Biogeochemical Cycles Of Nitrogen Research Articles

Variations of Soil Cyanobacteria Communities Accompanied by Different Habitat Types in Abandoned Ion-Absorbed Rare Earth Tailings

Soil fertility declined, companied with the poor capacity of water holding, after rare earth mining for a long time, mine tailings remediation produced in conventional rare earth elements REEs exploitation are currently urgent issues. Cyanobacteria are prokaryotic oxygenic phototrophs, played a vital role in the biogeochemical cycling of carbon (C) and nitrogen (N), reducing soil loss, are the pioneer organisms of biological soil crusts BSCs. However, studies on cyanobacteria inoculation in abandoned ionic rare earth mines were still insufficient. The abundance and composition of microorganisms in the topsoil of abandoned ionic rare earth mines were analyzed using high-throughput sequencing of the 16S rRNA V4 gene in this study. Fifteen samples were selected in and around rare earth mine tailings based on five different habitat types to assess the difference of main bacteria component and the dominant cyanobacteria caused by environmental factors. A total of 713,057 effective 16S rRNA genes were classified into 30 bacteria phyla, and 7 cyanobacterial genera were determined in phyla of Cyanobacteria. Proteobacteria, Chloroflexi, Acidobacteria, Actinobacteria, Planctomycetes, and Cyanobacteria were dominant groups in all samples (>5% of total effective sequences). Only Microcoleus, Leptolyngbya, Microcystis, Stigonema, Chroococcidiopsis, Phormidium, and Lyngbya were determined in rare earth mine tailings. Leptolyngbya was wildly distributed in rare earth tailings except for natural woodland, while Microcoleus was found in all of the studying areas in this study. Mining activities could cause the amount of unknown cyanobacteria specie pnr_Chloroplast abnormal. Additionally, results showed that cyanobacteria community composition was not correlated with soil organic matter SOM, but cyanobacteria richness and diversity were limited by the high content of ammonia nitrogen, and Leptolyngbya and Microcoleus could apply to abandoned ion-absorbed rare earth mining tailings repairment.

Nitrogen atmospheric deposition driven by seasonal processes in a Brazilian region with agricultural background.

Understanding the seasonal patterns and influencing factors of nitrogen atmospheric deposition is essential to evaluate human impacts on the air quality and nitrogen biogeochemical cycle. However, evaluation of the nitrogen deposition flux, especially in South America agricultural regions, has not been fully investigated. In this paper, we quantified the atmospheric wet deposition fluxes of total dissolved nitrogen (TDN), dissolved organic nitrogen (DON), and dissolved inorganic nitrogen (DIN), in a region with agricultural and livestock predominance in the Southern Minas Gerais region, Brazil, from May 2018 to April 2019. Deposition fluxes of nitrogen species in the wet season (October-March) were on average 4.8-fold higher than those in the dry season, which revealed significant seasonal variations driven largely by the seasonality of rainfall and agricultural operations. We also found high NO3-/NH4+ ratios (average = 8.25), with higher values in dry season (NO3-/NH4+ = 12.8) in comparison with wet season (NO3-/NH4+ = 4.48), which revealed a higher relative contribution of NOx emissions from traffic sources in dry season. We also estimated the influence of atmospheric deposition of inorganic nitrogen (N-DIN) on environmental ecosystems, being 2.01 kgNha-1year-1 with potential risk of acidification and eutrophication of 30%. Therefore, attention should be paid to the role of wet atmospheric deposition of nitrogen as a source of nitrogen environmental pollution in agricultural regions.

Lake stability and anoxia dynamics revealed from high frequency vertical profiling in a eutrophic polymictic reservoir

ABSTRACT High-frequency water quality monitoring is rapidly growing in freshwater research and management. Sensor deployments at fixed depths in naturally formed lakes dominate the current understanding of ecosystem dynamics, with comparatively few studies focusing on reservoirs. Here we examined how environmental parameters affect the water column stability and dissolved oxygen (DO) dynamics in a North Texas eutrophic reservoir. We used an autonomous water quality monitoring profiler that recorded temperature, DO, and pH as well as wind speed, wind direction, and air temperature from an on-platform mounted weather station. Data were collected every 2 h from 25 April to 29 October 2019, with each profile spanning from the surface to 10 m depth at 0.5 m steps. Throughout most of the sampling period, the lake had no distinct thermocline, but we observed an oxycline throughout most of the monitoring period, with multiple mixing events. Near the sediments, periods of anoxia lasting hours to days were first observed in May, and July through September were chronically anoxic. In August and September, DO was frequently <2.0 mg L−1 throughout the entire water column, with 2 periods of total water column anoxia. Surface DO measurements for each 2 h interval were highly synchronous with Schmidt stability. During decreasing and low Schmidt stability or Lake Number, the oxycline destabilized in association with changes in wind direction and speed. Our results indicate that DO can be highly dynamic in polymictic reservoirs, with strong implications for carbon and nitrogen biogeochemical cycles.

