Nitrogen Cycling In Ecosystems Research Articles

Empirical evidence and theoretical understanding of ecosystem carbon and nitrogen cycle interactions.

Interactions between carbon (C) and nitrogen (N) cycles in terrestrial ecosystems are simulated in advanced vegetation models, yet methodologies vary widely, leading to divergent simulations of past land C balance trends. This underscores the need to reassess our understanding of ecosystem processes, given recent theoretical advancements and empirical data. We review current knowledge, emphasising evidence from experiments and trait data compilations for vegetation responses to CO2 and N input, alongside theoretical and ecological principles for modelling. N fertilisation increases leaf N content but inconsistently enhances leaf-level photosynthetic capacity. Whole-plant responses include increased leaf area and biomass, with reduced root allocation and increased aboveground biomass. Elevated atmospheric CO2 also boosts leaf area and biomass but intensifies belowground allocation, depleting soil N and likely reducing N losses. Global leaf traits data confirm these findings, indicating that soil N availability influences leaf N content more than photosynthetic capacity. A demonstration model based on the functional balance hypothesis accurately predicts responses to N and CO2 fertilisation on tissue allocation, growth and biomass, offering a path to reduce uncertainty in global C cycle projections.

Global Ecosystem Nitrogen Cycling Reciprocates Between Land-Use Conversion and Its Reversal.

Anthropogenic land-use practices influence ecosystem functions and the environment. Yet, the effect of global land-use change on ecosystem nitrogen (N) cycling remains unquantified despite that ecosystem N cycling plays a critical role in maintaining food security. Here, we analysed 2430 paired observations globally to show that converting natural to managed ecosystems increases ratios of autotrophic nitrification to ammonium immobilisation and nitrate to ammonium, but decreases soil immobilisation of mineral N, causing increased N losses via leaching and gaseous N emissions, such as nitrous oxide (e.g., via denitrification), resulting in a leaky N cycle. Changing land use from intensively managed to one that resembles natural ecosystems reversed N losses by 108% on average, resulting in a more conservative N cycle. Structural equation modelling revealed that changes in soil organic carbon, pH and carbon to N ratio were more important than changes in soil moisture content and temperature in predicting ecosystem N retention capacities following land-use conversion and its reversion. The hotspots of leaky N cycles were mostly in equatorial and tropical regions, as well as in Western Europe, the United States and China. Our results suggest that whether an ecosystem exhibits a conservative N cycle after land-use reversion depends on management practices.

Symbiotic nitrogen fixation reduces belowground biomass carbon costs of nitrogen acquisition under low, but not high, nitrogen availability.

Many plant species form symbiotic associations with nitrogen-fixing bacteria. Through this symbiosis, plants allocate photosynthate belowground to the bacteria in exchange for nitrogen fixed from the atmosphere. This symbiosis forms an important link between carbon and nitrogen cycles in many ecosystems. However, the economics of this relationship under soil nitrogen availability gradients is not well understood, as plant investment toward symbiotic nitrogen fixation tends to decrease with increasing soil nitrogen availability. Here, we used a manipulation experiment to examine how costs of nitrogen acquisition vary under a factorial combination of soil nitrogen availability and inoculation with Bradyrhizobium japonicum in Glycine max L. (Merr.). We found that inoculation decreased belowground biomass carbon costs to acquire nitrogen and increased total leaf area and total biomass, but these patterns were only observed under low fertilization and were the result of increased plant nitrogen uptake and no change in belowground carbon allocation. These results suggest that symbioses with nitrogen-fixing bacteria reduce carbon costs of nitrogen acquisition by increasing plant nitrogen uptake, but only when soil nitrogen is low, allowing individuals to increase nitrogen allocation to structures that support aboveground growth. This pattern may help explain the prevalence of plants capable of forming these associations in less fertile soils and provides useful insight into understanding the role of nutrient acquisition strategy on plant nitrogen uptake across nitrogen availability gradients.

