Nitrate is an important nitrogen source for Arctic tundra plants

Plant nitrogen uptake preference and drivers in natural ecosystems at the global scale

Elucidating plant nitrogen (N) acquisition is crucial for understanding plant N strategies and ecosystem productivity. However, the variation in plant N uptake preference and its controlling factors on a global scale remain unclear. We conducted a global synthesis to explore plant N preference patterns and driving factors. Globally, the average contributions of ammonium (NH4 +), nitrate (NO3 -), and glycine N to the total plant N uptake were 41.6 ± 1.1%, 32.8 ± 1.2%, and 25.6 ± 0.9%, respectively. However, plant N uptake preferences differed significantly among climatic regions and vegetation types. Soil NH4 + was the most preferred N form by plants in (sub)tropical regions, whereas NO3 - preference was significantly higher in high-latitude than low-latitude regions. Plant functional type was one of the most important factors driving NO3 - preference, with significantly higher NO3 - preference of nonwoody species than broadleaf-evergreen, conifer, and shrub species. Organic N preference was lowest in (sub)tropics and significantly lower than that in temperate and alpine regions. This study shows clear climatic patterns and different influencing factors of plant NH4 + and NO3 - preference, which can contribute to the accurate prediction of N constraints on ecosystem productivity and soil carbon dynamics.

Effects of Freeze-Thaw Cycles on Uptake Preferences of Plants for Nutrient: A Review

Freeze-thawing is an abiotic climatic force prevalent at mid-to-high latitudes or high altitudes, significantly impacting ecosystem nitrogen (N) and phosphorus (P) cycling, which is receiving increasing attention due to ongoing global warming. The N and P nutrients are essential for plant growth and development, and the uptake and utilization of these nutrients by plants are closely linked to external environmental conditions. Additionally, the availability of N and P nutrients influences the ecological adaptability of plants. Adapting plants to diverse external environments for the efficient uptake and utilization of N and P nutrients represents a main focus in contemporary ecological research on plant nutrient utilization in the ecosystems of mid-to-high latitudes or high altitudes. Through a comprehensive analysis of the experimental results regarding plant nutrient uptake and utilization in mid-to-high-latitude or high-altitude ecosystems, this paper discussed the processes of soil N and P cycling and the different utilization strategies of nutrient forms employed by plants during freezing and thawing. Freeze-thaw cycles affect the availability of N and P in the soil. Under freeze-thaw conditions, plants preferentially take up readily available N sources (e.g., nitrate (NO3--N) or ammonium (NH4+-N)) and adjust their root growth and timing of N uptake, developing specific physiological and biochemical adaptations to meet their growth needs. When nutrient conditions are poor or N sources are limited, plants may rely more on low-molecular-weight organic nitrogen (e.g., amino acids) as N sources. Plants adapt to changes in their environment by adjusting root growth, making changes in root secretions, and utilizing microbial communities associated with the P cycle to support more efficient P utilization. Future research should (i) enhance the monitoring of plant roots and nutrient dynamics in the subterranean layers of the soil; (ii) incorporate a broader range of nutrients; (iii) examine specific freeze-thaw landscape types, along with the spatial and temporal heterogeneity of climate change within seasons, which is essential for minimizing uncertainty in our understanding of plant nutrient utilization strategies.

High-Resolution Measurements of Multi-Isotopic Signatures (δ15N, δ18O, and Δ17O) of Winter NO2 in a Megacity in Central China

Isotopic compositions (δ15N, δ18O, and Δ17O) of nitrogen dioxide (NO2) are crucial proxies for elucidating the sources and transformations of atmospheric NO2, yet observations remain scarce. Here, we reported high-resolution (2 h interval) measurements of NO2 isotopic compositions in a megacity in Central China. Δ17O(NO2) exhibited a clear diurnal cycle with elevated values during the day versus night. Daytime Δ17O(NO2) varying from 20.2 to 40.1‰ can be explained by variations in ozone (O3) versus peroxyl radical oxidation of nitric oxide (NO). The estimated peroxyl radicals were positively correlated with photochemical oxidants (Ox = O3 + NO2), revealing the ability of Δ17O(NO2) to reflect changes in the atmospheric oxidation environment. Nighttime Δ17O(NO2) variations (12.9 to 29.8‰) can be categorized into three regimes: the first regime when Δ17O(NO2) stabilized at approximately 20‰ can be well explained by Δ17O transfer from O3; the increases of Δ17O(NO2) above 20‰ were attributed to the vertical mixing of residual layer air with elevated Δ17O(NO2) and the exchange process between NO2 and dinitrogen pentoxide; and as low as 12.9‰ Δ17O(NO2) can be explained by primary NO2 emissions and the exchange process between NO and NO2. The NO-NO2 exchange also elevated δ15N(NO2), consistent with the observed negative relationships between Δ17O(NO2) and δ15N(NO2) at night.

Deciphering the Dynamic Interplay between Rhizobacteria and Root Exudates via Cerium Oxide Nanomaterials Modulation for Promoting Soybean Yield and Quality

The interplay between root exudates and rhizobacteria is essential for enhancing agricultural productivity. Herein, the impacts of cerium dioxide nanomaterials (CeO2 NMs) on these interactions in soybean plants were investigated. Following 3-5 weeks of exposure to 5 mg·kg-1 CeO2 NMs, the composition of root exudates changed over time, with isoflavone levels increasing by 6.3-21.7 folds, potentially manipulating the rhizobacteria. Correspondingly, rhizobacteria such as Ensifer, Allorhizobium, Nitrospira, and Bradyrhizobium were enriched by 40.7-367.3% at three time points. CeO2 NMs stimulated isoflavone biosynthesis in soybean plants and their excretion into the rhizosphere via upregulating the expressions of MYB transcription factors, biosynthesis, and transporter genes. The interactions of root exudates and rhizobacteria mediated by CeO2 NMs enhanced plant biomass (45.5-75.9%), nodulation (85.7%), nitrogen fixation, nutrient acquisition, and soil health, improving soybean quality (34.4-223.9%) and yield (16.2%). This study provides insights into root exudate-rhizobacteria interactions in leguminous plants facilitated by NMs for sustainable agriculture.

Genome analysis reveals diverse novel psychrotolerant Mucilaginibacter species in Arctic tundra soils

As Arctic soil ecosystems warm due to climate change, enhanced microbial activity is projected to increase the rate of soil organic matter degradation. Delineating the diversity and activity of Arctic tundra microbial communities active in decomposition is thus of keen interest. Here, we describe novel cold-adapted bacteria in the genus Mucilaginibacter (Bacteroidota) isolated from Artic tundra soils in Finland. These isolates are aerobic chemoorganotrophs and appear well adapted to the low-temperature environment, where they are also exposed to desiccation and a wide regime of annual temperature variation. Initial 16S ribosomal RNA (rRNA)-based phylogenetic analysis suggested that five isolated strains represent new species of the genus Mucilaginibacter, confirmed by whole genome-based phylogenomic and average nucleotide identity. Five novel species are described: Mucilaginibacter geliditolerans sp. nov., Mucilaginibacter tundrae sp. nov., Mucilaginibacter empetricola sp. nov., Mucilaginibacter saanensis sp. nov., and Mucilaginibacter cryoferens sp. nov. Genome and phenotype analysis showed their potential in complex carbon degradation, nitrogen assimilation, polyphenol degradation, and adaptation to their tundra heath habitat. A pangenome analysis of the newly identified species alongside known members of the Mucilaginibacter genus sourced from various environments revealed the distinctive characteristics of the tundra strains. These strains possess unique genes related to energy production, nitrogen uptake, adaptation, and the synthesis of secondary metabolites that aid in their growth, potentially accounting for their prevalence in tundra soil. By uncovering novel species and strains within the Mucilaginibacter, we enhance our understanding of this genus and elucidate how environmental fluctuations shape the microbial functionality and interactions in Arctic tundra ecosystems.

