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

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

Drought mitigates the adverse effects of O3 on plant photosynthesis rather than growth: A global meta-analysis considering plant functional types.

Tropospheric ozone (O3 ) is a phytotoxic air pollutant adversely affecting plant growth. High O3 exposures are often concurrent with summer drought. The effects of both stresses on plants are complex, and their interactions are not yet well understood. Here, we investigate whether drought can mitigate the negative effects of O3 on plant physiology and growth based on a meta-analysis. We found that drought mitigated the negative effects of O3 on plant photosynthesis, but the modification of the O3 effect on the whole-plant biomass by drought was not significant. This is explained by a compensatory response of water-deficient plants that leads to increased metabolic costs. Relative to water control condition, reduced water treatment decreased the effects of O3 on photosynthetic traits, and leaf and root biomass in deciduous broadleaf species, while all traits in evergreen coniferous species showed no significant response. This suggested that the mitigating effects of drought on the negative impacts of O3 on the deciduous broadleaf species were more extensive than on the evergreen coniferous ones. Therefore, to avoid over- or underestimations when assessing the impact of O3 on vegetation growth, soil moisture should be considered. These results contribute to a better understanding of terrestrial ecosystem responses under global change.

Opposing effects of warming on the stability of above- and belowground productivity in facing an extreme drought event.

Climate warming, often accompanied by extreme drought events, could have profound effects on both plant community structure and ecosystem functioning. However, how warming interacts with extreme drought to affect community- and ecosystem-level stability remains a largely open question. Using data from a manipulative experiment with three warming treatments in an alpine meadow that experienced one extreme drought event, we investigated how warming modulates resistance and recovery of community structural and ecosystem functional stability in facing with extreme drought. We found warming decreased resistance and recovery of aboveground net primary productivity (ANPP) and structural resistance but increased resistance and recovery of belowground net primary productivity (BNPP), overall net primary productivity (NPP), and structural recovery. The findings highlight the importance of jointly considering above- and belowground processes when evaluating ecosystem stability under global warming and extreme climate events. The stability of dominant species, rather than species richness and species asynchrony, was identified as a key predictor of ecosystem functional resistance and recovery, except for BNPP recovery. In addition, structural resistance of common species contributed strongly to the resistance changes in BNPP and NPP. Importantly, community structural resistance and recovery dominated the resistance and recovery of BNPP and NPP, but not for ANPP, suggesting the different mechanisms underlie the maintenance of stability of above- versus belowground productivity. This study is among the first to explain that warming modulates ecosystem stability in the face of extreme drought and lay stress on the need to investigate ecological stability at the community level for a more mechanistic understanding of ecosystem stability in response to climate extremes.

Phylogenetic Conservation of Soil Microbial Responses to Elevated Tropospheric Ozone and Nitrogen Fertilization.