Differential Responses of Soil Nitrogen Forms to Climate Warming in Castanopsis hystrix and Quercus aliena Forests of China

Climate warming impacts soil nitrogen cycling in forest ecosystems, thus influencing their productivity, but this has not yet been sufficiently studied. Experiments commenced in January 2012 in a subtropical Castanopsis hystrix Hook. f. and Thomson ex A. DC. plantation and in May 2011 in a temperate Quercus aliena Blume forest, China. Four treatments were established comprising trenching, artificial warming (up to 2 °C), artificial warming + trenching, and untreated control plots. The plots were 2 × 3 m in size. In 2021 and 2022, soil nitrogen mineralization, soil nutrient availability, fine root biomass and microbial biomass were measured at 0–20 cm soil depth in 6 replicate plots per treatment. Warming significantly increased soil temperature in both forests. In the C. hystrix plantation, warming significantly increased available phosphorus (AP) and fine root biomass (FRB), but it did not affect soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass phosphorus (MBP) and their ratios. Warming depressed the net mineralization rate (NMR) and net ammonification rate (NAR) of the C. hystrix plantation, probably because the competition for nitrogen uptake by fine roots and microorganisms increased, thus decreasing substrates for nitrogen mineralization and ammonification processes. Trenching and warming + trenching increased the net nitrification rate (NNR), which might be related to decreased NH4+-N absorption of trees in the trenched plots and the increased microbial activity involved in soil nitrification. In the Q. aliena forest, warming significantly increased NH4+-N, MBC/MBN, Root C/N, Root N/P, and decreased pH, MBN, MBN/MBP and Root P; and there was no effect of trenching. Notably, the NAR, NNR and NMR were largely unaffected by long-term warming. We attributed this to the negative effect of increasing NH4+-N and decreasing MBN/MBP offsetting the positive effect of soil warming. This study highlights the vulnerability of subtropical forest stands to long-term warming due to decreased soil N mineralization and increased NO3−-N leaching. In contrast, the soil N cycle in the temperate forest was more resilient to a decade of continuous warming.

Species pool and local assembly processes drive β diversity of ammonia-oxidizing and denitrifying microbial communities in rivers along a latitudinal gradient.

Both regional species pool and local community assembly mechanism drive the microbial diversity patterns across geographical gradients. However, little has been done to separate their effects on the β diversity patterns of microbial communities involved in nitrogen (N) cycling in river ecosystems. Here, we use high-throughput sequencing of the archaeal amoA, bacterial amoA, nirK, and nirS genes, null model, and neutral community model to distinguish the relative importance of species pool and local assembly processes for ammonia-oxidizing and denitrifying communities in river wetlands along a latitudinal gradient in eastern China. Results indicated that the β diversity of the nirS-type denitrifying community co-varied with γ diversity and environmental heterogeneity, implying that regional species pool and heterogeneous selection explained variation in β diversity. However, the β diversity of ammonia-oxidizing and nirK-type denitrifying communities did not correlate with γ diversity and environmental heterogeneity. The continuous hump distribution of β deviation along the latitudinal gradient and the lower species dispersal rate indicated that the dispersal limitation shaped the variation in β diversity of ammonia-oxidizing and nirK-type denitrifying communities. Additionally, biotic interactions drove ammonia-oxidizing and nirS-type denitrifying communities by influencing species co-occurrence patterns. Our study highlights the importance of regional species pool and local community assembly processes in shaping geographical patterns of N-cycling microorganisms and extends knowledge of their adaptability to a continuously changing environment on a large scale.

Nitrogen Deposition Reduces the Rate of Leaf Litter Decomposition: A Global Study

The litter decomposition of plant leaves is a vital process in carbon (C) and nitrogen (N) cycling in global terrestrial ecosystems. However, previous assessments of the key determinants of the N deposition effects of litter decomposition have been more controversial. In this meta-analysis, we compared the overall effects of N addition on the litter decomposition rates, litter nutrient content (C and N), and litter constituent (lignin and cellulose) residual rates using a log response ratio approach. Our results showed that exogenous N addition increased the N content and inhibited lignin degradation in litter. N deposition decreased the leaf litter decomposition rate by increasing the lignin and N residues and decreasing the litter C content and soil pH. The analysis also concluded that the initial litter C/N ratio, lignin content, and soil pH were main factors in mediating the effect of N deposition on litter decomposition rate. Overall, the results of this study indicate that N deposition can slow decomposition rates by inhibiting N release and lignin degradation of litter. Notably, these results emphasize that the effect of N deposition on litter decomposition mainly depends on the endogenous quality of the litter and soil pH in the decomposition environment.