Mechanisms of biodiversity loss under nitrogen enrichment: unveiling a shift from light competition to cation toxicity

The primary mechanisms contributing to nitrogen (N) addition induced grassland biodiversity loss, namely light competition and soil cation toxicity, are often examined separately in various studies. However, their relative significance in governing biodiversity loss along N addition gradient remains unclear. We conducted a 4-yr field experiment with five N addition rates (0, 2, 10, 20, and 50 g N m-2 yr-1) and performed a meta-analysis using global data from 239 observations in N-fertilized grassland ecosystems. Results from our field experiment and meta-analysis indicate that both light competition and soil cation (e.g. Mn2+ and Al3+) toxicity contribute to plant diversity loss under N enrichment. The relative importance of these mechanisms varied with N enrichment intensity. Light competition played a more significant role in influencing species richness under low N addition (≤ 10 g m-2 yr-1), while cation toxicity became increasingly dominant in reducing biodiversity under high N addition (>10 g m-2 yr-1). Therefore, a transition from light competition to cation toxicity occurs with increasing N availability. These findings imply that the biodiversity loss along the N gradient is regulated by distinct mechanisms, necessitating the adoption of differential management strategies to mitigate diversity loss under varying intensities of N enrichment.

Global distribution and drivers of relative contributions among soil nitrogen sources to terrestrial plants

Soil extractable nitrate, ammonium, and organic nitrogen (N) are essential N sources supporting primary productivity and regulating species composition of terrestrial plants. However, it remains unclear how plants utilize these N sources and how surface-earth environments regulate plant N utilization. Here, we establish a framework to analyze observational data of natural N isotopes in plants and soils globally, we quantify fractional contributions of soil nitrate (fNO3-), ammonium (fNH4+), and organic N (fEON) to plant-used N in soils. We find that mean annual temperature (MAT), not mean annual precipitation or atmospheric N deposition, regulates global variations of fNO3-, fNH4+, and fEON. The fNO3- increases with MAT, reaching 46% at 28.5 °C. The fNH4+ also increases with MAT, achieving a maximum of 46% at 14.4 °C, showing a decline as temperatures further increase. Meanwhile, the fEON gradually decreases with MAT, stabilizing at about 20% when the MAT exceeds 15 °C. These results clarify global plant N-use patterns and reveal temperature rather than human N loading as a key regulator, which should be considered in evaluating influences of global changes on terrestrial ecosystems.

Unveiling unique microbial nitrogen cycling and nitrification driver in coastal Antarctica

Largely removed from anthropogenic delivery of nitrogen (N), Antarctica has notably low levels of nitrogen. Though our understanding of biological sources of ammonia have been elucidated, the microbial drivers of nitrate (NO3-) cycling in coastal Antarctica remains poorly understood. Here, we explore microbial N cycling in coastal Antarctica, unraveling the biological origin of NO3- via oxygen isotopes in soil and lake sediment, and through the reconstruction of 1968 metagenome-assembled genomes from 29 microbial phyla. Our analysis reveals the metabolic potential for microbial N2 fixation, nitrification, and denitrification, but not for anaerobic ammonium oxidation, signifying a unique microbial N-cycling dynamic. We identify the predominance of complete ammonia oxidizing (comammox) Nitrospira, capable of performing the entire nitrification process. Their adaptive strategies to the Antarctic environment likely include synthesis of trehalose for cold stress, high substrate affinity for resource utilization, and alternate metabolic pathways for nutrient-scarce conditions. We confirm the significant role of comammox Nitrospira in the autotrophic, nitrification process via 13C-DNA-based stable isotope probing. This research highlights the crucial contribution of nitrification to the N budget in coastal Antarctica, identifying comammox Nitrospira clade B as a nitrification driver.

Geochemical landscapes as drivers of wildlife reproductive success: Insights from a high-Arctic ecosystem

The bioavailability of essential and non-essential elements in vegetation is expected to influence the performance of free-ranging terrestrial herbivores. However, attempts to relate the use of geochemical landscapes by animal populations directly to reproductive output are currently lacking. Here we measured concentrations of 14 essential and non-essential elements in soil and vegetation samples collected in the Zackenberg valley, northeast Greenland, and linked these to environmental conditions to spatially predict and map geochemical landscapes. We then used long-term (1996-2021) survey data of muskoxen (Ovibos moschatus) to quantify annual variation in the relative use of essential and non-essential elements in vegetated sites and their relationship to calf recruitment the following year. Results showed that the relative use of the geochemical landscape by muskoxen varied substantially between years and differed among elements. Selection for vegetated sites with higher levels of the essential elements N, Cu, Se, and Mo was positively linked to annual calf recruitment. In contrast, selection for vegetated sites with higher concentrations of the non-essential elements As and Pb was negatively correlated to annual calf recruitment. Based on the concentrations measured in our study, we found no apparent associations between annual calf recruitment and levels of C, Mn, Co, Zn, Cd, Ba, Hg, and C:N ratio in the vegetation. We conclude that the spatial distribution and access to essential and non-essential elements are important drivers of reproductive output in muskoxen, which may also apply to other wildlife populations. The value of geochemical landscapes to assess habitat-performance relationships is likely to increase under future environmental change.

Nitrogen palaeo-isoscapes: Changing spatial gradients of faunal δ15N in late Pleistocene and early Holocene Europe

Nitrogen isotope ratio analysis (δ15N) of animal tissue is widely used in archaeology and palaeoecology to investigate diet and ecological niche. Data interpretations require an understanding of nitrogen isotope compositions at the base of the food web (baseline δ15N). Significant variation in animal δ15N has been recognised at various spatiotemporal scales and related to changes both in baseline δ15N, linked to environmental and climatic influence on the terrestrial nitrogen cycle, and animal ecology. Isoscapes (models of isotope spatial variation) have proved a useful tool for investigating spatial variability in biogeochemical cycles in present-day marine and terrestrial ecosystems, but so far, their application to palaeo-data has been more limited. Here, we present time-sliced nitrogen isoscapes for late Pleistocene and early Holocene Europe (c. 50,000 to 10,000 years BP) using herbivore collagen δ15N data. This period covers the Last Glacial-Interglacial Transition, during which significant variation in the terrestrial nitrogen cycle occurred. We use generalized linear mixed modelling approaches for interpolation and test models which both include and exclude climate covariate data. Our results show clear changes in spatial gradients of δ15N through time. Prediction of the lowest faunal δ15N values in northern latitudes after, rather than during, the Last Glacial Maximum is consistent with the Late Glacial Nitrogen Excursion (LGNE). We find that including climatic covariate data does not significantly improve model performance. These findings have implications for investigating the drivers of the LGNE, which has been linked to increased landscape moisture and permafrost thaw, and for understanding changing isotopic baselines, which are fundamental for studies investigating diets, niche partitioning, and migration of higher trophic level animals.