Seasonal source identification and formation processes of marine particulate water soluble organic nitrogen over an offshore island in the East China Sea

Water soluble organic nitrogen (WSON) had great influences on the aerosol chemistry, hygroscopicity, marine primary productivity, as well as nitrogen biogeochemical cycles. Aerosol sampling was conducted over an offshore island in the East China Sea in four seasons of 2019, aiming to reveal the seasonal sources and secondary formation processes of marine WSON. The annual mean WSON concentration reached 1.05 ± 1.72 μg/m3 with a mean WSON/WSTN fraction of 27 %. In spring, WSON was associated with combustion emissions. The liquid-phase reaction of NH3/NH4+ with VOCs was a potential secondary formation process of WSON. In summer, WSON was mainly formed through the gaseous phase oxidation of marine biogenic precursors. In autumn, WSON showed miscellaneous sources from agricultural activities, biomass burning, and fossil fuel combustion. In addition to the contribution from primary urea, WSON could be also affected by the oxidation of biological proteinaceous matters. This explained the highest WSON concentrations and WSON/WSOC ratios in autumn. In winter, WSON was probably emitted from sea spray aerosols via the bubble-bursting processes. This study indicated that the sources of WSON over the coastal waters in the East China Sea were quite diverse, highlighting the need of more detailed characterization of marine WSON at the molecular level.

Influence of the Interdomain Interface on Structural and Redox Properties of Multiheme Proteins.

Multiheme proteins are important in energy conversion and biogeochemical cycles of nitrogen and sulfur. A diheme cytochrome c4 (c4) was used as a model to elucidate roles of the interdomain interface on properties of iron centers in its hemes A and B. Isolated monoheme domains c4-A and c4-B, together with the full-length diheme c4 and its Met-to-His ligand variants, were characterized by a variety of spectroscopic and stability measurements. In both isolated domains, the heme iron is Met/His-ligated at pH 5.0, as in the full-length c4, but becomes His/His-ligated in c4-B at higher pH. Intradomain contacts in c4-A are minimally affected by the separation of c4-A and c4-B domains, and isolated c4-A is folded. In contrast, the isolated c4-B is partially unfolded, and the interface with c4-A guides folding of this domain. The c4-A and c4-B domains have the propensity to interact even without the polypeptide linker. Thermodynamic cycles have revealed properties of monomeric folded isolated domains, suggesting that ferrous (FeII), but not ferric (FeIII) c4-A and c4-B, is stabilized by the interface. This study illustrates the effects of the interface on tuning structural and redox properties of multiheme proteins and enriches our understanding of redox-dependent complexation.

Competition between Ag+ and Ni2+ in nickel enzymes: Implications for the Ag+ antibacterial activity

Silver's antimicrobial properties have been known for centuries, but exactly how it kills bacteria is still a mystery. Information on the competition between the native Ni2+ and abiogenic Ag+ cations in bacterial systems is also critically lacking. For example, urease, a famous nickel-containing enzyme that hydrolyzes urea into carbon dioxide and ammonia (a key step in the biogeochemical nitrogen cycle on Earth), is inhibited by Ag+ cations, but the molecular mechanism of silver’s action is poorly understood. By employing density functional theory (DFT) calculations combined with the polarizable continuum model (PCM) computations we assess the susceptibility of the mono/binuclear Ni2+ binding sites in the nickel enzymatic centers to Ni2+→Ag+ substitution. The active centers in the mononuclear glyoxalase I and acireductone dioxygenase enzymes appear to be well protected against Ag+ attack and, presumably, stay functional even in its presence. On the other hand, the binuclear nickel binding site in urease appears vulnerable to silver attack - the results obtained are in line with available experimental data and give reason to assume a possible substitution of the essential Ni2+ cation from the urease metal center by Ag+.