Response Characteristics and Community Assembly Mechanisms of nirS-Type Denitrifiers in the Alpine Wetland under Simulated Precipitation Conditions.

The nitrogen cycling process in alpine wetlands is profoundly affected by precipitation changes, yet the dynamic response mechanism of denitrifiers to long-term precipitation shifts in the alpine wetland of the Qinghai-Tibet Plateau remains enigmatic. Utilizing high-throughput sequencing analysis of nirS-type functional genes, this study delved into the dynamic response mechanism of nirS-type denitrifiers to precipitation changes in the alpine wetland of Qinghai Lake. The findings revealed that nirS-type denitrifiers in the alpine wetland of Qinghai Lake were primarily Proteobacteria, and Alpha diversity exhibited a negative correlation with the precipitation gradient, with deterministic processes predominating in the community assembly of denitrifying microbes. A 50% increase in rainfall shifted the community assembly process of denitrifiers from deterministic to stochastic. Dominant microflora at the genus level responded significantly to precipitation changes, with aerobic bacteria comprising the majority of differentially abundant taxa (55.56%). As precipitation increased, the complexity of the microbial interaction network decreased, and a 25% reduction in precipitation notably elevated the relative abundance of three key functional groups: chemoheterotrophic, aerobic chemoheterotrophic, and nitrogen fixation. Precipitation notably emerged as the primary regulator of nirS-type denitrifiers in the alpine wetland of Qinghai Lake, accounting for 51% of the variation in community composition. In summary, this study offers a fresh perspective for investigating the ecological processes of nitrogen cycling in alpine ecosystems by examining the diversity and community composition of nirS-type denitrifiers in response to precipitation changes.

Enterobacter sp. HIT-SHJ4 isolated from wetland with carbon, nitrogen and sulfur co-metabolism and its implication for bioremediation

Both autotrophic and heterotrophic denitrification are known as important bioprocesses of microbe-mediated nitrogen cycle in natural ecosystems. Actually, mixotrophic denitrification co-driven by organic matter and reduced sulfur substances are also common, especially in hypoxic environments such as estuarine sediments. However, carbon, nitrogen and sulfur co-metabolism during mixotrophic denitrification in natural water ecosystems has rarely been reported in detail. Therefore, this study investigated the co-metabolism of carbon, nitrogen and sulfur using samples collected from four distinct natural water ecosystems. Results demonstrated that samples from various sources all exhibited the ability for co-metabolism of carbon, nitrogen and sulfur. Microbial community analysis showed that Pseudomonas and Paracoccus were dominant bacteria ranging from 65.6% to 75.5% in mixotrophic environment. Enterobacter sp. HIT-SHJ4, a mixotrophic denitrifying strain which owned the capacity for co-metabolism of carbon, nitrogen and sulfur, was isolated and reported here for the first time. The strain preferred methanol as its carbon source and demonstrated remarkable efficiency for removing sulfide and nitrate with below 100 mg/L sulfide. Under weak acid conditions (pH 6.5–7.0), it exhibited enhanced capability in converting sulfide to elemental sulfur. Its bioactivity was evident within a temperature from 25 °C to 40 °C and C/N ratios from 0.75 to 3. This study confirmed the widespread presence of microbial-mediated synergistic carbon, nitrogen and sulfur metabolism in natural aquatic ecosystems. HIT-SHJ4 emerges as a novel strain, shedding light on carbon, nitrogen and sulfur co-metabolism in natural water bodies. Furthermore, it also serves as a promising candidate microorganism for in-situ ecological remediation, particularly in dealing with contamination posed by nitrate, sulfide, and organic matter.

Aridity threshold for alpine soil nitrogen isotope signature and ecosystem nitrogen cycling.

