Nitrogen Budget Research Articles

Nitrogen budgets for freshwater aquaculture and mariculture in a large tropical island – A case study for Hainan Island 1998–2018

Nitrogen is an essential nutrient in aquaculture. It is also an important factor in coastal and river eutrophication. We present an island-scale model to study the nitrogen flows in different aquaculture systems in Hainan Island during 1998–2018. The result indicated that nitrogen losses associated with pond sludge, wastewater discharge and gaseous emission increased by a factor of 1.4, 4.6 and 3.2, respectively. Sludge and wastewater account for 84% of the total losses to the environment. During the past 20 years, aquacultural yields and the nitrogen use efficiency (NUE) improved considerably in Hainan Island. Nevertheless, nitrogen losses to the environment increased significantly as well, with negative effects for local ecosystems. In the future, sustainable aquacultural practices are needed to improve NUE and to reduce nitrogen losses to the environment.

Distribution and Oxidation Rates of Ammonia-Oxidizing Archaea Influenced by the Coastal Upwelling off Eastern Hainan Island.

Coastal upwelling causes variations in temperature, salinity and inorganic nutrients in the water column, consequently leading to the shift of microbial populations and their metabolic activities. Impacts of the eastern Hainan upwelling (EHU) on the ammonia-oxidizing archaea (AOA) were investigated based on the amoA gene using pyrosequencing and quantitative PCR at both DNA and cDNA levels, together with the determination of the ammonia oxidation (AO) rate measured with 15N-labelled ammonium. By comparing stations with and without upwelling influence, we found that coastal upwelling correlated with an increase in amoA gene abundance, the dominance of distinct clades for AOA communities at the respective gene and transcript levels, and a large increase in the proportion of the SCM1-like (Nitrosopumilus maritimus-like) cluster as well. The AO rates were generally higher in the deeper water (~25 m), which was in significant positive correlation with the proportion of cluster Water Column A (WCA) at the transcript level, indicating the potential contribution of this cluster to in situ ammonia oxidization. Our study demonstrated that coastal upwelling had a significant impact on the AOA community and ammonia oxidization rate; therefore, this physical forcing should be considered in the future assessment of the global nitrogen budgets and biogeochemical nitrogen cycles.

Nutrient budgets for the Bohai Sea: Implication for ratio imbalance of nitrogen to phosphorus input under intense human activities

Eutrophication is a global problem for coastal ecosystems, one that the Bohai Sea (BHS), China, is severely afflicted by due to rapid economic and social development over the last forty years. For sustainable nutrients management in the BHS, comprehensive budgets for Nitrogen (N) and Phosphorus (P) was characterized in 2017, and the relative contributions of river input, submarine fresh groundwater discharge, atmospheric deposition, sediment diffusion, and exchange with the Yellow Sea were quantified. The annual N and P fluxes into the BHS were 362 × 103 t and 10.4 × 103 t, respectively. The terrigenous N inputs occupied the highest proportion, while the largest P input was from sediment diffusion. The ratio of N:P was 77 for total external inputs, while that of the Yellow River was 680; both exceeded the Redfield ratio, indicating an imbalance in the nutrient structure and a P limitation in the BHS.

Active Nitrogen Cycle Driven by Solar Radiation in Clean Desert Air

AbstractDeserts have been recognized as constantly releasing reactive nitrogen compounds to the atmosphere due to the subsoil‐storage leaching and surface‐deposit evaporation induced by solar radiative heating. However, this release and the consequent response of the nitrogen cycles in desert air have not been evaluated and are overlooked in most regional and global models. In our nitrate measurements, including nitric acid and particulate nitrate, in clean air, that is, in the absence of or with minor anthropogenic influences, in a Chinese desert, diurnal nitrate variations consisting of daytime increases and nighttime decreases were observed in the range of 1.3–3.7 μgm−3 in summer, 0.1–2.6 μgm−3 in spring, and 0.2–1.3 μgm−3 in autumn. These values are considerably higher than those observed in remote marine and forest areas. Simulations with a regional transport model demonstrate that the variation could be largely accounted for by the oxidation of soil‐emitted reactive nitrogen compounds by photochemical‐reactions products. These results indicate an active nitrogen cycle in clean desert air, which is the consequence of the coordination of solar radiation‐induced surface soil emission and subsequent photochemical reactions. Satellite data of NO2 further indicate possible similar cycles in most major desert areas worldwide. By including this nitrogen cycle in the vast desert areas and its subsequent effects, such as those on ozone chemistry, climate models may thus provide a better understanding of the global nitrogen budget in the historical and developing Earth ecosystems.