Plant primary productivity and crop yields have been reduced due to the doubled level of global tropospheric ozone. Little is known about how elevated ozone affects soil microbial communities in the cropland ecosystem and whether such effects are sensitive to the nitrogen (N) supply. Here, we examined the responses of bacterial and fungal communities in maize soils to elevated ozone (+60 ppb ozone) across different levels of N fertilization (+60, +120, and +240 kg N ha-1yr-1). The fungal alpha diversity was decreased (P < 0.05), whereas the bacterial alpha diversity displayed no significant change under elevated ozone. Significant (P < 0.05) effects of N fertilization and elevated ozone on both the bacterial and fungal communities were observed. However, no interactive effects between N fertilization and elevated ozone were observed for bacterial and fungal communities (P > 0.1). The bacterial responses to N fertilization as well as the bacterial and fungal responses to elevated ozone were all phylogenetically conserved, showing universal homogeneous selection (homogeneous environmental conditions leading to more similar community structures). In detail, bacterial Alphaproteobacteria, Actinobacteria, and Chloroflexi, as well as fungal Ascomycota, were increased by elevated ozone, whereas bacterial Gammaproteobacteria, Bacteroidetes, and Elusimicrobia, as well as fungal Glomeromycota, were decreased by elevated ozone (P < 0.05). These ozone-responsive phyla were generally correlated (P < 0.05) with plant biomass, plant carbon (C) uptake, and soil dissolved organic C, demonstrating that elevated ozone affects plant-microbe interactions. Our study highlighted that microbial responses to elevated ozone display a phylogenetic clustering pattern, suggesting that response strategies to elevated ozone stress may be phylogenetically conserved ecological traits. IMPORTANCE The interactions of plant and soil microbial communities support plant growth and health. The increasing tropospheric ozone decreases crop biomass and also alters soil microbial communities, but the ways in which crops and their associated soil microbial communities respond to elevated tropospheric ozone are not clear, and it is also obscure whether the interactions between ozone and the commonly applied N fertilization exist. We showed that the microbial responses to both elevated ozone and N fertilization were phylogenetically conserved. However, the microbial communities that responded to N fertilization and elevated ozone were different, and this was further verified by the lack of an interactive effect between N fertilization and elevated ozone. Given that the global tropospheric ozone concentration will continue to increase in the coming decades, the decrease of specific microbial populations caused by elevated ozone would result in the extinction of certain microbial taxa. This ozone-induced effect will further harm crop production, and awareness is urgently needed.

Variations in leaf anatomical characteristics drive the decrease of mesophyll conductance in poplar under elevated ozone.

Decline in mesophyll conductance (gm ) plays a key role in limiting photosynthesis in plants exposed to elevated ozone (O3 ). Leaf anatomical traits are known to influence gm , but the potential effects of O3 -induced changes in leaf anatomy on gm have not yet been clarified. Here, two poplar clones were exposed to elevated O3 . The effects of O3 on the photosynthetic capacity and anatomical characteristics were assessed to investigate the leaf anatomical properties that potentially affect gm . We also conducted global meta-analysis to explore the general response patterns of gm and leaf anatomy to O3 exposure. We found that the O3 -induced reduction in gm was critical in limiting leaf photosynthesis. Changes in liquid-phase conductance rather than gas-phase conductance drive the decline in gm under elevated O3, and this effect was associated with thicker cell walls and smaller chloroplast sizes. The effects of O3 on palisade and spongy mesophyll cell traits and their contributions to gm were highly genotype-dependent. Our results suggest that, while anatomical adjustments under elevated O3 may contribute to defense against O3 stress, they also cause declines in gm and photosynthesis. These results provide the first evidence of anatomical constraints on gm under elevated O3 .

Enhanced immobility of Pb(II) during ferrihydrite-Pb(II) coprecipitates aging impacted by malic acid or phosphate.

Metastable ferrihydrite is omnipresent in environments and can influence the fate of Pb(II) during ferrihydrite transformation. Ferrihydrite is rarely pure and often coexists with impurities, which may influence the mineralogical changes of ferrihydrite and Pb(II) behavior. In this work, we investigated the effect of malic acid or phosphate on Pb(II)-ferrihydrite coprecipitates (Fh-Pb) transformation and the subsequent fate of Pb(II) during the 10-day aging of Fh-Pb. Results showed that both malic acid and phosphate retarded Fh-Pb transformation and prevented the release of Pb(II) from Fh-Pb back into solutions. Pb(II) was beneficial to goethite formation by inhibiting hematite formation while both malic acid and phosphate inhibited goethite formation since they could act as templates of nucleation. Besides, malic acid and phosphate improved the proportion of non-extracted Pb(II) during Fh-Pb transformation, indicating that Pb(II) was incorporated into secondary minerals. Pb(II) could not replace Fe(III) within the crystal lattice due to its large radius but was occluded into pores and defect structures within the secondary mineral lattices. This work can advance our understanding of the influences of malic acid and phosphate on Pb(II) immobility during Fh-Pb aging.

