Foliar N2 O emissions constitute a significant source to atmosphere.

Nitrous oxide (N2 O) is a potent greenhouse gas and causes stratospheric ozone depletion. While the emissions of N2 O from soil are widely recognized, recent research has shown that terrestrial plants may also emit N2 O from their leaves under controlled laboratory conditions. However, it is unclear whether foliar N2 O emissions are universal across varying plant taxa, what the global significance of foliar N2 O emissions is, and how the foliage produces N2 O in situ. Here we investigated the abilities of 25 common plant taxa, including trees, shrubs and herbs, to emit N2 O under in situ conditions. Using 15 N isotopic labeling, we demonstrated that the foliage-emitted N2 O was predominantly derived from nitrate. Moreover, by selectively injecting biocide in conjunction with the isolating and back-inoculating of endophytes, we demonstrated that the foliar N2 O emissions were driven by endophytic bacteria. The seasonal N2 O emission rates ranged from 3.2 to 9.2 ng N2 O-N g-1 dried foliage h-1 . Extrapolating these emission rates to global foliar biomass and plant N uptake, we estimated global foliar N2 O emission to be 1.21 and 1.01 Tg N2 O-N year-1 , respectively. These estimates account for 6%-7% of the current global annual N2 O emission of 17 Tg N2 O-N year-1 , indicating that in situ foliar N2 O emission is a universal process for terrestrial plants and contributes significantly to the global N2 O inventory. This finding highlights the importance of measuring foliar N2 O emissions in future studies to enable the accurate assigning of mechanisms and the development of effective mitigation.

Anthropogenic N input increases global warming potential by awakening the "sleeping" ancient C in deep critical zones.

Even a small net increase in soil organic carbon (SOC) mineralization will cause a substantial increase in the atmospheric CO2 concentration. It is widely recognized that the SOC mineralization within deep critical zones (2 to 12 m depth) is slower and much less influenced by anthropogenic disturbance when compared to that of surface soil. Here, we showed that 20 years of nitrogen (N) fertilization enriched a deep critical zone with nitrate, almost doubling the SOC mineralization rate. This result was supported by corresponding increases in the expressions of functional genes typical of recalcitrant SOC degradation and enzyme activities. The CO2 released and the SOC had a similar 14C age (6000 to 10,000 years before the present). Our results indicate that N fertilization of crops may enhance CO2 emissions from deep critical zones to the atmosphere through a previously disregarded mechanism. This provides another reason for markedly improving N management in fertilized agricultural soils.

Nitrate as an alternative electron acceptor destabilizes the mineral associated organic carbon in moisturized deep soil depths.

Numerous studies have investigated the effects of nitrogen (N) addition on soil organic carbon (SOC) decomposition. However, most studies have focused on the shallow top soils <0.2 m (surface soil), with a few studies also examining the deeper soil depths of 0.5-1.0 m (subsoil). Studies investigating the effects of N addition on SOC decomposition in soil >1.0 m deep (deep soil) are rare. Here, we investigated the effects and the underlying mechanisms of nitrate addition on SOC stability in soil depths deeper than 1.0 m. The results showed that nitrate addition promoted deep soil respiration if the stoichiometric mole ratio of nitrate to O2 exceeded the threshold of 6:1, at which nitrate can be used as an alternative acceptor to O2 for microbial respiration. In addition, the mole ratio of the produced CO2 to N2O was 2.57:1, which is close to the theoretical ratio of 2:1 expected when nitrate is used as an electron acceptor for microbial respiration. These results demonstrated that nitrate, as an alternative acceptor to O2, promoted microbial carbon decomposition in deep soil. Furthermore, our results showed that nitrate addition increased the abundance of SOC decomposers and the expressions of their functional genes, and concurrently decreased MAOC, and the ratio of MAOC/SOC decreased from 20% before incubation to 4% at the end of incubation. Thus, nitrate can destabilize the MAOC in deep soils by stimulating microbial utilization of MAOC. Our results imply a new mechanism on how above-ground anthropogenic N inputs affect MAOC stability in deep soil. Mitigation of nitrate leaching is expected to benefit the conservation of MAOC in deep soil depths.