Determination of tipping points in nitrogen (N) isotope (δ15N) natural abundance, especially soil δ15N, with increasing aridity, is critical for estimating N-cycling dynamics and N limitation in terrestrial ecosystems. However, whether there are linear or nonlinear responses of soil δ15N to increases in aridity and if these responses correspond well with soil N cycling remains largely unknown. In this study, we investigated soil δ15N and soil N-cycling characteristics in both topsoil and subsoil layers along a drought gradient across a 3000-km transect of drylands on the Qinghai-Tibetan Plateau. We found that the effect of increasing aridity on soil δ15N values shifted from negative to positive with thresholds at aridity index (AI) = 0.27 and 0.29 for the topsoil and subsoil, respectively, although soil N pools and N transformation rates linearly decreased with increasing aridity in both soil layers. Furthermore, we identified markedly different correlations between soil δ15N and soil N-cycling traits above and below the AI thresholds (0.27 and 0.29 for topsoil and subsoil, respectively). Specifically, in wetter regions, soil δ15N positively correlated with most soil N-cycling traits, suggesting that high soil δ15N may result from the "openness" of soil N cycling. Conversely, in drier regions, soil δ15N showed insignificant relationships with soil N-cycling traits and correlated well with factors, such as soil-available phosphorus and foliage δ15N, demonstrating that pathways other than typical soil N cycling may dominate soil δ15N under drier conditions. Overall, these results highlight that different ecosystem N-cycling processes may drive soil δ15N along the aridity gradient, broadening our understanding of N cycling as indicated by soil δ15N under changing drought regimes. The aridity threshold of soil δ15N should be considered in terrestrial N-cycling models when incorporating 15N isotope signals to predict N cycling and availability under climatic dryness.

The pivotal role of phosphorus level gradient in regulating nitrogen cycle in wetland ecosystems

Phosphorus (P) is one of key drivers in Earth's nitrogen (N) cycle, however, the global overview of the P-regulated microbial community structure and gene abundance carrying wetland N process remains to be investigated. The key environmental factors that influenced wetland N cycle were initially screened, verifying the central role P. More complex and stable community interaction can be established in rich (20 mg/kg < P ≤ 100 mg/kg) and surplus P groups (P > 100 mg/kg) compared to that in deficient P group (P ≤ 20 mg/kg), with enhanced participation of betaproteobacteria and actinobacteria (i.e., changed hub microorganisms). Accordingly, P-mediated variations in gene expression patterns can be expected. On the one hand, the gene responses to carbon (C), N, and P factors presented nearly synchronous variation, highlighting the potential C-N-P coupling cycle in wetland ecosystem. On the other hand, the gene sensitivity towards environmental factors was changed at different P levels. Overall, the P level gradient can influence N cycle in direct (i.e., influences on gene abundances) and indirect (i.e., influences on gene response to environmental factors) manners. These findings provide important insights for controlling the N cycle in wetland ecosystems, particularly in cases where P levels are limiting factors.

Nitrate Chemodenitrification by Iron Sulfides to Ammonium under Mild Conditions and Transformation Mechanism.

Autotrophic denitrification utilizing iron sulfides as electron donors has been well studied, but the occurrence and mechanism of abiotic nitrate (NO3-) chemodenitrification by iron sulfides have not yet been thoroughly investigated. In this study, NO3- chemodenitrification by three types of iron sulfides (FeS, FeS2, and pyrrhotite) at pH 6.37 and ambient temperature of 30 °C was investigated. FeS chemically reduced NO3- to ammonium (NH4+), with a high reduction efficiency of 97.5% and NH4+ formation selectivity of 82.6%, but FeS2 and pyrrhotite did not reduce NO3- abiotically. Electrochemical Tafel characterization confirmed that the electron release rate from FeS was higher than that from FeS2 and pyrrhotite. Quenching experiments and density functional theory calculations further elucidated the heterogeneous chemodenitrification mechanism of NO3- by FeS. Fe(II) on the FeS surface was the primary site for NO3- reduction. FeS possessing sulfur vacancies can selectively adsorb oxygen atoms from NO3- and water molecules and promote water dissociation to form adsorbed hydrogen, thereby forming NH4+. Collectively, these findings suggest that the NO3- chemodenitrification by iron sulfides cannot be ignored, which has great implications for the nitrogen, sulfur, and iron cycles in soil and water ecosystems.