Nitrogen driven niche differentiation in bacterioplankton communities of northeast coastal Bay of Bengal

The Bay of Bengal receives nitrogen inputs from multiple sources and the potential role of nitrogen-metabolizing microbial communities in the surface water is not well understood. The nitrogen budget estimate shows a deficit of 4.7 ± 2.4 Tg N yr-1, suggesting a significant role of dissolved organic nitrogen remineralization in fuelling ecosystem processes. Unravelling the process of remineralization leading to increasing concentrations of dissolved inorganic nitrogen (DIN) in coastal ecosystems such as in mangroves require a better understanding of the composition of functional resident bacterioplankton communities. Bacterioplankton communities were elucidated from eight stations along different estuaries spanning west to east of northeast coastal Bay of Bengal to understand the influence of DIN on shaping these communities. The eight stations were differentiated into ‘low’ and ‘high’ DIN stations based on DIN concentration, with five stations with High DIN concentration (>45 μM) and three stations with Low DIN concentration (<40μM). The V3–V4 region of 16S rRNA was amplified and sequenced to elucidate resident bacterioplankton community structure from environmental DNA. Proteobacteria, Bacteroidetes, and Firmicutes were the dominant bacterioplankton phyla across all stations. Nitrogen-fixing groups such as Nitrospirae, Lentisphaerae, Chloroflexi, and Planctomycetes make up about 1% of the bacterioplankton communities. Abundances of Spirochaetes and Tenericutes showed a positive correlation with DIN. Pseudomonadales, Alteromonadales, and Desulfovibrionales were found to distinctly vary in abundance between Low and High DIN stations. Predicted metagenomic profiles from taxonomically derived community structures indicated bacterial nitrate-nitrite reductase to be negatively correlated with prevalent DIN concentration in High DIN stations but positively correlated in Low DIN stations. This trend was also consistent for genes encoding for nitrate/nitrite response regulators and transporter proteins. This indicates the need to delineate functional bacterioplankton community structures to better understand their role in influencing rates and fluxes of nitrogen within mangroves.

The origin of nitrogen in Earth's mantle: Constraints from basalts 15N/14N and N2/3He ratios

Plate tectonics is thought to be a major driver of volatile redistribution on Earth. The budget of nitrogen in Earth's mantle has been suggested to be almost entirely surface-derived. Recycling would contribute nitrogen with relatively heavy 15N/14N isotope ratios to Earth's mantle. This could explain why the Earth's mantle 15N/14N isotope ratio is substantially higher than both solar gases and chondritic parent bodies akin to enstatite chondrites. Here, published nitrogen isotope data of mid-ocean ridge and ocean island basalts are compiled and used to evaluate the nitrogen subduction hypothesis. Nitrogen isotope ratios are used in conjunction with published N2/3He and K2O/TiO2 ratios on the same basalts. Assuming that 3He is not recycled, N2/3He ratios are argued to trace nitrogen addition to mantle sources via subduction. Various mantle source enrichments for basalts are tracked with K2O/TiO2 ratios: elevated K2O/TiO2 ratios are assumed to primarily reflect the contributions of recycled components in the basalts mantle sources. The main result of our data compilation is that for most basalts, δ15N and N2/3He remain constant across a vast range of K2O/TiO2 ratios. Mid-ocean ridge basalts have δ15N signatures that are lower than air by ~4‰ and an average N2/3He ratio of 3.7 (±1.2) x106 (95% confidence, n = 30). Published δ15N and N2/3He are invariant across K2O/TiO2 ratios that vary over a factor of ~20. Using estimates of slab K2O/TiO2 and [TiO2], the observed invariant δ15N and N2/3He may be fit with slabs containing ~0.1 ppm N. A mass balance shows that adding ~10% recycled slabs to the convective mantle only raises the N2/3He by <5%. Lavas from Iceland, Galapagos and Hawaii have high 3He/4He and 15N/14N ratios relative to the convective mantle. Only seven samples show nitrogen isotopic signatures that are unaffected by air contamination, although those samples are poorly characterized for N2/3He. The seven basalts show δ15N between −2 and 0‰ that do not vary systematically with K2O/TiO2 ratios that vary over a factor of ~5. The N2/3He ratios of these seven basalts is unknown, but the high 3He/4He mantle may be estimated by combining published N2/36Ar to 3He/36Ar ratios. This yields a N2/3He of 2.3 (±1.2) x 106 (1σ uncertainty). This is indistinguishable from the MORB estimate of 3.7 (±1.2) x 106. Invariant δ15N across variable degrees of mantle enrichments and MORB-like N2/3He for the high 3He/4He mantle are not consistent with nitrogen addition to plume sources with elevated 3He/4He ratios. A δ15N between −2 and 0‰ for plume sources, only marginally higher than MORB, could be a primordial feature of undegassed mantle reservoirs. Nonetheless, nitrogen subduction may have contributed to a specific array of mantle sources, as revealed by the few published data on basalts with low 3He/4He ratios. Lavas from the Society plume with low 3He/4He ratios show an enriched mantle source, and they have elevated δ15N ≥ +0.5‰ and N2/3He > 107. For those, the addition of slabs with concentrations of ~0.1 ppm N to a mantle source can account for the integrated dataset. To summarize, the published data suggest that nitrogen subduction may explain a sub-set of published N isotope data on basalts, but that N recycling has an overall more limited impact on mantle nitrogen than previously thought.