Nitrogen enrichment alters climate sensitivity of biodiversity and productivity differentially and reverses the relationship between them in an alpine meadow.

Biodiversity and productivity that highly determine ecosystem services are varying largely under global change. However, the climate sensitivity of them and their relationship are not well understood, especially in the context of increasing nitrogen (N) deposition. Here, based on a six-year N manipulation experiment in an alpine meadow, we quantified interannual climate sensitivity of species richness (SR) and above-ground net primary productivity (ANPP) as well as SR-ANPP relationship as affected by six N addition rate (Nrate) gradients. We found that interannual variations in ANPP and SR were mainly driven by temperature instead of precipitation. In the plots without N addition, higher temperature substantially increased ANPP but reduced SR across years, thus resulting in a negative SR-ANPP relationship. However, the negative and positive responses of SR and ANPP to temperature increased and declined significantly with increasing Nrate, respectively, leading to a shift of the negative relationship between SR and ANPP into a positive one under high Nrate. Moreover, the adverse influence of drought on SR and ANPP would be aggravated by N fertilization, as indicated by the increased positive effect of precipitation on them under N enrichment. Our findings indicate that climate sensitivity of productivity and biodiversity may be misestimated if the impact of N deposition is not considered, and the importance of biodiversity to maintain productivity would enhance as N deposition increases. This study provides a new insight to explain variation of biodiversity-productivity relationship along with environmental changes.

Elevated ozone decreases the activity of Rubisco in poplar but not its activation under fluctuating light.

Increasing tropospheric ozone (O3) is well-known to decrease leaf photosynthesis under steady-state light through reductions in biochemical capacity. However, the effects of O3 on photosynthetic induction and its biochemical limitations in response to fluctuating light remain unclear, despite the rapid fluctuations of light intensity occurring under field conditions. In this study, two hybrid poplar clones with different O3 sensitivities were exposed to elevated O3. Dynamic photosynthetic CO2 response measurements were conducted to quantify the impact of elevated O3 and exposure duration on biochemical limitations during photosynthetic induction. We found that elevated O3 significantly reduced the steady-state light-saturated photosynthetic rate, the maximum rate of carboxylation (Vcmax) and Rubisco content. In addition, elevated O3 significantly decreased the time constants for slow phases and weighting of the fast phase of the Vcmax induction in poplar clone '546' but not in clone '107'. However, elevated O3 did not affect the time, it took to reach a given percentage of full Vcmax activation or photosynthetic induction in either clone. Overall, photosynthetic induction was primarily limited by the activity of Rubisco rather than the regeneration of ribulose-1,5-biphosphate regardless of O3 concentration and exposure duration. The lack of O3-induced effects on the activation of Rubisco observed here would simplify the simulation of impacts of O3 on nonsteady-state photosynthesis in dynamic photosynthetic models.

Interaction between Se(IV) and fulvic acid and its impact on Se(IV) immobility in ferrihydrite-Se(IV) coprecipitates during aging.

Selenium (Se) is regarded as a trace element for humans, but it is toxic in excess. In natural environments, the mobility of Se is dominantly controlled by the Se oxyanions with high solubility such as selenite (Se(IV)). Se(IV) is often associated with the omnipresent ferrihydrite and coexisting organic matter. However, there is little information on the dynamic interactions among Se(IV), fulvic acid, and ferrihydrite. This study investigated the influence of fulvic acid on ferrihydrite-Se(IV) coprecipitates (Fh-Se) transformation for 8 days and the subsequent behavior of Se(IV) at varied pH (5.0, 7.5, and 10.0). Results showed that fulvic acid had different effects on Fh-Se transformation at varied pH values. Fh-Se transformation was promoted by fulvic acid at initial pH 5.0 whereas it was inhibited at initial pH 10.0. Interestingly, at initial pH 7.5, Fh-Se transformation was promoted at a low C/Fe ratio while it was suppressed at a high C/Fe ratio. Besides, fulvic acid induced the generation of more extractable Se(IV) at initial pH 5.0 and more coprecipitated Se(IV) at initial pH 7.5 and blocked the release of Se(IV) at initial pH 10.0. Fulvic acid possibly interacted with Se(IV) via carboxyl complexation and weakened the inhibition of Se(IV) on Fh-Se transformation. Thus, fulvic acid increased the transformation rate of Fh-Se. These findings help to uncover the environmental behavior of Se(IV) and organic matter during ferrihydrite transformation.