Nitrogen deposition from aviation emissions

Excess nitrogen deposition from anthropogenic sources of atmospheric emissions, such as agriculture and transportation, can have negative effects on natural environments. Designing effective conservation efforts requires knowledge of the contribution of individual sectors. This study utilizes a global atmospheric chemistry-transport model to quantify, for the first time, the contribution of global aviation NO_x emissions to nitrogen deposition for 2005 and 2019. We find that aviation led to an additional 1.39 Tg of nitrogen deposited globally in 2019, up 72 % from 2005, with 67 % of each year's total occurring through wet deposition. In 2019, aviation was responsible for an average of 0.66 %, 1.13 %, and 1.61 % of modeled nitrogen deposition from all sources over Asia, Europe, and North America, respectively. These impacts are spatially widespread, with 56 % of deposition occurring over water. Emissions during the landing, taxi and takeoff (LTO) phases of flight are responsible for 8 % of aviation's nitrogen deposition impacts on average globally, and between 16 and 32 % over most land in regions with high aviation activity. Despite currently representing less than 1.2 % of nitrogen deposition globally, further growth of aviation emissions would result in increases in aviation's contribution to nitrogen deposition and associated critical loads.

Source oxygen contributions of primary nitrate emitted from biomass burning

Atmospheric nitrate (NO3-) produced by photochemical oxidation in the atmosphere has high oxygen isotope ratios (δ18O values). Recently, the primary NO3- emitted from combustion sources was found to have much lower δ18O values. However, it is unclear how and to what extents the low δ18O signatures were controlled by major O sources during the primary NO3- formation of combustion processes. Here, we first measured concentrations and δ18O values of NO3- from burning five biomass materials (bb-NO3- and δ18Obb-NO3-, respectively) in China. Distinctly higher concentration levels of the bb-NO3- emissions (42.1 ± 8.1 μmol m-3) than ambient NO3- suggest it is a potential source of atmospheric NO3- pollution. Much lower δ18Obb-NO3- signatures (27.6 ± 2.7 ‰) than ambient NO3- support it as a primary emission source with different O sources and formation mechanism from secondary NO3-. Isotope mass-balance modeling revealed that atmospheric O2 and the biomass O dominated the O of bb-NO3- (53 ± 7 % and 40 ± 4 %, respectively) over the aqueous vapor (7 ± 3 %). Besides, we found increasing δ18Obb-NO3- values with the biomass N contents and relatively lower δ18Obb-NO3- values for biomasses with higher carbon (C) and lower O contents, indicating that biomass C, N, and O contents may influence the source O contributions of the bb-NO3-. This work provides a novel isotope analysis on the O source contribution of the bb-NO3-, which is useful for understanding the formation mechanism of combustion-related NO3- sources and evaluating the primary NO3- emissions.

Plant nitrogen-use strategies and their responses to the urban elevation of atmospheric nitrogen deposition in southwestern China

The elevation of nitrogen (N) deposition by urbanization profoundly impacts the structure and function of surrounding forest ecosystems. Plants are major biomass sinks of external N inputs into forests. Yet, the N-use strategies of forest plants in many areas remain unconstrained in city areas, so their responses and adapting mechanisms to the elevated N deposition are open questions. Here we investigated concentrations and N isotope (δ¹⁵N) of total N (TN) and nitrate (NO₃^-) in leaves and roots of four plant species in subtropical shrubberies and pine forests under N deposition levels of 13 kg-N ha^-1 yr^-1 and 29 kg-N ha^-1 yr^-1 at the Guiyang area of southwestern China, respectively. The δ¹⁵N differences between plant NO₃^- and soil NO₃^- revealed a meager NO₃^- reduction in leaves but a preferentially high NO₃^- reduction in roots. δ¹⁵N mass-balance analyses between plant TN and soil dissolved N suggested that soil NO₃^- contributed more than reduced N, and dissolved organic N contributed comparably with ammonium to plant TN, and the study plants preferred NO₃^- over reduced N. The elevation of N deposition induced root but not leaf NO₃^- reduction and enhanced the contribution of soil NO₃^- to plant TN, but plant NO₃^- preference decreased due to much higher magnitudes of soil NO₃^- enrichment than plant NO₃^- utilization. We conclude that plants in subtropical forests of southwestern China preferred NO₃^- over reduced N, and NO₃^- was reduced more in roots than in leaves, anthropogenic N pollution enhanced soil NO₃^- enrichment and plant NO₃^- utilization but reduced plant NO₃^- preference.

Microbiogeochemical Traits to Identify Nitrogen Hotspots in Permafrost Regions

Permafrost-affected tundra soils are large carbon (C) and nitrogen (N) reservoirs. However, N is largely bound in soil organic matter (SOM), and ecosystems generally have low N availability. Therefore, microbial induced N-cycling processes and N losses were considered negligible. Recent studies show that microbial N processing rates, inorganic N availability, and lateral N losses from thawing permafrost increase when vegetation cover is disturbed, resulting in reduced N uptake or increased N input from thawing permafrost. In this review, we describe currently known N hotspots, particularly bare patches in permafrost peatland or permafrost soils affected by thermokarst, and their microbiogeochemical characteristics, and present evidence for previously unrecorded N hotspots in the tundra. We summarize the current understanding of microbial N cycling processes that promote the release of the potent greenhouse gas (GHG) nitrous oxide (N2O) and the translocation of inorganic N from terrestrial into aquatic ecosystems. We suggest that certain soil characteristics and microbial traits can be used as indicators of N availability and N losses. Identifying N hotspots in permafrost soils is key to assessing the potential for N release from permafrost-affected soils under global warming, as well as the impact of increased N availability on emissions of carbon-containing GHGs.

Temporal Variations Rather than Long-Term Warming Control Extracellular Enzyme Activities and Microbial Community Structures in the High Arctic Soil

In Arctic soils, warming accelerates decomposition of organic matter and increases emission of greenhouse gases (GHGs), contributing to a positive feedback to climate change. Although microorganisms play a key role in the processes between decomposition of organic matter and GHGs emission, the effects of warming on temporal responses of microbial activity are still elusive. In this study, treatments of warming and precipitation were conducted from 2012 to 2018 in Cambridge Bay, Canada. Soils of organic and mineral layers were collected monthly from June to September in 2018 and analyzed for extracellular enzyme activities and bacterial community structures. The activity of hydrolases was the highest in June and decreased thereafter over summer in both organic and mineral layers. Bacterial community structures changed gradually over summer, and the responses were distinct depending on soil layers and environmental factors; water content and soil temperature affected the shift of bacterial community structures in both layers, whereas bacterial abundance, dissolved organic carbon, and inorganic nitrogen did so in the organic layer only. The activity of hydrolases and bacterial community structures did not differ significantly among treatments but among months. Our results demonstrate that temporal variations may control extracellular enzyme activities and microbial community structure rather than the small effect of warming over a long period in high Arctic soil. Although the effects of the treatments on microbial activity were minor, our study provides insight that microbial activity may increase due to an increase in carbon availability, if the growing season is prolonged in the Arctic.