Cloud and fog deposition: Monitoring in high elevation and coastal ecosystems. The past, present, and future

Deposition of pollutants by cloud water exceeds deposition by precipitation and dry deposition in many high elevation settings. This larger loading of pollutants in these environments is due to a combination of factors such as high frequency of cloud immersion, high wind speeds, orographic enhancement of precipitation, and large leaf area of tree species typical in these environments. Fog-impacted coastal ecosystems also experience increased pollutant deposition similar to cloud-impacted high elevation sites. Therefore, development of meaningful critical load values and total nitrogen budgets for high-elevation and fog-impacted sites requires reliable cloud and fog water deposition estimates. The cost and labor intensity of cloud/fog water sample collection have made it difficult to conduct long-term studies that would provide the data needed to develop accurate estimates. Current understanding of fog formation, transport, and the role of fog and cloud deposition in hydrogeological and biogeochemical cycles is incomplete due, in part, to lack of a concerted interdisciplinary approach to the problem. Historically, these obstacles have limited interest in and collection of cloud and fog water samples. In addition to measurements of cloud/fog chemical composition, documenting fog/cloud deposition fluxes of pollutant and nutrient species requires knowledge of cloud/fog physical properties, frequency and duration of fog/cloud interception with landscapes, properties of vegetation on those landscapes, and properties of the wind that drive droplet/vegetation interactions. Because drop deposition efficiency is dependent on drop size, it is also important to consider variations in fog/cloud drop composition with drop size as species enriched in larger/smaller drops will experience enhanced/reduced deposition rates.This paper presents summary results from a small U.S. cloud water monitoring network that operated from the mid-nineties through 2011, as well as a brief qualitative review of other cloud and fog water studies conducted in the United States (including Puerto Rico), Europe, South America/Pacific, and Asia. Current collection methods are also reviewed. Recent scientific efforts by the National Atmospheric Deposition Program's (NADP) Total Deposition Science Committee and NADP's Critical Loads of Atmospheric Deposition Science Committee have identified occult (cloud/fog) deposition as a “need” in developing critical loads for ecosystems that experience significant cloud and fog impaction. To this end, remaining knowledge gaps and suggestions for future research to investigate these gaps are explored.

Nitrogen Budget and Statistical Entropy Analysis of the Tiber River Catchment, a Highly Anthropized Environment

Modern farming causes a decline in the recycling of the soil’s inorganic matter due to losses by leaching, runoff, or infiltration into the groundwater. The Soil System Budget approach was applied to evaluate the net N budget at the catchment and sub-catchment levels of the Tiber River (central Italy) in order to establish the causes for different N budgets among the sub-catchments. Statistical Entropy Analysis (SEA) was used to evaluate the N efficiency of the Tiber River and its sub-catchments, providing information on the dispersion of different N forms in the environment. The total N inputs exceeded the total outputs, showing a low N retention (15.8%) at the catchment level, although some sub-catchments showed higher N retention values. The Utilized Agricultural Area was important in the determination of the N balance, as it was linked to zoo- and agricultural activities, although the Random Forest analysis showed that the importance ranking changed with the land use. The low N retention of the Tiber catchment was due to the soil characteristics (Cambisols and Leptosols), loads from atmospheric deposition, biological fixation, and the livestock industry. The SEA simulations showed a reduction of the N released into the atmosphere and groundwater compartments from 34% to 6% through a reduction of the N loads by 50%.

