matter and total soil nitrogen were significantly associated with the composition of archaeal and bacterial communities as well as with their gene abundance. This study revealed that soil microorganisms putatively involved in nitrogen cycling in riparian zones were more affected by land-use type than flooding.