[Spatiotemporal Distribution and Health Impacts of PM2.5 and O3 in Beijing, from 2014 to 2020].

In China, fine particulate matter (PM2.5) and tropospheric ozone (O3) have become major air pollutants that threaten human health. Since 2013, the government has strengthened air pollution controls in Beijing and achieved significant effects. A spatial-temporal analysis was conducted of the distribution and health impacts of PM2.5 and O3 in Beijing, using data collected from 34 air quality monitoring sites between 2014 and 2020. In 2014, the annual average PM2.5 and seasonal (April to September) average of daily one-hourly maximum O3 concentrations (O3_max) were 92.0 µg·m-3 and 81.9 nmol·mol-1, respectively. From 2014 to 2020, annual average PM2.5 decreased at a rate of 7.5 µg·m-3. However, there was no significant difference in O3_max over the years. The concentrations of PM2.5 were highest in December and January (in winter) and lowest in August (in summer). On the contrary, O3_max was highest in June. The diurnal variations of PM2.5 were affected by meteorological conditions and emission sources, and maximum concentrations occurred between 22:00 to 00:00, while minimum concentrations occurred between 14:00 to 16:00. The concentration of O3_max showed an opposite pattern, with minimum vales occurring at 07:00 and maximum values occurring in the afternoon. The spatial distribution of PM2.5 showed similar patterns in 2014 and 2019, with the south of Beijing exhibiting the highest concentrations, and the north the lowest. The concentration of O3_max was higher in suburban areas than in traffic areas. In terms of health impacts, 1580 cases of cardiovascular disease and 821 of respiratory disease were attributed to PM2.5 in 2014, while 2180 cases of respiratory disease were attributable to O3 in 2014. In 2019, mortalities attributable to PM2.5 had decreased by 50% compared to 2014. While the number of disease cases attributable to O3 were similar in 2014 and 2019. the results indicate that PM2.5 pollution in Beijing has been successfully controlled, while O3 pollution has become more severe, and was the primary air pollutant threatening human health in 2019. Therefore, the synchronous control of PM2.5 and O3 should be implemented in the future.

[Relationship Between Relative Crop Yield/Woody Plant Biomass and Ground-level Ozone Pollution in China].

Currently, ground-level ozone (O3) pollution is increasingly serious in China, and highly threatens plant productivity. In this study, we summarized the relationship between relative crop yield or woody plant biomass and O3 metrics, including M7 (the mean of hourly concentrations from 09:00 to 16:00), SUM06 (sum of hourly concentrations above 60 nmol·mol-1), W126 (Sigmoidal weighted sum of the hourly concentrations during a specified period), PODY[phytotoxic O3 dose above a threshold flux of Y nmol·(m2·s)-1], and the most commonly used AOT40 (accumulated O3 concentrations over an hourly threshold of 40 nmol·mol-1), and proposed their critical level to protect plants against O3 damage. Based on the AOT40 metric, we found that the O3 risk threshold that caused a 5% decrease in yield was 5.93 µmol·mol-1·h for rice, 2.69 µmol·mol-1·h for winter wheat, 8.67 µmol·mol-1·h for maize, and 4.17 µmol·mol-1·h for soybean, indicating that maize may be more tolerant to O3 than the other three crops in China. The O3 risk threshold that led to a 5% reduction in total biomass was 12.20 µmol·mol-1·h across five poplar clones (based on experiment) and 10.87 µmol·mol-1·h across 17 woody plants (based on integrated analysis). On the other hand, some perspectives were presented concerning the establishment of O3 response relationships for important productivity-related parameters, and the improvement of accuracy in O3 regional risk assessment. It is also important to consider how to couple other important factors affecting plant O3 sensitivity (such as soil nitrogen and plant age) into the regional assessment model.