Influence of rewetting on N2O emissions in three different fen types.

In recent years, many peatlands in Europe have been rewetted for nature conservation and global warming mitigation. However, the effects on emissions of the greenhouse gas nitrous oxide (N2O) have been found to be highly variable and driving factors are poorly understood. Therefore, we measured N2O fluxes every two weeks over three years on pairs of sites (one drained, one rewetted) of three important peatland types in North-Eastern Germany, namely, percolation fen, alder forest and coastal fen. Additionally, every three months, sources of N2O were determined using a stable isotope mapping approach. Overall, fluxes were under the very dry conditions of the study years usually small with large temporal and spatial variations. Ammonium concentrations consistently and significantly correlated positively with N2O fluxes for all sites. Cumulative fluxes were often not significantly different from zero and apart from the rewetted alder forest, which was always a source of N2O, sites showed varying cumulative emission behavior (insignificant, source, potentially sink in one case) among years. Precipitation was positively correlated with cumulative fluxes on all drained sites and the rewetted alder forest. Isotope mapping indicated that N2O was always produced by more than one process simultaneously, with the estimated contribution of denitrification varying between 20 and 80%. N2O reduction played a potentially large role, with 5 to 50% of total emissions, showing large variations among sites and over time. Overall, neither the effect of rewetting, water level nor seasonality was clearly reflected in the fluxes or sources. Emissions were concentrated in hotspots and hot moments. A better understanding of the driving factors of N2O production and reduction in (rewetted) fens is essential and stable isotope methods including measurements of 15N and 18O as well as site preferences can help foster the necessary comprehension of the underlying mechanisms. Supplementary Information: The online version contains supplementary material available at 10.1007/s10705-022-10244-y.

Long-term vegetation change in the Western Tien-Shan Mountain pastures, Central Asia, driven by a combination of changing precipitation patterns and grazing pressure.

In mountain pastures worldwide, studies investigating vegetation changes due to long-term grazing and environmental changes are sparse, especially regarding the effects of changes in snowmelt patterns. The outstanding availability of historical vegetation data from Kyrgyz mountain pastures creates unique opportunities to study past and forecast future changes, making them ideal model ecosystems. Using a resurvey approach, we explored the response of mountain vegetation to management and environmental changes in the Western Tien-Shan to investigate whether plant communities of six vegetation types (ecozones) had changed over 42 years, whether changes were related to management or ecological causes and whether species' mean elevational ranges had changed. We assembled historic vegetation data (1973-1987) in six ecozones that were resurveyed annually from 2008 to 2015 and connected them with species' management-related traits and ecological indicator values. Overall, a homogenization of vegetation within and among ecozones was observed. Mountain steppe, meadow-steppe, and subalpine meadows showed the strongest convergence towards a dominance of mesic shrubs, related to increasing precipitation changing soil moisture and soil-salt regimes. In the high mountain steppe and the alpine ecozone, cushion dwarf shrubs increased, driven by increased soil moisture following faster snowmelt. Changes in the semidesert were related to highly variable spring soil moisture. Compositional changes accelerated over time. Mostly palatable species declined in abundance. More competitive unpalatable species replaced abundant (1973) unpalatable species. Mean elevation shifted significantly for 35 species (out of 136), with 60% shifting >100 m, more often upward (low and high elevations) than downward (mid-elevations). These mountain ecosystems seem more sensitive to changing precipitation than temperature- or grazing-induced changes, making climatic change a more important driver than management. Further adaptive management should consider the response of the vegetation to environmental changes and promote alternative land-use options to maintain ecosystem functioning. In mountain ecosystems worldwide, the observed acceleration of changes might go unnoticed, calling for long-term studies and global climate-vegetation-management interaction models.

Application methods of tracers for N2O source determination lead to inhomogeneous distribution in field plots.