Land use modified impacts of global change factors on soil microbial structure and function: A global hierarchical meta-analysis

Nitrogen cycling in terrestrial ecosystems is critical for biodiversity, vegetation productivity and biogeochemical cycling. However, little is known about the response of functional nitrogen cycle genes to global change factors in soils under different land uses. Here, we conducted a multiple hierarchical mixed effects meta-analyses of global change factors (GCFs) including warming (W+), mean altered precipitation (MAP+/−), elevated carbon dioxide concentrations (eCO2), and nitrogen addition (N+), using 2706 observations extracted from 200 peer-reviewed publications. The results showed that GCFs had significant and different effects on soil microbial communities under different types of land use. Under different land use types, such as Wetland, Tundra, Grassland, Forest, Desert and Agriculture, the richness and diversity of soil microbial communities will change accordingly due to differences in vegetation cover, soil management practices and environmental conditions. Notably, soil bacterial diversity is positively correlated with richness, but soil fungal diversity is negatively correlated with richness, when differences are driven by GCFs. For functional genes involved in nitrification, eCO2 in agricultural soils and the interaction of N+ with other GCFs in grassland soils stimulate an increase in the abundance of the AOA-amoA gene. In agricultural soil, MAP+ increases the abundance of nifH. W+ in agricultural soils and N+ in grassland soils decreased the abundance of nifH. The abundance of the genes nirS and nirK, involved in denitrification, was mainly negatively affected by W+ and positively affected by eCO2 in agricultural soil, but negatively affected by N+ in grassland soil. This meta-analysis was important for subsequent research related to global climate change. Considering data limitations, it is recommended to conduct multiple long-term integrated observational experiments to establish a scientific basis for addressing global changes in this context.

Seawater intrusion effects on nitrogen cycling in the regulated Nakdong River Estuary, South Korea

Research on the impact of seawater intrusion on nitrogen (N) cycling in coastal estuarine ecosystems is crucial; however, there is still a lack of relevant research conducted under in-situ field conditions. The effects of elevated salinity on N cycling processes and microbiomes were examined in situ seawater intrusion experiments conducted from 2019 to 2021 in the Nakdong River Estuary (South Korea), where an estuarine dam regulates tidal hydrodynamics. After the opening of the Nakdong Estuary Dam (seawater intrusion event), the density difference between seawater and freshwater resulted in varying degrees of seawater trapping at topographically deep stations. Bottom-water oxygen conditions had been altered in normoxia, hypoxia, and weak hypoxia due to the different degrees of seawater trapping in 2019, 2020, and 2021, respectively. Denitrification mostly dominated the nitrate (NO3-) reduction process, except in 2020 after seawater intrusion. However, denitrification rates decreased because of reduced coupled nitrification after seawater intrusion due to the dissolved oxygen limitation in 2020. Dissimilatory nitrate reduction to ammonium (DNRA) rates immediately increased after seawater intrusion in 2020, replacing denitrification as the dominant pathway in the NO3- reduction process. The enhanced DNRA rate was mainly due to the abundant organic matter associated with seawater invasion and more reducing environment (maybe sulfide enhancement effects) under high seawater-trapping conditions. Denitrification increased in 2021 after seawater intrusion during weak hypoxia; however, DNRA did not change. Small seawater intrusion in 2019 caused no seawater trapping and overall normoxic condition, though a slight shift from denitrification to DNRA was observed. Metagenomic analysis revealed a decrease in overall denitrification-associated genes in response to seawater intrusion in 2019 and 2020, while DNRA-associated gene abundance increased. In 2021 after seawater intrusion, microbial gene abundance associated with denitrification increased, while that of DNRA did not change significantly. These changes in gene abundance align mostly with alterations in nitrogen transformation rates. In summary, ecological change effects in N cycling after the dam opening (N retention or release, that is, eutrophication deterioration or mitigation) depend on the degree of seawater intrusion and the underlying freshwater conditions, which constitute the extent of seawater-trapping.