In-depth characterization of denitrifier communities across different soil ecosystems in the tundra

BACKGROUND

In contrast to earlier assumptions, there is now mounting evidence for the role of tundra soils as important sources of the greenhouse gas nitrous oxide (N2O). However, the microorganisms involved in the cycling of N2O in this system remain largely uncharacterized. Since tundra soils are variable sources and sinks of N2O, we aimed at investigating differences in community structure across different soil ecosystems in the tundra.

RESULTS

We analysed 1.4 Tb of metagenomic data from soils in northern Finland covering a range of ecosystems from dry upland soils to water-logged fens and obtained 796 manually binned and curated metagenome-assembled genomes (MAGs). We then searched for MAGs harbouring genes involved in denitrification, an important process driving N2O emissions. Communities of potential denitrifiers were dominated by microorganisms with truncated denitrification pathways (i.e., lacking one or more denitrification genes) and differed across soil ecosystems. Upland soils showed a strong N2O sink potential and were dominated by members of the Alphaproteobacteria such as Bradyrhizobium and Reyranella. Fens, which had in general net-zero N2O fluxes, had a high abundance of poorly characterized taxa affiliated with the Chloroflexota lineage Ellin6529 and the Acidobacteriota subdivision Gp23.

CONCLUSIONS

By coupling an in-depth characterization of microbial communities with in situ measurements of N2O fluxes, our results suggest that the observed spatial patterns of N2O fluxes in the tundra are related to differences in the composition of denitrifier communities.

Increased Arctic NO3− Availability as a Hydrogeomorphic Consequence of Permafrost Degradation and Landscape Drying

Climate-driven permafrost thaw alters the strongly coupled carbon and nitrogen cycles within the Arctic tundra, influencing the availability of limiting nutrients including nitrate (NO3−). Researchers have identified two primary mechanisms that increase nitrogen and NO3− availability within permafrost soils: (1) the ‘frozen feast’, where previously frozen organic material becomes available as it thaws, and (2) ‘shrubification’, where expansion of nitrogen-fixing shrubs promotes increased soil nitrogen. Through the synthesis of original and previously published observational data, and the application of multiple geospatial approaches, this study investigates and highlights a third mechanism that increases NO3− availability: the hydrogeomorphic evolution of polygonal permafrost landscapes. Permafrost thaw drives changes in microtopography, increasing the drainage of topographic highs, thus increasing oxic conditions that promote NO3− production and accumulation. We extrapolate relationships between NO3− and soil moisture in elevated topographic features within our study area and the broader Alaskan Coastal Plain and investigate potential changes in NO3− availability in response to possible hydrogeomorphic evolution scenarios of permafrost landscapes. These approximations indicate that such changes could increase Arctic tundra NO3− availability by ~250–1000%. Thus, hydrogeomorphic changes that accompany continued permafrost degradation in polygonal permafrost landscapes will substantially increase soil pore water NO3− availability and boost future fertilization and productivity in the Arctic.

Unexpectedly minor nitrous oxide emissions from fluvial networks draining permafrost catchments of the East Qinghai-Tibet Plateau

Streams and rivers emit substantial amounts of nitrous oxide (N₂O) and are therefore an essential component of global nitrogen (N) cycle. Permafrost soils store a large reservoir of dormant N that, upon thawing, can enter fluvial networks and partly degrade to N₂O, yet the role of waterborne release of N₂O in permafrost regions is unclear. Here we report N₂O concentrations and fluxes during different seasons between 2016 and 2018 in four watersheds on the East Qinghai-Tibet Plateau. Thawing permafrost soils are known to emit N₂O at a high rate, but permafrost rivers draining the East Qinghai-Tibet Plateau behave as unexpectedly minor sources of atmospheric N₂O. Such low N₂O fluxes are associated with low riverine dissolved inorganic N (DIN) after terrestrial plant uptake, unfavorable conditions for N₂O generation via denitrification, and low N₂O yield due to a small ratio of nitrite reductase: nitrous oxide reductase in these rivers. We estimate fluvial N₂O emissions of 0.432 - 0.463 Gg N₂O-N yr^-1 from permafrost landscapes on the entire Qinghai-Tibet Plateau, which is marginal (~0.15%) given their areal contribution to global streams and rivers (0.7%). However, we suggest that these permafrost-affected rivers can shift from minor sources to strong emitters in the warmer future, likely giving rise to the permafrost non-carbon feedback that intensifies warming.

A review on moss nitrogen and isotope signatures evidence for atmospheric nitrogen deposition

Moss nitrogen (N) concentration and isotopic composition (δ¹⁵N) values can reveal a better understanding of atmospheric N deposition patterns. Here, we summarize the moss N content and δ¹⁵N signatures using data compiled from 104 papers. Based on the dataset, we summarize the models for assessing the level and reduced (NH_x): oxidised compounds (NO_x) ratio of atmospheric N deposition. Results showed a historical increase in N concentration and ¹⁵N depletion of specimen mosses close to anthropogenic N sources from intensive animal production and agricultural activities (NH_x emission) since the 1800s. However, an increase of moss N with a less negative ¹⁵N observed in the last three decades could be due to a substantial fossil fuel combustion contributed NO_x emission. Spatially, N deposition in Europe decreased due to successful control actions, but Asia has become a hotspot for NH_x emission from agriculture. The present results highlight the importance of moss N and δ¹⁵N values for estimating atmospheric N deposition patterns at spatio-temporal trends.

A new isotope framework to decipher leaf-root nitrogen allocation and assimilation among plants in a tropical invaded ecosystem

Exotic plant invasion is an urgent issue occurring in the biosphere, which can be stimulated by environmental nitrogen (N) loading. However, the allocation and assimilation of soil N sources between leaves and roots remain unclear for plants in invaded ecosystems, which hampers the understanding of mechanisms behind the expansion of invasive plants and the co-existence of native plants. This work established a new framework to use N concentrations and isotopes of soils, roots, and leaves to quantitatively decipher intra-plant N allocation and assimilation among plant species under no invasion and under the invasion of Chromolaena odorata and Ageratina adenophora in a tropical ecosystem of SW China. We found that the assimilation of N derived from both soil ammonium (NH₄⁺) and nitrate (NO₃^-) were higher in leaves than in roots for invasive plants, leading to higher leaf N levels than native plants. Compared with the same species under no invasion, most native plants under invasion showed higher N concentrations and NH₄⁺ assimilations in both leaves and roots, and increases in leaf N were higher than in root N for native plants under invasion. These results inform that preferential N allocation, dominated by NH₄⁺-derived N, to leaves over roots as an important N-use strategy for plant invasion and co-existence in the studied tropical ecosystem.