Source determination of N2O has often been performed using stable isotope incubation experiments. In situ experiments with isotopic tracers are an important next step. However, the challenge is to distribute the tracers in the field as homogeneously as possible. To examine this, a bromide solution was applied as a stand-in tracer using either a watering can, a sprayer, or syringes to a relatively dry (25% gravimetric moisture content) or wet (30%) silt loam. After 1 h, samples were taken from three soil depths (0-10 cm), and analyzed for their water content and bromide concentration. The application with syringes was unsuccessful due to blocked cannulas. Therefore, further laboratory experiments were conducted with side-port cannulas. Despite a larger calculated gravimetric soil moisture difference with watering can application, more Br- tracer was recovered in the sprayer treatment, probably due to faster transport of Br- through macropore flow in the wetter conditions caused by the watering can treatment. The losses of Br- (33% for the watering can, 28% for the sprayer treatment) are equivalent to potential losses of isotopic tracer solutions. For application of 60 at% 15NH4 +, this resulted in theoretical enrichments of 44-53 at% in the upper 2.5 cm and 7-48 at% in 5-10 cm. As there was hardly any NO3 - in the soil, extrapolations for 15NO3 - calculated enrichments were 57-59 at% in the upper 2.5 cm and 26-57 at% in 5-10 cm. Overall, no method, including the side-port cannulas, was able to achieve a homogeneous distribution of the tracer. Future search for optimal tracer application should therefore investigate methods that utilize capillary forces and avoid overhead pressure. We recommend working on rather dry soil when applying tracers, as tracer recovery was larger here. Furthermore, larger amounts of tracer lead to more uniform distributions. Further studies should also investigate the importance of plant surfaces.

Electrodes Donate Electrons for Nitrate Reduction in a Soil Matrix via DNRA and Denitrification.

Microbial strains and indigenous microbiota in soil slurries have been reported to use electrons from electrodes for nitrate (NO3-) reduction. However, few studies have confirmed this in a soil matrix hitherto. This study investigated if, and how, an electric potential affected NO3- reduction in a soil matrix. The results showed that, compared to a control treatment, applying an electric potential of -0.5 V versus the standard hydrogen electrode (SHE) significantly increased the relative abundance of NO3--reducing microbes (e.g., Alcaligenaceae and Pseudomonadaceae) and the abundances of the nrfA, nirK, nirS, and nosZ genes in soil matrices. Meanwhile, the electric potential treatment doubled the NO3- reduction rate and significantly increased the rates of production of ammonium (NH4+), dinitrogen (N2), and nitrous oxide (N2O). The amount of NO3--N reduced under the electric potential treatment was comparable to the sum of the amounts of N observed in the increased N2O, N2, NH4+, and nitrite (NO2-) pools. An open-air experiment showed that the electric potential treatment promoted soil NO3- reduction with a spatial scale of at least 38 cm. These results demonstrated that an electric potential treatment could enhance NO3- reduction via both denitrification and dissimilatory NO3- reduction to ammonium (DNRA) in the soil matrix. The mechanisms revealed in this study have implications for the future development of potential techniques for enhancing NO3- reduction in the vadose zone and consequently reducing the risk of NO3- leaching.

Biochar, soil and land-use interactions that reduce nitrate leaching and N2O emissions: A meta-analysis.

Biochar can reduce both nitrous oxide (N2O) emissions and nitrate (NO3-) leaching, but refining biochar's use for estimating these types of losses remains elusive. For example, biochar properties such as ash content and labile organic compounds may induce transient effects that alter N-based losses. Thus, the aim of this meta-analysis was to assess interactions between biochar-induced effects on N2O emissions and NO3- retention, regarding the duration of experiments as well as soil and land use properties. Data were compiled from 88 peer-reviewed publications resulting in 608 observations up to May 2016 and corresponding response ratios were used to perform a random effects meta-analysis, testing biochar's impact on cumulative N2O emissions, soil NO3- concentrations and leaching in temperate, semi-arid, sub-tropical, and tropical climate. The overall N2O emissions reduction was 38%, but N2O emission reductions tended to be negligible after one year. Overall, soil NO3- concentrations remained unaffected while NO3- leaching was reduced by 13% with biochar; greater leaching reductions (>26%) occurred over longer experimental times (i.e. >30 days). Biochar had the strongest N2O-emission reducing effect in paddy soils (Anthrosols) and sandy soils (Arenosols). The use of biochar reduced both N2O emissions and NO3- leaching in arable farming and horticulture, but it did not affect these losses in grasslands and perennial crops. In conclusion, the time-dependent impact on N2O emissions and NO3- leaching is a crucial factor that needs to be considered in order to develop and test resilient and sustainable biochar-based N loss mitigation strategies. Our results provide a valuable starting point for future biochar-based N loss mitigation studies.