Influencing factors and significance of organic and inorganic nitrogen isotopic compositions in lacustrine sedimentary rocks

Comprehensive nitrogen biogeochemical cycle has been reconstructed for representative lacustrine organic-rich sedimentary rock in China, namely the Triassic Yanchang Formation (YF, 199–230 Ma) in Ordos and the Cretaceous Qingshankou Formation (QF, 86–92 Ma) in Songliao basins, by evaluating the organic and inorganic nitrogen isotopic compositions rather than only organic or bulk nitrogen isotopic compositions. The results indicate that the nitrogen isotope values of bulk rock (δ15Nbulk) in the non-metamorphic stage are significantly different from that of kerogen, which challenge the conceptual framework of sedimentary nitrogen isotope interpretation. The δ15Nbulk from the YF and QF were lower than their respective the nitrogen isotope values of kerogen (δ15Nker), with offsets up to ∼5.1‰, which have the inverse relationship for the metamorphosed rock. Thermal evolution did not significantly modify the δ15N of bulk rock and kerogen. The δ15N of sediments from the YF (δ15Nbulk, 1.6‰–5.6‰) were lower than that of rock from the QF (δ15Nbulk, 10.2‰–15.3‰). The nitrogen isotope values of silicate incorporated nitrogen (δ15Nsil) were slightly lower than those of the δ15Nker in the YF and obviously lower for the QF. The fact that different nitrogen cycles occur in the YF and QF due to the different depositional redox conditions leads to different isotopic results. The YF water environment dominated by oxic conditions is not conducive to the occurrence of denitrification and anammox, and no abundant N2 loss leads to the relatively light δ15Nbulk. In the stratified water for the QF, redox transition zone promotes denitrification and anammox, resulting in the heavy δ15Nbulk of rock and promotes the DNRA, resulting in heavy δ15Nker and low δ15Nsil.

Conversion of coastal wetland to aquaculture ponds decreased N2O emission: Evidence from a multi-year field study

Land reclamation is a major threat to the world's coastal wetlands, and it may influence the biogeochemical cycling of nitrogen in coastal regions. Conversion of coastal marshes into aquaculture ponds is common in the Asian Pacific region, but its impacts on the production and emission of nitrogen greenhouse gases remain poorly understood. In this study, we compared N2O emission from a brackish marsh and converted shrimp aquaculture ponds in the Shanyutan wetland, the Min River Estuary in Southeast China over a three-year period. We also measured sediment and porewater properties, relevant functional gene abundance, sediment N2O production potential and denitrification potential in the two habitats. Results indicated that the pond sediment had lower N-substrate availability, lower ammonia oxidation (AOA and comammox Nitrospira amoA), nitrite reduction (nirK and nirS) and nitrous oxide reduction (nosZ Ⅰ and nosZ Ⅱ) gene abundance and lower N2O production and denitrification potentials than in marsh sediments. Consequently, N2O emission fluxes from the aquaculture ponds (range 5.4–251.8 μg m–2 h–1) were significantly lower than those from the marsh (12.6–570.7 μg m–2 h–1). Overall, our results show that conversion from marsh to shrimp aquaculture ponds in the Shanyutan wetland may have diminished nutrient input from the catchment, impacted the N-cycling microbial community and lowered N2O production capacity of the sediment, leading to lower N2O emissions. Better post-harvesting management of pond water and sediment may further mitigate N2O emissions caused by the aquaculture operation.

Hillslope Geodiversity Impact on Biocrusts’ Biogeochemical Functions

Geodiversity integrates physical parameters such as geological, geomorphological, and pedological components. It represents the abiotic diversity of the earth surface layer. It incorporates geological (bedrocks and mineral sediments), geomorphological (geography, land surface formations, physical processes), pedological (soils), and hydrological characteristics. Biological soil crusts (biocrusts) play an essential role in regulating the biogeochemical cycles of carbon and nitrogen. Their ability is dependent on habitat conditions, composition, and cover percentage of the ground surface, all of which are affected by geodiversity. This study’s objective was to assess the effects of geodiversity on the biogeochemical functions of biocrusts by regulating the soil water dynamics and the subsequent impact on readily available nitrogen and carbon. Hillslope geodiversity is determined by the geodiversity found in the stone cover on the ground surface and in the stone content throughout the soil profile, as well as by the soil profile thickness of the underlying bedrock. We hypothesized that in dry environments, the physical conditions in high-geodiversity hillslopes, compared to low-geodiversity hillslopes, positively affect the soil water budget, which would affect the biocrusts and their readily available nitrogen and carbon. The results showed higher soil moisture content in the heterogeneous hillslopes. The ammonium and labile organic carbon in the biocrusts were more substantial in the heterogeneous than in the homogeneous hillslopes, while soil protein, nitrite, and soil organic matter were similar. We suggest that the comparatively high soil moisture content in the heterogeneous hillslopes stimulates biocrust community activities and increases the readily available nitrogen and carbon, thus improving the survival of shrubs in these ecosystems under long-term drought conditions.