New insights into the role of chrysanthemum calcineurin B-like interacting protein kinase CmCIPK23 in nitrate signaling in Arabidopsis roots

Nitrate is an important source of nitrogen and also acts as a signaling molecule to trigger numerous physiological, growth, and developmental processes throughout the life of the plant. Many nitrate transporters, transcription factors, and protein kinases participate in the regulation of nitrate signaling. Here, we identified a gene encoding the chrysanthemum calcineurin B-like interacting protein kinase CmCIPK23, which participates in nitrate signaling pathways. In Arabidopsis, overexpression of CmCIPK23 significantly decreased lateral root number and length and primary root length compared to the WT when grown on modified Murashige and Skoog medium with KNO₃ as the sole nitrogen source (modified MS). The expression of nitrate-responsive genes differed significantly between CmCIPK23-overexpressing Arabidopsis (CmCIPK23-OE) and the WT after nitrate treatment. Nitrate content was significantly lower in CmCIPK23-OE roots, which may have resulted from reduced nitrate uptake at high external nitrate concentrations (≥ 1 mM). Nitrate reductase activity and the expression of nitrate reductase and glutamine synthase genes were lower in CmCIPK23-OE roots. We also found that CmCIPK23 interacted with the transcription factor CmTGA1, whose Arabidopsis homolog regulates the nitrate response. We inferred that CmCIPK23 overexpression influences root development on modified MS medium, as well as root nitrate uptake and assimilation at high external nitrate supply. These findings offer new perspectives on the mechanisms by which the chrysanthemum CBL interacting protein kinase CmCIPK23 influences nitrate signaling.

Amino acids dominate diffusive nitrogen fluxes across soil depths in acidic tussock tundra

Organic nitrogen (N) is abundant in soils, but early conceptual frameworks considered it nonessential for plant growth. It is now well recognised that plants have the potential to take up organic N. However, it is still unclear whether plants supplement their N requirements by taking up organic N in situ: at what rate is organic N diffusing towards roots and are plants taking it up? We combined microdialysis with live-root uptake experiments to measure amino acid speciation and diffusion rates towards roots of Eriophorum vaginatum. Amino acid diffusion rates (321 ng N cm^-2  h^-1 ) were c. 3× higher than those for inorganic N. Positively charged amino acids made up 68% of the N diffusing through soils compared with neutral and negatively charged amino acids. Live-root uptake experiments confirmed that amino acids are taken up by plants (up to 1 µg N g^-1  min^-1 potential net uptake). Amino acids must be considered when forecasting plant-available N, especially when they dominate the N supply, and when acidity favours proteolysis over net N mineralisation. Determining amino acid production pathways and supply rates will become increasingly important in projecting the extent and consequences of shrub expansion, especially considering the higher C : N ratio of plants relative to soil.

Biochar and urease inhibitor mitigate NH3 and N2O emissions and improve wheat yield in a urea fertilized alkaline soil

In this study, we explored the role of biochar (BC) and/or urease inhibitor (UI) in mitigating ammonia (NH₃) and nitrous oxide (N₂O) discharge from urea fertilized wheat cultivated fields in Pakistan (34.01°N, 71.71°E). The experiment included five treatments [control, urea (150 kg N ha^-1), BC (10 Mg ha^-1), urea + BC and urea + BC + UI (1 L ton^-1)], which were all repeated four times and were carried out in a randomized complete block design. Urea supplementation along with BC and BC + UI reduced soil NH₃ emissions by 27% and 69%, respectively, compared to sole urea application. Nitrous oxide emissions from urea fertilized plots were also reduced by 24% and 53% applying BC and BC + UI, respectively, compared to urea alone. Application of BC with urea improved the grain yield, shoot biomass, and total N uptake of wheat by 13%, 24%, and 12%, respectively, compared to urea alone. Moreover, UI further promoted biomass and grain yield, and N assimilation in wheat by 38%, 22% and 27%, respectively, over sole urea application. In conclusion, application of BC and/or UI can mitigate NH₃ and N₂O emissions from urea fertilized soil, improve N use efficiency (NUE) and overall crop productivity.

Methyl Jasmonate and Sodium Nitroprusside Jointly Alleviate Cadmium Toxicity in Wheat (Triticum aestivum L.) Plants by Modifying Nitrogen Metabolism, Cadmium Detoxification, and AsA-GSH Cycle

The principal intent of the investigation was to examine the influence of joint application of methyl jasmonate (MeJA, 10 μM) and a nitric oxide-donor sodium nitroprusside (SNP, 100 μM) to wheat plants grown under cadmium (Cd as CdCl2, 100 μM) stress. Cd stress suppressed plant growth, chlorophylls (Chl), and PSII maximum efficiency (F v /F m ), but it elevated leaf and root Cd, and contents of leaf proline, phytochelatins, malondialdehyde, and hydrogen peroxide, as well as the activity of lipoxygenase. MeJA and SNP applied jointly or singly improved the concentrations of key antioxidant biomolecules, e.g., reduced glutathione and ascorbic acid and the activities of the key oxidative defense system enzymes such as catalase, superoxide dismutase, dehydroascorbate reductase, glutathione S-transferase, and glutathione reductase. Exogenously applied MeJA and SNP jointly or singly also improved nitrogen metabolism by activating the activities of glutamine synthetase, glutamate synthase, and nitrate and nitrite reductases. Compared with individual application of MeJA or SNP, the combined application of both showed better effect in terms of improving plant growth and key metabolic processes and reducing tissue Cd content, suggesting a putative interactive role of both compounds in alleviating Cd toxicity in wheat plants.

MAIN FINDINGS

The main findings are that exogenous application of methyl jasmonate and nitric oxide-donor sodium nitroprusside alleviated the cadmium (Cd)-induced adverse effects on growth of wheat plants grown under Cd by modulating key physiological processes and up-regulating enzymatic antioxidants and the ascorbic acid-glutathione cycle-related enzymes.

Oxidation and sources of atmospheric NOx during winter in Beijing based on δ18O-δ15N space of particulate nitrate

The determination of both stable nitrogen (δ¹⁵N-NO₃^-) and stable oxygen (δ¹⁸O-NO₃^-) isotopic signatures of nitrate in PM_2.5 has shown potential for an approach of assessing the sources and oxidation pathways of atmospheric NOx (NO+NO₂). In the present study, daily PM_2.5 samples were collected in the megacity of Beijing, China during the winter of 2017-2018, and this new approach was used to reveal the origin and oxidation pathways of atmospheric NOx. Specifically, the potential of field δ¹⁵N-NO₃^- signatures for determining the NOx oxidation chemistry was explored. Positive correlations between δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- were observed (with R² between 0.51 and 0.66, p < 0.01), and the underlying environmental significance was discussed. The results showed that the pathway-specific contributions to NO₃^- formation were approximately 45.3% from the OH pathway, 46.5% from N₂O₅ hydrolysis, and 8.2% from the NO₃+HC channel based on the δ¹⁸O-δ¹⁵N space of NO₃^-. The overall nitrogen isotopic fractionation factor (εN) from NOx to NO₃^- on a daily scale, under winter conditions, was approximately +16.1‰±1.8‰ (consistent with previous reports). Two independent approaches were used to simulate the daily and monthly ambient NOx mixtures (δ¹⁵N-NOx), respectively. Results indicated that the monthly mean values of δ¹⁵N-NOx compared well based on the two approaches, with values of -5.5‰ ± 2.6‰, -2.7‰ ± 1.9‰, and -3.2‰ ± 2.2‰ for November, December, and January (2017-2018), respectively. The uncertainty was in the order of 5%, 5‰ and 5.2‰ for the pathway-specific contributions, the εN, and δ¹⁵N-NOx, respectively. Results also indicated that vehicular exhaust was the key contributor to the wintertime atmospheric NOx in Beijing (2017-2018). Our advanced isotopic perspective will support the future assessment of the origin and oxidation of urban atmospheric NOx.