Ammonium removal through anaerobic ammonium oxidation coupled to iron(III) reduction along the Yangtze river–estuary continuum

Anaerobic ammonium oxidation coupled to iron(III) reduction (Feammox) process has recently been recognized as an important pathway for removing ammonium in various natural habitats. However, our understanding for Feammox in river–estuary continuum is limited. In this study, stable isotope tracers and high-throughput amplicon sequencing were employed to determine Feammox rates and identify associated microbial communities in sediments along the Yangtze river–estuary continuum. Feammox rates averaged 0.0058 ± 0.0069 mg N/(kg·d) and accounted for approximately 22.3% of the ammonium removed from the sediments. Sediment Fe(III), ammonium (NH4+), total organic carbon (TOC), and pH were identified as important factors influencing Feammox rates. Additionally, Spirochaeta, Caldilineaceae_uncultured, and Ignavibacterium were found potentially associated with Feammox, which had not been documented as Feammox-associated microbial taxa previously. This study demonstrates that Feammox plays a vital role in ammonium removal within the Yangtze river–estuary continuum, providing greater insight into nitrogen removal and cycling in aquatic ecosystems.

Historical development, impact mechanism and future trends of nitrogen footprint in Wuxi City, China

Human activities have changed the biogeochemical cycle of nitrogen, leading to a large amount of reactive nitrogen (Nr) into the environment, aggravating a series of environmental problems, affecting human and ecosystem health. Cities are the core areas driving nitrogen cycling in terrestrial ecosystems, however, there are numerous influencing factors and their contributions are unclear. The nitrogen footprint is an important index to understand the impact of human activities on the environment, however, the calculation of urban nitrogen footprint needs a simplified and accurate system method. Here we use a nitrogen footprint calculation model at the urban system level based on system nitrogen balance, and a multi-factor extended STIRPAT (stochastic impact by regression on population, affluence, and technology) model suitable for analyzing the impact mechanism of nitrogen footprint to estimate nitrogen footprint of Wuxi City during 1990–2050. We find that: (1) from 1990 to 2020, the total nitrogen footprint of Wuxi City was in an increasing trend, but the per capita nitrogen footprint was in a decreasing trend. The per capita nitrogen footprint of 22.36 kg capita−1 in 2020 was at a lower level globally. (2) Nr discharge from fossil fuel combustion and Haber-Bosch nitrogen fixation accounted for the main proportion of nitrogen footprint. (3) Dietary choice (Ad), GDP per capita (Ag), urbanization rate (Au), population (P), and fossil energy productivity (Te) were the key factors contributing to the increase of the nitrogen footprint, which resulted in an annual increase of 1.39 %. While nitrogen footprint productivity (Tn), nitrogen use efficiency in crop farming (Tc), and nitrogen use efficiency in animal breeding (Ta) were the key inhibit factors that inhibit the increase of nitrogen footprint, and these factors slow down the annual growth rate of nitrogen footprint by 0.39 %. (4) The continuous growth of nitrogen footprint in the baseline and population growth scenarios will bring more environmental problems and greater environmental governance pressure to Wuxi City, while the sustainable scenario that includes comprehensive means such as economic adaptation and technological improvement is more in line with the requirements of high-quality development in China. Several mitigation measures are then proposed by considering Wuxi's realities from both key impact factors and potential for nitrogen footprint reduction in different scenarios, which can provide valuable policy insights to other cities, especially lakeside cities to mitigate nitrogen footprint.

Simultaneous methane mitigation and nitrogen removal by denitrifying anaerobic methane oxidation in lake sediments