The importance of alkyl nitrates and sea ice emissions to atmospheric NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; sources and cycling in the summertime Southern Ocean marine boundary layer

Abstract. Atmospheric nitrate originates from the oxidation of nitrogen oxides (NOx=NO+NO2) and impacts both tropospheric chemistry and climate. NOx sources, cycling and NOx to nitrate formation pathways are poorly constrained in remote marine regions, especially the Southern Ocean, where pristine conditions serve as a useful proxy for the pre-industrial atmosphere. Here, we measured the isotopic composition (δ15N and δ18O) of atmospheric nitrate in coarse-mode (&gt;1 µm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5 to 70∘ S and across the northern edge of the Weddell Sea. The δ15N–NO3- decreased with latitude from −2.7 ‰ to −42.9 ‰. The decline in δ15N with latitude is attributed to changes in the dominant NOx sources: lightning at the low latitudes, oceanic alkyl nitrates at the mid-latitudes and photolysis of nitrate in snow at the high latitudes. There is no evidence of any influence from anthropogenic NOx sources or equilibrium isotope fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N signature of -21.8±7.6 ‰. Given that measurements of alkyl nitrate contributions to remote nitrogen budgets are scarce, this may be a useful tracer for detecting their contribution in other oceanic regions. The δ18O–NO3- was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low δ18O–NO3- values (less than 31 ‰) were observed at the western edge of the Weddell Sea. The air mass history of these samples indicates extensive interaction with sea-ice-covered ocean, which is known to enhance peroxy radical production. The observed low δ18O–NO3- is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ18O, relative to ozone, which has a high δ18O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NOx source than previously thought and that the ice-covered surface ocean impacts the reactive nitrogen budget as well as the oxidative capacity of the marine boundary layer.

Responses of nitrous oxide fluxes to autumn freeze-thaw cycles in permafrost peatlands of the Da Xing'an Mountains, Northeast China.

Climate warming has intensified changes of permafrost freeze-thaw process and postponed the starting period of soil freezing, which significantly affected the processes of N2O production and emission from the soils. However, responses of soil N2O fluxes to freeze-thaw cycles (FTCS) during autumn freezing period in permafrost peatlands in field remain unclear. Therefore, the static chamber-GC techniques were used to explore the effects of autumn FTCS on N2O fluxes in the three permafrost peatlands [Calamagrostis angustifolia peatland (CA), Larix gmelini-Sphagnum swamp (LS), and Eriophorum vaginatum peatland (EV)] in Da Xing'an Mountains, Northeast China, from September to November 2019. The response peaks of N2O fluxes to autumn FTCS in CA (29.22 ± 14.90μgm-2h-1) and EV (19.70 ± 7.26μgm-2h-1) occurred in the autumn FTCS prophase, whereas LS (11.33 ± 0.90μgm-2h-1) appeared in the autumn FTCS metaphase. CA (394.90μgm-2) and EV (497.82μgm-2) acted as a N2O source, and LS (- 1321.43μgm-2) was a N2O sink. The effects of autumn FTCS on N2O fluxes were significantly different (p < 0.001) in the three permafrost peatlands. N2O emissions during autumn FTCS were mainly driven by soil NH4+-N0-50cm, DOC30-40cm and 40-50cm content and soil NO3--N0-50cm content. The results implied that autumn FTCS could stimulate soil N2O emissions in permafrost peatlands and confirmed the important contribution of N2O emissions during autumn FTCS to annual nitrogen budget. This study could improve the accuracy of regional estimates of annual nitrogen budget.