Insights into ecological roles of uncultivated bacteria in Katase hot spring sediment from long-read metagenomics.

Diverse yet-uncultivated bacteria and archaea, i.e., microbial dark matter, are present in terrestrial hot spring environments. Numerous metagenome-assembled genomes (MAGs) of these uncultivated prokaryotes by short-read metagenomics have been reported so far, suggesting their metabolic potential. However, more reliable MAGs, i.e., circularized complete MAGs (cMAGs), have been rarely reported from hot spring environments. Here, we report 61 high-quality (HQ)-MAGs, including 14 cMAGs, of diverse uncultivated bacteria and archaea retrieved from hot spring sediment (52°C, pH 7.2) by highly accurate long-read sequencing using PacBio Sequel II. The HQ MAGs were affiliated with one archaeal and 13 bacterial phyla. Notably, nine of the 14 cMAGs were the first reported cMAGs for the family- to class-level clades that these cMAGs belonged to. The genome information suggests that the bacteria represented by MAGs play a significant role in the biogeochemical cycling of carbon, nitrogen, iron, and sulfur at this site. In particular, the genome analysis of six HQ MAGs including two cMAGs of Armatimonadota, of which members are frequently abundant in hot spring environments, predicts that they are aerobic, moderate thermophilic chemoorganoheterotrophs, and potentially oxidize and/or reduce iron. This prediction is consistent with the environmental conditions where they were detected. Our results expand the knowledge regarding the ecological potential of uncultivated bacteria in moderately-high-temperature environments.

First insights into the prokaryotic community structure of Lake Cote, Costa Rica: Influence on nutrient cycling.

Prokaryotic diversity in lakes has been studied for many years mainly focusing on community structure and how the bacterial assemblages are driven by physicochemical conditions such as temperature, oxygen, and nutrients. However, little is known about how the composition and function of the prokaryotic community changes upon lake stratification. To elucidate this, we studied Lake Cote in Costa Rica determining prokaryotic diversity and community structure in conjunction with physicochemistry along vertical gradients during stratification and mixing periods. Of the parameters measured, ammonium, oxygen, and temperature, in that order, were the main determinants driving the variability in the prokaryotic community structure of the lake. Distinct stratification of Lake Cote occurred (March 2018) and the community diversity was compared to a period of complete mixing (March 2019). The microbial community analysis indicated that stratification significantly altered the bacterial composition in the epi-meta- and hypolimnion. During stratification, the Deltaproteobacteria, Chloroflexi, Bacteroidetes, Nitrospirae, and Euryarchaeota were dominant in the hypolimnion yet largely absent in surface layers. Among these taxa, strict or facultative anaerobic bacteria were likely contributing to the lake nitrogen biogeochemical cycling, consistent with measurements of inorganic nitrogen measurements and microbial functional abundance predictions. In general, during both sampling events, a higher abundance of Alphaproteobacteria, Betaproteobacteria, Actinobacteria, and Cyanobacteria was found in the oxygenated layers. Lake Cote had a unique bacterial diversity, with 80% of Amplicon Sequence Variant (ASV) recovered similar to unclassified/uncultured strains and exhibits archetypal shallow lake physicochemical but not microbial fluctuations worthy of further investigation. This study provides an example of lake hydrodynamics impacts to microbial community and their function in Central American lakes with implications for other shallow, upland, and oligotrophic lake systems.