Effects of elevated ozone on maize under varying soil nitrogen levels: Biomass, nitrogen and carbon, and their allocation to kernel.

Effects of ozone (O3) on maize have been increasingly studied, but only few studies have focused on the combined impacts of O3 and nitrogen (N) on this important crop with C4 carbon (C) fixation. In this study, a maize cultivar with the largest acreage in China was exposed to two O3 treatments (NF: ambient air O3 concentration; NF60: NF plus 60 ppb O3) and four N levels (farmers' N practice: 240 kg N ha-1 yr-1; 150%, 50% and 25% of farmers' N practice). Generally, O3 and N significantly influenced biomass, N and C, but did not change their allocation to kernel. There were significant interactions between O3 and N in stem biomass, C concentration and uptake, and leaf biomass and C uptake, with significant O3 effects mainly occurring at N120 and N240. Based on the coefficient of determination (R2), root C:N ratio rather than the most commonly used leaf C:N ratio was the best trait to indicate maize productivity. Furthermore, O3 significantly increased the regression slopes between root C:N ratio and kernel N uptake, kernel C uptake and plant N uptake, strengthened the correlation of C:N ratio and kernel C uptake, and weakened the correlation of C:N ratio and hundred-kernels weight. These suggest that O3 pollution can change the relationship of C:N ratio and productivity in maize. The weak correlation between kernel harvest index (HI) and N harvest index (NHI) indicated that future breeding researches should consider how to improve the coupling between biomass and N-related nutrition allocations in crop edible parts. Our results not only are helpful to accurately estimate O3 impacts on maize with consideration of N but also provide a new insight into the relationship between plant traits and its productivity under O3 pollution.

Stomatal response drives between-species difference in predicted leaf water-use efficiency under elevated ozone.

Ozone-induced changes in the relationship between photosynthesis (An) and stomatal conductance (gs) vary among species, leading to inconsistent water use efficiency (WUE) responses to elevated ozone (O3). Thus, few vegetation models can accurately simulate the effects of O3 on WUE. Here, we conducted an experiment exposing two differently O3-sensitive species (Cotinus coggygria and Magnolia denudata) to five O3 concentrations and investigated the impact of O3 exposure on predicted WUE using a coupled An-gs model. We found that increases in stomatal O3 uptake caused linear reductions in the maximum rates of Rubisco carboxylation (Vcmax) and electron transport (Jmax) in both species. In addition, a negative linear correlation between O3-induced changes in the minimal gs of the stomatal model (g0) derived from the theory of optimal stomatal behavior and light-saturated photosynthesis was found in the O3-sensitive M. denudata. When the O3 dose-based responses of Vcmax and Jmax were included in a coupled An-gs model, simulated An under elevated O3 were in good agreement with observations in both species. For M. denudata, incorporating the O3 response of g0 into the coupled model further improved the accuracy of the simulated gs and WUE. In conclusion, the modified Vcmax, Jmax and g0 method presented here provides a foundation for improving the prediction for O3-induced changes in An, gs and WUE.

High nitrogen addition decreases the ozone flux by reducing the maximum stomatal conductance in poplar saplings.