Biogeochemical Budgets of Nutrients and Metabolism in the Curonian Lagoon (South East Baltic Sea): Spatial and Temporal Variations

Estuaries are biogeochemical reactors able to modulate the transfer of energy and matter from the watershed to the coastal zones and to retain or remove large amounts of terrestrially generated nutrients. However, they may switch from nutrient sink to source depending upon interannual variability of the nutrient supply and internal processes driving whole system metabolism (e.g., net autotrophic or heterotrophic). We tested this hypothesis in the Curonian Lagoon, a hypertrophic estuary located in the south east Baltic Sea, following the budget approach developed in the Land-Ocean Interactions in the Coastal Zone (LOICZ) project. Annual budgets for nitrogen (N), phosphorus (P), and silica (Si) were calculated for the 2013–2015 period. The lagoon was divided in a flushed, nutrient loaded area, and in a confined, less loaded area. The lagoon was always a sink for dissolved inorganic Si and P whereas it was a N sink in the confined area, dominated by denitrification, and a N source in the flushed area, due to dinitrogen (N2) fixation. The net ecosystem metabolism (NEM) indicated that the Curonian Lagoon was mainly autotrophic because of high primary production rates. In this turbid system, low N:P ratio, high summer temperatures, and calm weather conditions support high production of N2-fixing cyanobacteria, suppressing the estuarine N-sink role.

Cycles, Nitrogen Budget, and Sustainability of Agroecosystems after Applying Organic Fertilizers (Labeled with 15N)

Cycles, Nitrogen Budget, and Sustainability of Agroecosystems after Applying Organic Fertilizers (Labeled with 15N)

The Flux of Nitrate from England and Controls on Total Nitrogen Budget

The Flux of Nitrate from England and Controls on Total Nitrogen Budget

The effects of long-term application of organic and inorganic fertilizers on soil and plant Nitrogen-Carbon contents

Soil and crop management such as chemical fertilizer applications hurt the soil’s biological quality and productivity. The most well-known organic fertilizer such as animal manure, compost and mycorrhizal fungi are significantly contributing to soil organic carbon sink and consequently, organic fertilizers have a positive effect on the soil biological diversity and productivity. To increase soil carbon content, long-term organic fertilizer applications increase soil carbon budget hypothesis was tested under field conditions. The aim is to investigate the effects of different organic and inorganic fertilizer applications on the soil and wheat plant carbon and nitrogen content. A long-term field experiment was established in 1996 until the present time. In 2018, before cultivation control, Mineral fertilizer (NPK), Animal manure (25 ton ha-1), Compost (25 ton ha-1) and Compost+Mycorrhiza (10 ton ha-1) were applied. Adana-99 varieties of wheat seeds were sown. After harvest, soil and plant samples were taken and soil-plant carbon-nitrogen analyses were made. Results show that C % and N % contents of the plant seed, shoot, and root have higher concentrations than that of the control treatment. When the TC %, OC %, and N % contents of the soil at different depths (0-15 cm and 15-30 cm) and in the rhizosphere (R) and no rhizosphere (NR) area were examined, compost and animal manure treatments have higher content. At the rhizosphere area 0-15 cm depth the soil % OC, and % IC contents were statistically significant. The highest values of soil OC %, C: N and IC % contents at Rhizosphere 0-15 cm depth were obtained in animal manure with 2.94 %, 10.06, and 3.90 % respectively. Organic fertilizers application increased soil TC %, OC % and N% contents as well as contribute to the carbon and nitrogen budget. The results found support our hypothesis.

The carbon and nitrogen budget of Desmophyllum dianthus-a voracious cold-water coral thriving in an acidified Patagonian fjord.

In the North Patagonian fjord region, the cold-water coral (CWC) Desmophyllum dianthus occurs in high densities, in spite of low pH and aragonite saturation. If and how these conditions affect the energy demand of the corals is so far unknown. In a laboratory experiment, we investigated the carbon and nitrogen (C, N) budget of D. dianthus from Comau Fjord under three feeding scenarios: (1) live fjord zooplankton (100–2,300 µm), (2) live fjord zooplankton plus krill (>7 mm), and (3) four-day food deprivation. In closed incubations, C and N budgets were derived from the difference between C and N uptake during feeding and subsequent C and N loss through respiration, ammonium excretion, release of particulate organic carbon and nitrogen (POC, PON). Additional feeding with krill significantly increased coral respiration (35%), excretion (131%), and POC release (67%) compared to feeding on zooplankton only. Nevertheless, the higher C and N losses were overcompensated by the threefold higher C and N uptake, indicating a high assimilation and growth efficiency for the krill plus zooplankton diet. In contrast, short food deprivation caused a substantial reduction in respiration (59%), excretion (54%), release of POC (73%) and PON (87%) compared to feeding on zooplankton, suggesting a high potential to acclimatize to food scarcity (e.g., in winter). Notwithstanding, unfed corals ‘lost’ 2% of their tissue-C and 1.2% of their tissue-N per day in terms of metabolism and released particulate organic matter (likely mucus). To balance the C (N) losses, each D. dianthus polyp has to consume around 700 (400) zooplankters per day. The capture of a single, large krill individual, however, provides enough C and N to compensate daily C and N losses and grow tissue reserves, suggesting that krill plays an important nutritional role for the fjord corals. Efficient krill and zooplankton capture, as well as dietary and metabolic flexibility, may enable D. dianthus to thrive under adverse environmental conditions in its fjord habitat; however, it is not known how combined anthropogenic warming, acidification and eutrophication jeopardize the energy balance of this important habitat-building species.