Microplastics drive nitrification by enriching functional microorganisms in aquaculture pond waters

The plastisphere refers to biofilm formation on the microplastic (MP) surface, but its subsequent functions, especially driving the nitrogen biogeochemical cycle, are rarely studied. Here, MPs were incubated in the pelagic water and benthic water-sediment interface of an aquaculture pond, and the two corresponding microcosms amended with incubated plastisphere were simulated. The results showed decreased ammonia concentrations and increased nitrification rates in microcosms with either pelagic or benthic plastispheres. To uncover the possible mechanisms, the community structure and function of the plastisphere were investigated. As clarified by 16S rRNA, the community diversity of the pelagic plastisphere was significantly higher than that of the corresponding hydrosphere. Plastisphere communities, especially those incubated in pelagic water, were separated from the hydrosphere. Moreover, the abundance of Proteobacteria increased while the abundance of Cyanobacteria decreased in both plastispheres. Metagenome further revealed that the abundance of amoA and annotated Nitrososphaeraceae_archaeon and hao and affiliated Nitrosomonas_europaea, which contributed to ammonia oxidation to nitrite, was higher in the benthic plastisphere. Comparing the pelagic plastisphere with the corresponding hydrosphere, however, the abundance of nxrA and annotated Nitrobacter hamburgensis and nxrB and the affiliated Nitrospira moscoviensis, which are involved in nitrite oxidation, was more abundant in the plastisphere. These findings suggest that the plastisphere might selectively enrich functional microorganisms and genes in a habitat-dependent manner to promote nitrification in aquaculture ponds.

Decoupled cycling of carbon, nitrogen, and phosphorus in a grassland soil along a hillslope mediated by clay and soil moisture

Grasslands extend across a variety of topographies including non-flat hilly areas with varied soil texture and moisture that can mediate soil biogeochemical cycling of carbon (C), nitrogen (N) and phosphorus (P). In this study we examined soil organic C, total N and P pools (both inorganic and organic), as well as gross N mineralisation (GNM, as a measure of microbial activity and N dynamics) and microbial C, N and P in winter and spring along a grassland hillslope. Our results showed that variation in soil clay and associated soil moisture were the most important mediators affecting C and N dynamics along the grassland elevational gradient. Total organic C pools were highest where the clay content was highest, likely because of the sorption capacity of clay stabilising organic matter against microbial decomposition. Likewise, the variation in soil organic N pools along the hillslope was mostly controlled by clay and soil moisture effects on microbial stabilisation of organic N. Furthermore, microbial C, N and GNM increased with increased soil moisture, suggesting that microbial activity was limited by soil moisture. In contrast, soil P pools were not related to clay and soil moisture, while soil organic P was decoupled from soil organic C and N pools along the hillslope. Possibly, differences in microbial stabilisation and mineralisation pathways (oxidation for C and N, hydrolysis for P), microbial homeostatic regulation of C, N and P, and P fixation resulted in a decoupling of P from C and N cycling in this grassland. We conclude that variation in clay and associated soil moisture along grassland hillslopes can have important but non-unidirectional effects on soil C, N and P cycling.

Description of five novel thermophilic species of the genus Thermus: Thermus hydrothermalis sp. nov., Thermus neutrinimicus sp. nov., Thermus thalpophilus sp. nov., Thermus albus sp. nov., and Thermus altitudinis sp. nov., isolated from hot spring sediments

Biological denitrification is a significant process in nitrogen biogeochemical cycle of terrestrial geothermal environments, and Thermus species have been shown to be crucial heterotrophic denitrifier in hydrothermal system. Five Gram-stain negative, aerobic and rod-shaped thermophilic bacterial strains were isolated from hot spring sediments in Tibet, China. Phylogenetic analysis based on 16S rRNA gene and whole genome sequences indicated that these isolates should be assigned to the genus Thermus and were most closely related to Thermus caldifontis YIM 73026T, and Thermus brockianus YS38T. Average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between the five strains and the type strains of the genus Thermus were lower than the threshold values (95% and 70%, respectively) recommended for bacterial species, which clearly distinguished the five isolates from other species of the genus Thermus and indicated that they represent independent species. Colonies are circular, convex, non-transparent. Cell growth occurred at 37–80 °C (optimum, 60–65 °C), pH 6.0–8.0 (optimum, pH 7.0) and with 0–2.0% (w/v) NaCl (optimum, 0–0.5%). Denitrification genes (narG, nirK, nirS, and norB genes) detected in their genomes indicated their potential function in nitrogen metabolism. The obtained results combined with those of morphological, physiological, and chemotaxonomic characteristics, including the menaquinones, polar lipids, and cellular fatty acids showed that the isolates are proposed as representing five novel species of the genus Thermus, which are proposed as Thermus hydrothermalis sp. nov. SYSU G00291T, Thermus neutrinimicus sp. nov. SYSU G00388T, Thermus thalpophilus sp. nov. SYSU G00506T, Thermus albus sp. nov. SYSU G00608T, Thermus altitudinis sp. nov. SYSU G00630T.