Ground-level ozone (O3) and nitrogen (N) deposition are major environmental pollutants, often occurring concurrently. Ozone exposure- and flux-response relationships for tree biomass are used for regional O3 risk assessment. In order to investigate whether soil N addition affects stomatal O3 uptake of poplar, poplar saplings were exposed to treatment combinations of five O3 levels and four N addition levels. High N addition treatment reduced the accumulated stomatal O3 uptake in the leaf due to reduced maximum stomatal conductance (gs). Nitrogen addition also significantly reduced the steady-state light-saturated gs in August and September. Elevated O3 significantly reduced and N addition increased total plant biomass; however, there were no significant O3 × N interactions. The slopes of biomass-based O3 exposure- and flux-response relationships did not differ significantly among N treatments. The critical levels for a 5% biomass reduction were estimated at 15.4 ppm h and 17.1 mmol O3 m-2 projected leaf area (PLA) for Accumulated O3 exposure Over an hourly Threshold of 40 ppb (AOT40) and Phytotoxic Ozone Dose above a threshold 1 nmol O3 m-2 PLA s-1 (POD1). These results can facilitate the evaluations of O3 effect on the carbon-sink capacity and productivity of forest.

Impact of ozone pollution on nitrogen fertilization management during maize (Zea mays L.) production.

The impacts of ozone (O3) on crops have been extensively studied and are well understood. However, little information is available on the response of crops (especially maize) to the interactive effects of O3 and nitrogen (N) fertilizer. To this end, a maize cultivar (Zheng dan 958, ZD958) that is common in China was exposed to two O3 treatments and four N levels. We found that (1) the interactions between O3 and N were non-significant for grain yield, plant biomass, C and N, although N addition significantly increased all parameters except C concentrations in grain and plant; (2) compared to NF (non-filtered ambient air O3 concentration), NF60 (NF plus an extra 60 ppb O3) increased the optimum N application rates (Nopt) in grain yield and plant biomass, but not grain yield and plant biomass potentials, thus resulting in lower N use efficiencies (NUE) and a larger risk of N-related environmental pollution (e.g., increased N2O emission) under Nopt in NF60; (3) because of higher optimum plant N uptake (PNopt) in NF60, relative to NF, plant N-saturated conditions for grain yield potential can be gradually turned into N-limited conditions as O3 pollution increases. These findings manifest that O3 is a vital global change factor impacting the management of N fertilization. If current O3 pollution is substantially reduced, maize yield and biomass potentials can be increased under reductions in N input and N-related environmental pollution. In addition, these results can also contribute in developing and verifying Nopt model considering O3 pollution in the future.

Effects of ozone on maize (Zea mays L.) photosynthetic physiology, biomass and yield components based on exposure- and flux-response relationships.

Since the Industrial Revolution, the global ambient O3 concentration has more than doubled. Negative impact of O3 on some common crops such as wheat and soybeans has been widely recognized, but there is relatively little information about maize, the typical C4 plant and third most important crop worldwide. To partly compensate this knowledge gap, the maize cultivar (Zhengdan 958, ZD958) with maximum planting area in China was exposed to a range of chronic ozone (O3) exposures in open top chambers (OTCs). The O3 effects on this highly important crop were estimated in relation to two O3 metrics, AOT40 (accumulated hourly O3 concentration over a threshold of 40 ppb during daylight hours) and POD6 (Phytotoxic O3 Dose above a threshold flux of 6 nmol O3 m-2 s-1 during a specified period). We found that (1) the reduced light-saturated net photosynthetic rate (Asat) mainly caused by non-stomatal limitations across heading and grain filling stages, but the stomatal limitations at the former stage were stronger than those at the latter stage; (2) impact of O3 on water use efficiency (WUE) of maize was significantly dependent on developmental stage; (3) yield loss induced by O3 was mainly due to a reduction in kernels weight rather than in the number of kernels; (4) the performance of AOT40 and POD6 was similar, according to their determination coefficients (R2); (5) the order of O3 sensitivity among different parameters was photosynthetic parameters > biomass parameters > yield-related parameters; (6) Responses of Asat to O3 between heading and gran filling stages were significantly different based on AOT40 metric, but not POD6. The proposed O3 metrics-response relationships will be valuable for O3 risk assessment in Asia and also for crop productivity models including the influence of O3.

Ozone exposure- and flux-yield response relationships for maize.