Wheat production and partial nitrogen budget in response to herbicide and nitrogen application

Managing nitrogen (N) inputs to sustain yields benefits while minimizing N losses is an important agronomic approach. The effect of affinity herbicides (H) (i.e., 0, 1, 2, and 3 kg/ha) and N treatments (Unfertilized, 0:100, 33:67, 67:33, and 100:0% N derived from FYM:Urea, respectively to supplies 120 kg N/ha) on N balance and wheat production was studied. Plant height and yield was improved with 2 kg H/ha. Soil N increased with fertilization in proportion to the amount of added manure coupled with yield improvement. The N removal in 100% urea represented a positive N balance (+1.5 kg N/ha) and losses, whereas FYM/urea showed negative N balances (−18.4 to −68.5 kg/ha) and conservation. This study demonstrated the importance of N sources for evaluating the potential N losses and significance of FYM usage for reducing the losses of N into the ecosystem.

Chemical characteristics and sources of nitrogen-containing organic compounds at a regional site in the North China Plain during the transition period of autumn and winter

Organic nitrogen constitutes a significant fraction of the nitrogen budget in particulate matter (PM). However, the composition and sources of nitrogen-containing organic compounds (NOCs) in PM remain unclear currently in North China Plain (NCP), China. Rare local or regional studies on NOCs were conducted. In this study, ambient fine particles (PM2.5) were collected in Xianghe, a regional background site in NCP, from 26 October to 26 December 2017. The insights from this study include NOC molecule identification, concentration level, and NOC sources and origins. Specifically, we have identified and quantified >90 NOC species, with urea being the most abundant, accounting for 39.7 ± 4.7% of the total NOC followed by free amino acids (FAAs; 21.9 ± 1.5%), cyclic NOCs (15.3 ± 4.5%), amines (14.8 ± 1.5%), alkyl amides (5.8 ± 0.5%), isocyanates (1.7 ± 0.2%), and nitriles (1.1 ± 0.2%). The time series of FAAs was well correlated (r = 0.51–0.68, p < 0.01) with the organic marker of levoglucosan and was moderately correlated with Ox (r = 0.29–0.41, p < 0.01), suggesting biomass burning and secondary formation were important FAAs sources. We also show that amines can be oxidized and/or reacted by aqueous-phase processing to form secondary aerosols, which are further enhanced by the involvement of iron in the catalytic process. Using the receptor model of positive matrix factorization (PMF), six factors were identified including coal combustion, crustal sources, biomass burning, industry-related sources, traffic emissions, and secondary aerosols. Source apportionment of NOC shows biomass burning was the dominant factor, accounting for 31.8% of the total NOCs. This study provides a unique dataset of NOCs at this regional background site in the NCP, with the insights of NOC chemical composition and sources gained in this study being important for future NOC modeling as well as NOC health effects studies.

Effects of Seasonal Thermal Stratification on Nitrogen Transformation and Diffusion at the Sediment-Water Interface in a Deep Canyon Artificial Reservoir of Wujiang River Basin