Rapid, sensitive analysis method for determining the nitrogen stable isotope ratio of total free amino acids in soil.

The amino acid-nitrogen (AA-N) isotope analysis of naturally abundant or isotope-labeled samples is indispensable for tracing nitrogen transfer in soil nitrogen biogeochemical cycling processes. Despite the usefulness of AA-N isotope analysis, the preparation methods are complex and time-consuming, and necessitate the use of toxic reagents. We present an improved, rapid method for AA-N isotope analysis with high precision. At a high pH, AA-N was released and oxidized to N2 O using ClO- under vacuum. Additionally, purge-and-trap isotope ratio mass spectrometry was used to analyze N2 O. Moreover, we investigated the effect of various factors on the N2 O conversion process with glycine and applied the results to seven representative single-N AAs (alanine, serine, cysteine, aspartic acid, glutamic acid, leucine, and phenylalanine) and five poly-N AAs (lysine, arginine, histidine, tryptophan, and asparagine), as well as side-chain analogs, blank reagent, and other N forms. The concentration of ClO- and the pH were determined to be crucial factors for achieving desirable AA-N to N2 O conversion efficiencies. Glycine-N had the highest N2 O yield of 70%, with isotopic results consistent with those of the reference values at a high precision (within 0.5‰ for natural abundance and 0.01 atom% for 15 N-enrichment) at the nanomolar N level. Additionally, the α-NH2 AAs were labile, and the single-N AAs were more easily converted to N2 O than poly-N AAs. With the exception of γ-aminobutyric acid, the N2 O conversion efficiencies of the side-chain N analogs were very low (below 5%). This method was also applicable to the 15 N analysis of the total free AAs in complex soil samples without interference from analytical blanks and other forms of N. Our method is highly selective for the α-NH2 groups of an amino acid, and the oxidation of the side chain is difficult. In addition, the method is sensitive, rapid, and convenient, and does not require toxic reagents.

Nitric Oxide, Nitric Oxide Formers and Their Physiological Impacts in Bacteria.

Nitric oxide (NO) is an active and critical nitrogen oxide in the microbe-driven nitrogen biogeochemical cycle, and is of great interest to medicine and the biological sciences. As a gas molecule prior to oxygen, NO respiration represents an early form of energy generation via various reactions in prokaryotes. Major enzymes for endogenous NO formation known to date include two types of nitrite reductases in denitrification, hydroxylamine oxidoreductase in ammonia oxidation, and NO synthases (NOSs). While the former two play critical roles in shaping electron transport pathways in bacteria, NOSs are intracellular enzymes catalyzing metabolism of certain amino acids and have been extensively studied in mammals. NO interacts with numerous cellular targets, most of which are redox-active proteins. Doing so, NO plays harmful and beneficial roles by affecting diverse biological processes within bacterial physiology. Here, we discuss recent advances in the field, including NO-forming enzymes, the molecular mechanisms by which these enzymes function, physiological roles of bacterial NOSs, and regulation of NO homeostasis in bacteria.

A long-term overview of nitrogen in tropical rainwater and wet deposition fluxes in southern Brazil

This work reports inorganic and organic nitrogen concentrations in rainwater and estimates wet deposition fluxes in an agro-industrial region of São Paulo State (Brazil) impacted by biomass burning and vehicular public policies, spanning the period from 2005 to 2020. The overall volume weighted mean (VWM) concentration of nitrate in rainwater was 13.1 ± 0.5 μmol L−1 (n = 696). Despite decreasing vehicular emissions of NOx over the study period, there was no clear trend of nitrate concentrations in rainwater, possibly due to biomass burning emission sources. The overall ammonium VWM concentration was 24.3 ± 1.1 (n = 696). In contrast to nitrate, there was a 94% increase of ammonium in 16 years, attributed to increased emissions from vehicles and fertilizer use, together with variable biomass burning emissions. Dissolved organic nitrogen (DON) concentrations in individual samples ranged from 0.2 to 153 μmol N L−1, corresponding to between 1 and 73% of the total dissolved nitrogen, with VWM of 18.9 ± 0.9 μmol N L−1 (n = 377). Amino acids corresponded, on average, to 15% of the DON. For the majority of the years, the VWM concentrations of nitrate, ammonium, and DON were higher for the pool of samples collected during the dry season, compared to the wet season. These results highlighted the importance of biomass burning sources of reactive nitrogen in drier months, when wildfires are common. The wet deposition flux of total dissolved nitrogen was ∼12 kg N ha−1 yr−1, with ammonium corresponding to 43%, followed by DON (33%) and nitrate (23%). These results demonstrated the importance of including DON when modeling the biogeochemical cycle of nitrogen, especially in a world where wildfires may become more frequent and intense.