Biochar addition affects root morphology and nitrogen uptake capacity in common reed (Phragmites australis)

Nitrogen (N) is a key factor that limits plant growth in most terrestrial ecosystems, and biochar reportedly improves soil characteristics and grain yields. However, the effects of biochar on plant N uptake in wetland ecosystems and the underlying mechanisms of these effects remain unclear. Therefore, our study sought to characterise the effects of biochar addition on Phragmites australis N absorption rates at two different N deposition conditions [30 and 60 kg N hm^-2 yr^-1; i.e., "low" and "high" N treatments, respectively]. Our results demonstrated that biochar significantly promoted root biomass growth in P. australis in the high N treatment group. In contrast, the low N treatment group exhibited an increased proportion of fine roots and a decrease in the average P. australis root diameter. The N absorption rate of P. australis in the low N treatment group significantly increased with biochar addition and ammonium N became the preferred N source. The absorption rates of both ammonium and nitrate N were negatively correlated with the average P. australis root diameter. Therefore, our findings indicate that biochar may affect the N uptake strategy of P. australis by altering root morphogenesis, thereby providing new insights into potential restoration strategies for wetland vegetation.

Recent advances in the applications of nano-agrochemicals for sustainable agricultural development

Modern agricultural practices have triggered the process of agricultural pollution. This process can cause the degradation of eco-systems, land, and environment owing to the modern-day by-products of agriculture. The substantial use of chemical fertilizers, pesticides, and, contaminated water for irrigation cause further damage to agriculture. The current scenario of the agriculture and food sector has therefore become unsustainable. Nanotechnology has provided innovative and resourceful frontiers to the agriculture sector by contributing practical applications in conventional agricultural ways and practices. There is a large possibility that agri-nanotechnology can have a significant impact on the sustainable agriculture and crop growth. Recent research has shown the potential of nanotechnology in improving the agriculture sector by enhancing the efficiency of agricultural inputs and providing solutions to agricultural problems for improving food productivity and security. The prospective use of nanoscale agrochemicals such as nanofertilizers, nanopesticides, nanosensors, and nanoformulations in agriculture has transformed traditional agro-practices, making them more sustainable and efficient. However, the application of these nano-products in real field situations raises concern about nanomaterial safety, exposure levels, and toxicological repercussions to the environment and human health. The present review gives an insight into recent advancements in nanotechnology-based agrochemicals that have revolutionized the agriculture sector. Further, the implementation barriers related to the nanomaterial use in agriculture, their commercialization potential, and the need for policy regulations to assess possible nano-agricultural risks are also discussed.

Arbuscular Mycorrhizal Community in Roots and Nitrogen Uptake Patterns of Understory Trees Beneath Ectomycorrhizal and Non-ectomycorrhizal Overstory Trees

Nitrogen (N) is an essential plant nutrient, and plants can take up N from several sources, including via mycorrhizal fungal associations. The N uptake patterns of understory plants may vary beneath different types of overstory trees, especially through the difference in their type of mycorrhizal association (arbuscular mycorrhizal, AM; or ectomycorrhizal, ECM), because soil mycorrhizal community and N availability differ beneath AM (non-ECM) and ECM overstory trees (e.g., relatively low nitrate content beneath ECM overstory trees). To test this hypothesis, we examined six co-existing AM-symbiotic understory tree species common beneath both AM-symbiotic black locust (non-ECM) and ECM-symbiotic oak trees of dryland forests in China. We measured AM fungal community composition of roots and natural abundance stable isotopic composition of N (δ15N) in plant leaves, roots, and soils. The root mycorrhizal community composition of understory trees did not significantly differ between beneath non-ECM and ECM overstory trees, although some OTUs more frequently appeared beneath non-ECM trees. Understory trees beneath non-ECM overstory trees had similar δ15N values in leaves and soil nitrate, suggesting that they took up most of their nitrogen as nitrate. Beneath ECM overstory trees, understory trees had consistently lower leaf than root δ15N, suggesting they depended on mycorrhizal fungi for N acquisition since mycorrhizal fungi transfer isotopically light N to host plants. Additionally, leaf N concentrations in the understory trees were lower beneath ECM than the non-ECM overstory trees. Our results show that, without large differences in root mycorrhizal community, the N uptake patterns of understory trees vary between beneath different overstory trees.

Important contributions of non-fossil fuel nitrogen oxides emissions

Since the industrial revolution, it has been assumed that fossil-fuel combustions dominate increasing nitrogen oxide (NO_x) emissions. However, it remains uncertain to the actual contribution of the non-fossil fuel NO_x to total NO_x emissions. Natural N isotopes of NO₃⁻ in precipitation (δ¹⁵N_w-NO3−) have been widely employed for tracing atmospheric NO_x sources. Here, we compiled global δ¹⁵N_w-NO3− observations to evaluate the relative importance of fossil and non-fossil fuel NO_x emissions. We found that regional differences in human activities directly influenced spatial-temporal patterns of δ¹⁵N_w-NO3− variations. Further, isotope mass-balance and bottom-up calculations suggest that the non-fossil fuel NO_x accounts for 55 ± 7% of total NO_x emissions, reaching up to 21.6 ± 16.6Mt yr⁻¹ in East Asia, 7.4 ± 5.5Mt yr⁻¹ in Europe, and 21.8 ± 18.5Mt yr⁻¹ in North America, respectively. These results reveal the importance of non-fossil fuel NO_x emissions and provide direct evidence for making strategies on mitigating atmospheric NO_x pollution.

This study investigates in the importance of non-fossil fuel NO_x emissions in the surface-earth-nitrogen cycle. The study shows how changes of regional human activities directly influence δ¹⁵N signatures of deposited NO_x to terrestrial environments and that emissions have largely been underestimated.

Isotopic Interpretation of Particulate Nitrate in the Metropolitan City of Karachi, Pakistan: Insight into the Oceanic Contribution to NOx

Nitrogen oxide (NO_x) abatement has become the focus of air quality management strategies. In this study, we examined NO_x sources and the atmospheric conversion of NO_x in Karachi, Pakistan, a megacity in South Asia with serious particulate pollution problems. Oceanic contributions to NO_x were quantified for the first time based on a novel approach using nitrogen/oxygen isotopic analysis in nitrate (δ¹⁵N-NO₃^-; δ¹⁸O-NO₃^-) and a Bayesian model. Our results showed that δ¹⁵N-NO₃^- in Karachi varied between -10.2‰ and +12.4‰. As indicated by the δ¹⁸O-NO₃^- findings (+66.2 ± 7.8‰), the •OH pathway dominated NO_x conversion throughout the nearly two-year observation, but high NO₃^- events were attributed to the O₃ pathway. Coal combustion was the most significant source (32.0 ± 9.8%) of NO_x in Karachi, with higher contributions in the autumn and winter; a similar situation occurred for biomass burning + lightning (30.3 ± 6.5%). However, mobile sources (25.2 ± 6.4%) and microbial processes (12.5 ± 7.5%) exhibited opposite seasonal trends. The oceanic contributions to NO_x in Karachi were estimated to be 16.8%, of which lightning, shipping emissions, and microbial processes accounted for 20.3%, 46.3%, and 33.4%, respectively, emphasizing the dominance of shipping emissions as an oceanic NO_x source.