Methane (CH4) is a potent greenhouse gas, with lake ecosystems significantly contributing to its global emissions. Denitrifying anaerobic methane oxidation (DAMO) process, mediated by NC10 bacteria and ANME-2d archaea, links global carbon and nitrogen cycles. However, their potential roles in mitigating methane emissions and removing nitrogen from lake ecosystems remain unclear. This study explored the spatial variations in activities of nitrite- and nitrate-DAMO and their functional microbes in Changdanghu Lake sediments (Jiangsu Province, China). The results showed that although the average abundance of ANME-2d archaea (5.0 × 106 copies g−1) was significantly higher than that of NC10 bacteria (2.1 × 106 copies g−1), the average potential rates of nitrite-DAMO (4.59 nmol 13CO2 g−1 d−1) and nitrate-DAMO (5.01 nmol 13CO2 g−1 d−1) showed no significant difference across all sampling sites. It is estimated that nitrite- and nitrate-DAMO consumed approximately 6.46 and 7.05 mg CH4 m−2 d−1, respectively, which accordingly achieved 15.07–24.95 mg m−2 d−1 nitrogen removal from the studied lake sediments. Statistical analyses found that nitrite- and nitrate-DAMO activities were both significantly related to sediment nitrate contents and ANME-2d archaeal abundance. In addition, NC10 bacterial and ANME-2d archaeal community compositions showed significant correlations with sediment organic carbon content and water depth. Overall, this study underscores the dual roles of nitrite- and nitrate-DAMO processes in CH4 mitigation and nitrogen elimination and their key environmental impact factors (sediment organic carbon and inorganic nitrogen contents, and water depth) in shallow lake, enhancing the understanding of carbon and nitrogen cycles in freshwater aquatic ecosystems.

Disinfectant-induced ammonia oxidation disruption in microbial N-cycling process in aquatic ecosystem after the COVID-19 outbreak

Anthropogenic activities significantly impact the elemental cycles in aquatic ecosystems, with the N-cycling playing a critical role in potential nutrient turnover and substance cycling. We hypothesized that measures to prevent COVID-19 transmission profoundly altered the nitrogen cycle in riverine ecosystems. To investigate this, we re-analyzed metagenomic data and identified 60 N-cycling genes and 21 host metagenomes from four urban reaches (one upstream city, Wuhan, and two downstream cities) along the Yangtze River. Our analyses revealed a marked decrease in the abundance of bacterial ammonia monooxygenase genes, as well as in the host, ammonia-oxidizing autotrophic Nitrosomonas, followed by a substantial recovery post-pandemic. We posited that discharge of sodium hypochlorite (NaOCl) disinfectant may be a primary factor in the reduction of N-cycling process. To test this hypothesis, we exposed pure cultures of Nitrosomonas europaea to NaOCl to explore the microbial stress response. Results indicated that NaOCl exposure rapidly compromised the cell structure and inhibited ammonia oxidation of N. europaea, likely due to oxidative stress damage and reduced expression of nitrogen metabolism-related ammonia monooxygenase. Using the functional tagging technique, we determined that NaOCl directly destroyed the ammonia monooxygenase protein and DNA structure. This study highlights the negative impacts of chlorine disinfectants on the function of aquatic ecosystems and elucidates potential mechanisms of action.

Paucibacter sediminis sp. nov., isolated from sediment in a freshwater pond.

A Gram-stain-negative, aerobic, oxidase-positive, weakly catalase-positive, motile by means of a single polar flagellum, rod-shaped bacterium designated as strain S2-9T was isolated from sediment sampled in Wiyang pond, Republic of Korea. Growth of this strain was observed at 10-40 °C (optimum, 35 °C) and pH 5.5-9.5 (optimum, pH 7.0-8.0) and in the presence of 0-0.5 % NaCl in Reasoner's 2A broth. The major fatty acids (>10 %) of strain S2-9T were C16 : 0 and summed feature 3 (comprising a mixture of C16 : 1 ω7c and/or C16 : 1 ω6c). Ubiquinone-8 was detected as the respiratory quinone. The major polar lipids were phosphatidylethanolamine and phosphatidylglycerol. Strain S2-9T showed the highest 16S rRNA gene sequence similarity to Paucibacter oligotrophus CHU3T (98.7 %), followed by 'Paucibacter aquatile' CR182 (98.4 %), all type strains of Pelomonas species (98.1-98.3 %), Mitsuaria chitosanitabida NBRC 102408T (97.9 %), Kinneretia asaccharophila KIN192T (97.8 %), Mitsuaria chitinivorans HWN-4T (97.4 %), and Paucibacter toxinivorans 2C20T (97.4 %). Phylogenetic trees based on the 16S rRNA gene and whole-genome sequences showed that strain S2-9T formed a tight phylogenetic lineage with Paucibacter species (CHU3T, CR182, and 2C20T). Average nucleotide identity and digital DNA-DNA hybridization values between strain S2-9T and Paucibacter strains were 76.6-79.3% and 19.5-21.5 %, respectively. The genomic DNA G+C content of strain S2-9T was 68.3 mol%. Notably, genes responsible for both sulphur oxidation and reduction and denitrification were found in the genome of strain S2-9T, suggesting that strain S2-9T is involved in the nitrogen and sulphur cycles in pond ecosystems. Based on the polyphasic taxonomic results, strain S2-9T represents a novel species of the genus Paucibacter, for which the name Paucibacter sediminis sp. nov. is proposed. The type strain is S2-9T (= KACC 22267T= JCM 34541T).