Watershed-scale nitrogen pollution in aquatic systems has become a worldwide concern due to its continuous impact on water quality deterioration, while the knowledge of key influencing factors dominating nitrogen transportation and transformation at the sediment-water interface (SWI) remains limited, especially in impounded rivers with an artificial reservoir. Hence, for a better understanding of the effects of thermal stratification on nitrogen transformation, we investigated the nitrogen species and isotopes in the sediment of a deep reservoir in Southwest China. Our results confirmed a significant difference in nitrogen species and isotopic composition in sediment between those in the thermal stratification period and non-thermal stratification period and indicated that the sediment biogeochemical process and transportation were clearly linked to the variations in water temperature and dissolved oxygen dominated by the process of thermal stratification. Significant seasonal differences in NH4+-N and NO3−-N in pore water of the upper layer (0–19 cm) revealed that nitrification exhausted NH4+ in the non-stratified period (NSP), and a potential low mineralization rate appeared when compared with those in the stratified period (SP). Seasonal differences in nitrogen species and isotope fractionation of δ15N-PON (about 2.3‰ in SP) in the upper layer sediment indicated a higher anaerobic mineralization rate of organic matter in SP than that in NSP. The diffusion fluxes of NH4+-N at SWI were 9.48 and 15.66 mg·m−2·d−1 in NSP and SP, respectively, and annual NH4+-N diffusion accounted for 21.8% of total storage in the reservoir. This study demonstrated that the nitrogen cycling processes, especially nitrification, denitrification, and mineralization, have been largely altered along with the changes in dissolved oxygen and that the diffusion of nitrogen species varied with the presence of the oxygen. The results contribute to the future study of watershed nitrogen budget evaluation and suggest that the endogenous nitrogen released from the sediment-water interface should be emphasized when aiming to fulfil water management policies in deep reservoirs.

Divergent Pathways of Nitrogen-Fixing Trees through Succession Depend on Starting Nitrogen Supply and Priority Effects.

AbstractNitrogen-fixing trees are a major potential source of nitrogen in terrestrial ecosystems. The degree to which they persist in older forests has considerable implications for forest nitrogen budgets. We characterized nitrogen-fixing tree abundance across stand age in the contiguous United States and analyzed a theoretical model to help understand competitive outcomes and successional trajectories of nitrogen-fixing and nonfixing trees. Nitrogen-fixing tree abundance is bimodal in all regions except the northeastern United States, even in older forests, suggesting that competitive exclusion (including priority effects) is more common than coexistence at the spatial scale of our analysis. Our model analysis suggests conditions under which alternative competitive outcomes are possible and when they are transient (lasting decades or centuries) versus persistent (millennia). Critically, the timescale of the feedbacks between nitrogen fixation and soil nitrogen supply, which is thought to drive the exclusion of nitrogen-fixing trees through succession, can be long. Therefore, the long transient outcomes of competition are more relevant for real forests than the long-term equilibrium. Within these long-term transients, the background soil nitrogen supply is a major determinant of competitive outcomes. Consistent with the expectations of resource ratio theory, competitive exclusion is more likely at high and low nitrogen supply, while intermediate nitrogen supply makes coexistence or priority effects possible. However, these outcomes are modified by the nitrogen fixation strategy: obligate nitrogen fixation makes coexistence more likely than priority effects, compared with perfectly facultative fixation. These results advance our understanding of the successional trajectories of nitrogen-fixing trees and their effects on ecosystem development in secondary succession.

New Estimation of the NOx Snow‐Source on the Antarctic Plateau

AbstractTo fully decipher the role of nitrate photolysis on the atmospheric oxidative capacity in snow‐covered regions, NOx flux must be determined with more precision than existing estimates. Here, we introduce a method based on dynamic flux chamber measurements for evaluating the NOx production by photolysis of snowpack nitrate in Antarctica. Flux chamber experiments were conducted for the first time in Antarctica, at the French‐Italian station Concordia, Dome C (75°06'S, 123°20’E, 3233 m a.s.l) during the 2019–2020 summer campaign. Measurements were gathered with several snow samples of different ages ranging from newly formed drifted snow to 6‐year‐old firn. Contrary to existing literature expectations, the daily average photolysis rate coefficient, , did not significantly vary between differently aged snow samples, suggesting that the photolabile nitrate in snow behaves as a single‐family source with common photochemical properties, where a = (2.37 0.35) × 10−8 s−1 (1) has been calculated from December 10th 2019 to January 7th 2020. At Dome C summer daily average NOx flux, , based on measured NOx production rates was estimated to be (4.3 1.2) × 108 molecules cm−2 s−1, which is 1.5–7 times less than the net NOx flux observed previously above snow at Dome C using the gradient flux method. Using these results, we extrapolated an annual continental snow sourced NOx budget of 0.017 0.003 TgN y−1, 2 times the nitrogen budget, (N‐budget), of the stratospheric denitrification previously estimated for Antarctica. These quantifications of nitrate photolysis using flux chamber experiments provide a road‐map toward a new parameterization of the product that can improve future global and regional models of atmospheric chemistry.