Effects of biochar-based controlled release nitrogen fertilizer on nitrogen-use efficiency of oilseed rape (Brassica napus L.)

Biochar-based controlled release nitrogen fertilizers (BCRNFs) have received increasing attention due to their ability to improve nitrogen-use efficiency (NUE) and increase crop yields. We previously developed a novel BCRNF, but its effects on soil microbes, NUE, and crop yields have not been reported. Therefore, we designed a pot experiment with five randomised treatments: CK (without urea and biochar), B (addition biochar without urea), B + U (biochar mixed urea), Urea (addition urea without biochar), and BCRNF (addition BCRNF), to investigate the effects of BCRNF on nitrifiers and denitrifiers, and how these impact nitrogen supply and NUE. Results of high-throughput sequencing revealed bacterial community groups with higher nutrient metabolic cycling ability under BCRNF treatment during harvest stage. Compared to Urea treatment, BCRNF treatment stimulated nitrification by increasing the copy number of the bacterial amoA gene and reducing nitrous oxide emission by limiting the abundance of nirS and nirK. Eventually, BCRNF successfully enhanced the yield (~ 16.6%) and NUE (~ 58.79%) of rape by slowly releasing N and modulating the abundance of functional microbes through increased soil nitrification and reduced denitrification, as compared with Urea treatment. BCRNF significantly improved soil NO₃⁻, leading to an increase in N uptake by rape and NUE, thereby promoting rape growth and increasing grain yield.

Application of δ15N to trace the impact of penguin guano on terrestrial and aquatic nitrogen cycles in Victoria Land, Ross Sea region, Antarctica

Penguin colonies in Antarctica offer an ideal "natural laboratory" to investigate ecosystem function and the nitrogen (N) cycle. This study assessed the spatial distribution of penguin-derived N from guano and quantitatively assessed its impact on plant N utilization strategies in Victoria Land, Ross Sea region, Antarctica. Soil, moss, and aquatic microbial mats were collected inside and outside an active Adélie penguin (Pygoscelis adeliae) colony and analyzed for δ¹⁵N of total and inorganic nitrogen (NH₄⁺-N and NO₃^--N). The soil total nitrogen (TN), NH₄⁺-N, and NO₃^--N concentrations, as well as their δ¹⁵N values were significantly higher in guano-impacted areas than those in guano-free areas, verifying that guano is an important N source at and near penguin colonies. However, even far from the penguin colonies, soil δ¹⁵N values resembled those in penguin colonies, suggesting strong spatial impacts of penguin-derived N. The moss impacted by guano was more enriched in δ¹⁵N than in guano-free areas. The δ¹⁵N values of NH₄⁺-N and NO₃^--N in soils covered with moss revealed that the moss might prefer inorganic N in the absence of guano, while the dissolved organic N would become an important source for moss growing on ornithogenic soils. Aquatic microbial mat samples near penguin colonies were ¹⁵N-enriched, but ¹⁵N-depleted at upland sites.

Faster nitrogen cycling and more fungal and root biomass in cold ecosystems under experimental warming: a meta-analysis

Warming can alter the biogeochemistry and ecology of soils. These alterations can be particularly large in high northern latitude ecosystems, which are experiencing the most intense warming globally. In this meta-analysis, we investigated global trends in how experimental warming is altering the biogeochemistry of the most common limiting nutrient for biological processes in cold ecosystems of high northern latitudes (>50°): nitrogen (N). For comparison, we also analyzed cold ecosystems at intermediate and high southern latitudes. In addition, we examined N-relevant genes and enzymes, and the abundance of belowground organisms. Together, our findings suggest that warming in cold ecosystems increases N mineralization rates and N2 O emissions and does not affect N fixation, at least not in a consistent way across biomes and conditions. Changes in belowground N fluxes caused by warming lead to an accumulation of N in the forms of dissolved organic and root N. These changes seem to be more closely linked to increases in enzyme activity that target relatively labile N sources, than to changes in the abundance of N-relevant genes (e.g., amoA and nosZ). Finally, our analysis suggests that warming in cold ecosystems leads to an increase in plant roots, fungi, and (likely in an indirect way) fungivores, and does not affect the abundance of archaea, bacteria, or bacterivores. In summary, our findings highlight global trends in the ways warming is altering the biogeochemistry and ecology of soils in cold ecosystems, and provide information that can be valuable for prediction of changes and for management of such ecosystems.

Nitrogen isotope differences between atmospheric nitrate and corresponding nitrogen oxides: A new constraint using oxygen isotopes

Tracking of reactive nitrogen (N) sources is important for the effective mitigation of N emissions. By combining the N and oxygen (O) isotopes of atmospheric NO₃^-, stable isotope mixing models were recently applied to evaluate the relative contributions of major NO_x sources. However, it has long been unresolved how to accurately constrain the δ¹⁵N differences between NO₃^- and corresponding NO_x (ε_(NO2→NO3-) values). Here, we first incorporated the HC oxidation (NO₂ → NO₃^-) pathway by using Δ¹⁷O values to evaluate the ε_(NO2→NO3-) values, performed on NO₃^- in PM_2.5 collected during the day and at night from January 4-13, 2015 at an urban site in Beijing. We found that the Δ¹⁷O-based ε values (ε_{17O-based(NO2→NO3-)}) (15.6 ± 7.4‰) differed distinctly from δ¹⁸O-based ε values (ε_{18O-based(NO2→NO3-)}) (33.0 ± 9.5‰) so did not properly incorporate the isotopic effects of the HC oxidation (NO₂ → NO₃^-) pathway. Based on the ε_(NO2→NO3-) values, δ¹⁵N values of NO_x from coal combustion (CC), vehicle exhausts (VE), biomass burning (BB), and the microbial N cycle (MC), as well as NO₃^- in PM_2.5, we further quantified the source contributions by using Stable Isotope Analysis in R (the SIAR model). We found that the respective fractional contributions of CC-NO_x and MC-NO_x were underestimated by 64% and were overestimated by 216% by using ε_{18O-based(NO2→NO3-)} values. We concluded that the new ε_{17O-based(NO2→NO3-)} values reduced uncertainties in contribution analysis and the evaluation method for atmospheric NO₃^- sources.

Stable Isotopes of Water and Nitrate for the Identification of Groundwater Flowpaths: A Review

Nitrate contamination in stream water and groundwater is a serious environmental problem that arises in areas of high agricultural activities or high population density. It is therefore important to identify the source and flowpath of nitrate in water bodies. In recent decades, the dual isotope analysis (δ15N and δ18O) of nitrate has been widely applied to track contamination sources by taking advantage of the difference in nitrogen and oxygen isotope ratios for different sources. However, transformation processes of nitrogen compounds can change the isotopic composition of nitrate due to the various redox processes in the environment, which often makes it difficult to identify contaminant sources. To compensate for this, the stable water isotope of the H2O itself can be used to interpret the complex hydrological and hydrochemical processes for the movement of nitrate contaminants. Therefore, the present study aims at understanding the fundamental background of stable water and nitrate isotope analysis, including isotope fractionation, analytical methods such as nitrate concentration from samples, instrumentation, and the typical ranges of δ15N and δ18O from various nitrate sources. In addition, we discuss hydrograph separation using the oxygen and hydrogen isotopes of water in combination with the nitrogen and oxygen isotopes of nitrate to understand the relative contributions of precipitation and groundwater to stream water. This study will assist in understanding the groundwater flowpaths as well as tracking the sources of nitrate contamination using the stable isotope analysis in combination with nitrate and water.