A stomatal ozone (O3) flux-response relationship for relative yield of maize was established by parameterizing a Jarvis stomatal conductance model. For the function (fVPD) describing the limitation of stomatal conductance by vapor pressure deficit (VPD, kPa), cumulative VPD during daylight hours was superior to hourly VPD. The latter function is proposed as a methodological improvement of this multiplicative model when stomatal conductance peaks during the morning and it is reduced later as it is the case of maize in this experiment. The model agreed relatively well with the measured stomatal conductance (R2 = 0.63). Based on the comparison of R2 values of the response functions, POD6 (Phytotoxic Ozone Dose over an hourly threshold 6 nmol m-2 s-1) and AOT40 (accumulated hourly O3 concentrations over a threshold of 40 ppb) performed similarly. The critical levels based on POD6 and AOT40 for 5% reduction in maize yield were 1.17 mmol m-2 PLA and 8.70 ppm h, respectively. In comparison with other important crops, the ranking of sensitivity of maize strongly differed depending on the O3 metric used, AOT40 or POD6. The newly proposed response functions are relevant for O3 risk assessment for this crop in Asia.

Precision saturated absorption spectroscopy of H3.

In our previous work on the Lamb-dips of the ν2 fundamental band transitions of H3+, the saturated absorption spectrum was obtained by third-derivative spectroscopy using frequency modulation with an optical parametric oscillator (OPO). However, frequency modulation also caused errors in the absolute frequency determination. To solve this problem, we built a tunable offset locking system to lock the pump frequency of the OPO to an iodine-stabilized Nd:YAG laser. With this improvement, we were able to scan the OPO idler frequency precisely and obtain the saturated absorption profile using intensity modulation. Furthermore, ion concentration modulation was employed to subtract the background noise and increase the signal-to-noise ratio. To determine the absolute frequency of the idler wave, the OPO signal frequency was locked to an optical frequency comb. The absolute frequency accuracy of our spectrometer was better than 7 kHz, demonstrated by measuring the wavelength standard transition of methane at 3.39 µm. Finally, we measured 16 transitions of H3+ and our results agree very well with other precision measurements. This work successfully resolved the discrepancies between our previous measurements and other precision measurements.

Relative carrier-envelope phase stabilization of hybridly synchronized ultrafast Yb and Er fiber-laser systems with the feed-forward scheme.

Stabilization of the relative carrier-envelope (CE) phase for hybridly synchronized two-color fs Yb and Er fiber-laser systems is demonstrated for the first time by utilizing the feed-forward scheme based on an acousto-optic frequency shifter. The slow drift issues arising from the feed-forward scheme are solved by adding the in-loop relative CE frequency coarse stabilization via modulating the laser pump current. Sub-fs timing locking between the two-color pulses is still maintained due to the fast response and large locking range of hybrid synchronization. The approach provides an alternative way to obtain phase-stable synchronized two-color pulses with higher pulse energies.

Terahertz plasmonic waveguide based on metal rod arrays for nanofilm sensing.

A high-aspect-ratio metallic rod array is demonstrated to generate and propagate highly confined terahertz (THz) surface plasmonic waves under end-fire excitation. The transverse modal power distribution and spectral properties of the bound THz plasmonic wave are characterized in two metallic rod arrays with different periods and in two configurations with and without attaching a subwavelength superstrate. The integrated metallic rod array-based waveguide can be used to sense the various thin films deposited on the polypropylene superstrate based on the phase-sensitive mechanism. The sensor exhibits different phase detection sensitivities depending on the modal power immersed in the air gaps between the metallic rods. Deep-subwavelength SiO(2) and ZnO nanofilms with an optical path difference of 252 nm, which is equivalent to λ/3968 at 0.300 THz, are used as analytes to test the integrated plasmonic waveguide. Analysis of the refractive index and thickness of molecular membranes indicates that the metallic rod array-based THz waveguide can integrate various biochip platforms for minute molecular detection, which is extremely less than the coherent length of THz waves.