Combined effects of damming and drought on nitrogen dynamics in an ephemeral river of North China

Damming and drought have dramatically altered water cycle dynamics in rivers, which further affects nitrogen biogeochemical cycles; however, their combined effects have rarely been studied, especially for ephemeral rivers with variable water flows. Taking the Dagu River of North China as the study area, we examined the spatiotemporal variability of nitrogen in river water and its relation to damming and drought. The total nitrogen (TN), dissolved inorganic nitrogen (DIN) and NO3‾ concentrations varied significantly in space and time, but the NH4+ and NO2‾ concentrations changed little. Spatially, compared with the upper reaches, the TN, DIN and NO3‾ concentrations in the lower reaches decreased by 71%, 81% and 85%, respectively, which was concurrent with the isotopic enrichment of nitrate. Temporally, their concentrations in dry seasons were more than twice of wet seasons, and increased from 2018 (normal year) to 2019 (dry year). The cascade dams may promote the NO3‾ removal towards the lower reaches through assimilation, which combined with nitrification and strong evaporation to enhance δ18O–NO3‾ enrichment and disproportionate δ15N–NO3‾/δ18O–NO3‾ ratio. Further, the damming effects can be aggravated by droughts because of no flow, extended nitrogen residence time and increased groundwater discharge with higher nitrogen contents, which jointly elevated the nitrogen level. Hence, the combined effects of damming and drought should be fully considered for management of water environment and resources in regions with ephemeral rivers.

Lewis acid-assisted reduction of nitrite to nitric and nitrous oxides via the elusive nitrite radical dianion.

Reduction of nitrite anions (NO2-) to nitric oxide (NO), nitrous oxide (N2O) and ultimately dinitrogen (N2) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (-0.74 V versus NHE). Detailed characterization confirms the formation of the borane-capped radical nitrite dianion (NO22-), which features a N(II) oxidation state. Protonation of the nitrite dianion results in the facile loss of nitric oxide (NO), whereas its reaction with NO results in disproportionation to nitrous oxide (N2O) and nitrite (NO2-). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO2- to NO.

Periphytic biofilms-mediated microbial interactions and their impact on the nitrogen cycle in rice paddies

Rice paddies are unique waterlogged wetlands artificially constructed for agricultural production. Periphytic biofilms (PBs) at the soil-water interface play an important role in rice paddies characterized by high nutrient input but low utilization efficiency. PBs are composed of microbial aggregates, including a wide variety of microorganisms (algae, bacteria, fungi, protozoa, and metazoa), extracellular polymeric substances and minerals (iron, aluminum, and calcium), which form an integrated food web and energy flux within a relatively stable micro-ecosystem. PBs are crucial to regulate and streamline the nitrogen cycle by neutralizing nitrogen losses and improving rice production since PBs can serve as both a sink by capturing surplus nitrogen and a source by slowly re-releasing this nitrogen for reutilization. Here the ecological advantages of PBs in regulating the nitrogen cycle in rice paddies are illustrated. We summarize the key functional importance of PBs, including the intricate and delicate community structure, microbial interactions among individual phylotypes, a wide diversity of self-produced organics, the active adaptation of PBs to constantly changing environments, and the intricate mechanisms by which PBs regulate the nitrogen cycle. We also identify the future challenges of microbial interspecific cooperation in PBs and their quantitative contributions to agricultural sustainability, optimizing nitrogen utilization and crop yields in rice paddies. ● Periphytic biofilms (PBs) are active microbial aggregates ubiquitously distributed in wetlands. ● PBs form a stable and intricate food web catalyzing biogeochemical cycling of soil nitrogen. ● PBs in rice paddies can improve nitrogen utilization efficiency by plants. ● PBs-mediated microbial interactions facilitate the nitrogen cycle cooperation among microorganisms, while self-produced organics and genomic adaptation are beneficial for the nitrogen cycle regulation by PBs.