Development and Assessment of a Body Condition Score Scheme for European Bison (Bison bonasus).

Resettlement projects of the strongly threatened European bison (Bison bonasus) require a monitoring phase to assess both population status and habitat quality. Schemes of animal body condition scores (BCS) are robust tools to meet this requirement in practice. However, so far, no BCS scheme has been designed for European bison. Here, we suggest a body condition score scheme based on the extent of soft tissue around bony structures. The scoring system was developed with scores ranging from 1 (emaciated) to 5 (obese). Condition scores can be deduced after visually assessing the European bison both from the side and behind. Robustness of the scheme was evaluated: Unbiased people from different professional backgrounds were asked to assess the BCS of photographed semiwild European bison under field conditions and results were compared. Results demonstrate the suitability of the method. Nevertheless, variability of the results among assessors illustrates the necessity for training as well as for further research to validate the scheme as a true measure of physiological condition. We discuss the prospects and limits of a broad use of this scheme within the European bison community, and recommend the BCS scheme as a management tool.

Relationship between Remote Sensing Data, Plant Biomass and Soil Nitrogen Dynamics in Intensively Managed Grasslands under Controlled Conditions.

The sustainable use of grasslands in intensive farming systems aims to optimize nitrogen (N) inputs to increase crop yields and decrease harmful losses to the environment at the same time. To achieve this, simple optical sensors may provide a non-destructive, time- and cost-effective tool for estimating plant biomass in the field, considering spatial and temporal variability. However, the plant growth and related N uptake is affected by the available N in the soil, and therefore, N mineralization and N losses. These soil N dynamics and N losses are affected by the N input and environmental conditions, and cannot easily be determined non-destructively. Therefore, the question arises: whether a relationship can be depicted between N fertilizer levels, plant biomass and N dynamics as indicated by nitrous oxide (N2O) losses and inorganic N levels. We conducted a standardized greenhouse experiment to explore the potential of spectral measurements for analyzing yield response, N mineralization and N2O emissions in a permanent grassland. Ryegrass was subjected to four mineral fertilizer input levels over 100 days (four harvests) under controlled environmental conditions. The soil temperature and moisture content were automatically monitored, and the emission rates of N2O and carbon dioxide (CO2) were detected frequently. Spectral measurements of the swards were performed directly before harvesting. The normalized difference vegetation index (NDVI) and simple ratio (SR) were moderately correlated with an increasing biomass as affected by fertilization level. Furthermore, we found a non-linear response of increasing N2O emissions to elevated fertilizer levels. Moreover, inorganic N and extractable organic N levels at the end of the experiment tended to increase with the increasing N fertilizer addition. However, microbial biomass C and CO2 efflux showed no significant differences among fertilizer treatments, reflecting no substantial changes in the soil biological pool size and the extent of the C mineralization. Neither the NDVI nor SR, nor the plant biomass, were related to cumulative N2O emissions or inorganic N at harvesting. Our results verify the usefulness of optical sensors for biomass detection, and show the difficulty in linking spectral measurements of plant traits to N processes in the soil, despite that the latter affects the former.

Perturbation-free measurement of in situ di-nitrogen emissions from denitrification in nitrate-rich aquatic ecosystems.