Mitigated N2O emissions from submerged-plant-covered aquatic ecosystems on the Changjiang River Delta

Submerged plants affect nitrogen cycling in aquatic ecosystems. However, whether and how submerged plants change nitrous oxide (N2O) production mechanism and emissions flux remains controversial. Current research primarily focuses on the feedback from N2O release to variation of substrate level and microbial communities. It is deficient in connecting the relative contribution of individual N2O production processes (i.e., the N2O partition). Here, we attempted to offer a comprehensive understanding of the N2O mitigation mechanism in aquatic ecosystems on the Changjiang River Delta according to stable isotopic techniques, metagenome-assembly genome analysis, and statistical analysis. We found that the submerged plant reduced 45 % of N2O emissions by slowing down the dissolved inorganic nitrogen conversion velocity to N2O in sediment (Vf-[DIN]sed). It was attributed to changing the N2O partition and suppressing the potential capacity of net N2O production (i.e., nor/nosZ). The dominated production processes showed a shift with increasing excess N2O. Meanwhile, distinct shift thresholds of planted and unplanted habitats reflected different mechanisms of stimulated N2O production. The hotspot zone of N2O production corresponded to high nor/nosZ and unsaturated oxygen (O2) in unplanted habitat. In contrast, planted habitat hotspot has lower nor/nosZ and supersaturated O2. O2 from photosynthesis critically impacted the activities of N2O producers and consumers. In summary, the presence of submerged plants is beneficial to mitigate N2O emissions from aquatic ecosystems.

A theory for context-dependent effects of mammalian trampling on ecosystem nitrogen cycling.

Large mammalian herbivores substantially impact ecosystem functioning. As their populations are dramatically altered globally, disentangling their consumptive and non-consumptive effects is critical to advance mechanistic understanding and improve prediction of effects over ecosystem and Earth-system spatial extents. Mathematical models have played an important role in clarifying potential mechanisms of herbivore zoogeochemistry, based mostly on their consumptive effects as primary consumers and recyclers of organic and inorganic matter via defecation and urination. Trampling is a ubiquitous effect among walking vertebrates, but the consequences and potential mechanisms of trampling in diverse environments remain poorly understood. We derive a novel mathematical model of large mammalian herbivore effects on ecosystem nitrogen cycling, focusing on how trampling and environmental context impact soil processes. We model herbivore trampling with a linear positive or negative additive effect on soil-mediated nitrogen cycling processes. Combining analytical and numerical analyses, we find trampling by large mammalian herbivores is likely to decrease nitrogen mineralisation rate across diverse environments, such as temperate grassland and boreal forest. These effects are mediated by multiple potential mechanisms, including trampling-induced changes to detritivore biomass and functioning (e.g. rate of organic matter consumption). We also uncover scenarios where trampling can increase nitrogen mineralisation rate, contingent on the environment-specific relative sensitivity of detritivore mineral-nitrogen release and detritivore mortality, to trampling. In contrast to some consumptive mechanisms, our results suggest the pace of soil nitrogen cycling prior to trampling has little influence over the direction of the trampling net effect on nitrogen mineralisation, but that net effects may be greater in slow-cycling systems (e.g. boreal forests) than in fast-cycling systems (e.g. grasslands). Our model clarifies the potential consequences of previously overlooked mechanisms of zoogeochemistry that are common to all terrestrial biomes. Our results provide empirically testable predictions to guide future progress in empirical and theoretical studies of herbivore effects in diverse environmental contexts. Resolving ecological contingencies around animal consumptive and non-consumptive effects will improve whole-ecosystem management efforts such as restoration and rewilding.