Revealing biogeochemical signatures of Arctic landscapes with river chemistry

Riverine fluxes of carbon and inorganic nutrients are increasing in virtually all large permafrost-affected rivers, indicating major shifts in Arctic landscapes. However, it is currently difficult to identify what is causing these changes in nutrient processing and flux because most long-term records of Arctic river chemistry are from small, headwater catchments draining <200 km2 or from large rivers draining >100,000 km2. The interactions of nutrient sources and sinks across these scales are what ultimately control solute flux to the Arctic Ocean. In this context, we performed spatially-distributed sampling of 120 subcatchments nested within three Arctic watersheds spanning alpine, tundra, and glacial-lake landscapes in Alaska. We found that the dominant spatial scales controlling organic carbon and major nutrient concentrations was 3-30 km2, indicating a continuum of diffuse and discrete sourcing and processing dynamics. These patterns were consistent seasonally, suggesting that relatively fine-scale landscape patches drive solute generation in this region of the Arctic. These network-scale empirical frameworks could guide and benchmark future Earth system models seeking to represent lateral and longitudinal solute transport in rapidly changing Arctic landscapes.

Foliar uptake of atmospheric nitrate by two dominant subalpine plants: insights from in situ triple-isotope analysis

The significance of foliar uptake of nitrogen (N) compounds in natural conditions is not well understood, despite growing evidence of its importance to plant nutrition. In subalpine meadows, N-limitation fosters the dominance of specific subalpine plant species, which in turn ensures the provision of essential ecosystems services. Understanding how these plants absorb N and from which sources is important in predicting ecological consequences of increasing N deposition. Here, we investigate the sources of N to plants from subalpine meadows with distinct land-use history in the French Alps, using the triple isotopes (Δ¹⁷ O, δ¹⁸ O, and δ¹⁵ N) of plant tissue nitrate (NO₃^- ). We use this approach to evaluate the significance of foliar uptake of atmospheric NO₃^- (NO₃^-_atm ). The foliar uptake of NO₃^-_atm accounted for 4-16% of the leaf NO₃^- content, and contributed more to the leaf NO₃^- pool after peak biomass. Additionally, the gradual ¹⁵ N enrichment of NO₃^- from the soil to the leaves reflected the contribution of NO₃^-_atm assimilation to plants' metabolism. The present study confirms that foliar uptake is a potentially important pathway for NO₃^-_atm into subalpine plants. This is of major significance as N emissions (and deposition) are predicted to increase globally in the future.

Uptake Patterns of Glycine, Ammonium, and Nitrate Differ Among Four Common Tree Species of Northeast China

Fundamental questions of how plant species within secondary forests and plantations in northeast China partition limited nitrogen (N) resource remain unclear. Here we conducted a 15N tracer greenhouse study to determine glycine, ammonium, and nitrate uptake by the seedlings of two coniferous species, Pinus koraiensis (Pinus) and Larix keampferi (Larix), and two broadleaf species, Quercus mongolica (Quercus) and Juglans mandshurica (Juglans), that are common in natural secondary forests in northeast China. Glycine contributed 43% to total N uptake of Pinus, but only 20, 11, and 21% to N uptake by Larix, Quercus, and Juglans, respectively (whole plant), whereas nitrate uptake was 24, 74, 88, and 68% of total uptake for these four species, respectively. Retention of glycine carbon versus nitrogen in Pinus roots indicated that 36% of glycine uptake was retained intact. Nitrate was preferentially used by Larix, Quercus, and Juglans, with nitrate uptake constituting 68∼88% of total N use by these three species. These results demonstrated that these dominant tree species in secondary forests in northeast China partitioned limited N resource by varying uptake of glycine, ammonium and nitrate, with all species, except Pinus, using nitrate that are most abundant within these soils. Such N use pattern may also provide potential underlying mechanisms for the higher retention of deposited nitrate than ammonium into aboveground biomass in these secondary forests.

Prokaryotic community shifts during soil formation on sands in the tundra zone

A chronosequence approach, i.e., a comparison of spatially distinct plots with different stages of succession, is commonly used for studying microbial community dynamics during paedogenesis. The successional traits of prokaryotic communities following sand fixation processes have previously been characterized for arid and semi-arid regions, but they have not been considered for the tundra zone, where the environmental conditions are unfavourable for the establishment of complicated biocoenoses. In this research, we characterized the prokaryotic diversity and abundance of microbial genes found in a typical tundra and wooded tundra along a gradient of increasing vegetation—unfixed aeolian sand, semi-fixed surfaces with mosses and lichens, and mature soil under fully developed plant cover. Microbial communities from typical tundra and wooded tundra plots at three stages of sand fixation were compared using quantitative polymerase chain reaction (qPCR) and high-throughput sequencing of 16S rRNA gene libraries. The abundances of ribosomal genes increased gradually in both chronosequences, and a similar trend was observed for the functional genes related to the nitrogen cycle (nifH, bacterial amoA, nirK and nirS). The relative abundance of Planctomycetes increased, while those of Thaumarchaeota, Cyanobacteria and Chloroflexi decreased from unfixed sands to mature soils. According to β-diversity analysis, prokaryotic communities of unfixed sands were more heterogeneous compared to those of mature soils. Despite the differences in the plant cover of the two mature soils, the structural compositions of the prokaryotic communities were shaped in the same way. Thus, sand fixation in the tundra zone increases archaeal, bacterial and fungal abundances, shifts and unifies prokaryotic communities structure.

A new method for soil health assessment based on Analytic Hierarchy Process and meta-analysis

Understanding soil health condition is essential to the sustainability and stability of the entire ecosystem of farmland. The primary objective of this study was to improve the soil health index (SHI) based on principle component analysis (PCA) and develop a new analysis method for soil health assessment based on Meta-Analytic Hierarchy Process (Meta-AHP), which provides consistent minimum data sets (MDS), weight and scoring function for different locations, studies and management. The thirteen variables of MDS that exhibited sensitivity to management between organic and conventional soil were selected by meta-analysis. The indicator weight was assigned by a combination of experts scoring, AHP and meta-analysis. To test the applicability and sensitivity of the soil health assessment by Meta-AHP, a sixteen-year long-term test was assessed by the conventional SHI method (cSHI) and Meta-AHP. The results showed that similar evaluation results and significant positive correlations (**P < 0.01, n = 9) between the two evaluated methods were observed, and the results calculated using Meta-AHP had the best discrimination under different plant systems due to the higher F values when compared with the cSHI. This study developed a sensitive and consistent SH assessment framework that can be used applied to a variety of location, study, and soil management systems.