Increased production of reactive nitrogen (Nr) from atmospheric di-nitrogen (N2) has greatly contributed to increased food production. However, enriching the biosphere with Nr has also caused a series of negative effects on global ecosystems, especially aquatic ecosystems. The main pathway converting Nr back into the atmospheric N2 pool is the last step in the denitrification process. Despite several attempts, there is still a need for perturbation-free methods for measuring in situ N2 fluxes from denitrification in aquatic ecosystems at the field scale. Such a method is needed to comprehensively quantify the N2 fluxes from aquatic ecosystems. Here we observed linear relationships between the δ15N-N2O signatures and the logarithmically transformed N2O/(N2+N2O) emission ratios. Through independent measurements, we verified that the perturbation-free N2 flux from denitrification in nitrate-rich aquatic ecosystems can be inferred from these linear relationships. Our method allowed the determination of field-scale in situ N2 fluxes from nitrate-rich aquatic ecosystems both with and without overlaying water. The perturbation-free in situ N2 fluxes observed by the new method were almost one order of magnitude higher than those by the sediment core method. The ability of aquatic ecosystems to remove Nr may previously have been severely underestimated.

Dual isotope and isotopomer signatures of nitrous oxide from fungal denitrification--a pure culture study.

RATIONALE: The contribution of fungal denitrification to the emission of the greenhouse gas nitrous oxide (N2O) from soil has not yet been sufficiently investigated. The intramolecular (15)N site preference (SP) of N2O could provide a tool to distinguish between N2O produced by bacteria or fungi, since in previous studies fungi exhibited much higher SP values than bacteria. METHODS: To further constrain isotopic evidence of fungal denitrification, we incubated six soil fungal strains under denitrifying conditions, with either NO3(-) or NO2(-) as the electron acceptor, and measured the isotopic signature (δ(18)O, δ(15)Nbulk and SP values) of the N2O produced. The nitrogen isotopic fractionation was calculated and the oxygen isotope exchange associated with particular fungal enzymes was estimated. RESULTS: Five fungi of the order Hypocreales produced N2O with a SP of 35.1 ± 1.7 ‰ after 7 days of anaerobic incubation independent of the electron acceptor, whereas one Sordariales species produced N2O from NO2(-) only, with a SP value of 21.9 ± 1.4 ‰. Smaller isotope effects of (15)Nbulk were associated with larger N2O production. The δ(18)O values were influenced by oxygen exchange between water and denitrification intermediates, which occurred primarily at the nitrite reduction step. CONCLUSIONS: Our results confirm that SP of N2O is a promising tool to differentiate between fungal and bacterial N2O from denitrification. Modelling of oxygen isotope fractionation processes indicated that the contribution of the NO2(-) and NO reduction steps to the total oxygen exchange differed among the various fungal species studied. However, more information is needed about different biological orders of fungi as they may differ in denitrification enzymes and consequently in the SP and δ(18)O values of the N2O produced.

Fungal oxygen exchange between denitrification intermediates and water.

RATIONALE: Fungi can contribute greatly to N2O production from denitrification. Therefore, it is important to quantify the isotopic signature of fungal N2O. The isotopic composition of N2O can be used to identify and analyze the processes of N2O production and N2O reduction. In contrast to bacteria, information about the oxygen exchange between denitrification intermediates and water during fungal denitrification is lacking, impeding the explanatory power of stable isotope methods. METHODS: Six fungal species were anaerobically incubated with the electron acceptors nitrate or nitrite and (18)O-labeled water to determine the oxygen exchange between denitrification intermediates and water. After seven days of incubation, gas samples were analyzed for N2O isotopologues by isotope ratio mass spectrometry. RESULTS: All the fungal species produced N2O. N2O production was greater when nitrite was the sole electron acceptor (129 to 6558 nmol N2O g dw(-1) h(-1)) than when nitrate was the electron acceptor (6 to 47 nmol N2O g dw(-1) h(-1)). Oxygen exchange was complete with nitrate as electron acceptor in one of five fungi and with nitrite in two of six fungi. Oxygen exchange of the other fungi varied (41 to 89% with nitrite and 11 to 61% with nitrate). CONCLUSIONS: This is the first report on oxygen exchange with water during fungal denitrification. The exchange appears to be within the range previously reported for bacterial denitrification. This adds to the difficulty of differentiating N2O producing processes based on the origin of N2O-O. However, the large oxygen exchange repeatedly observed for bacteria and now also fungi could lead to less variability in the δ(18)O values of N2O from soils, which could facilitate the assessment of the extent of N2O reduction.