Bi-directional exchange of ammonia above a corn crop canopy: from flux measurements to model parameterizations

<p>Emissions of ammonia (NH<sub>3</sub>) from agriculture have a significant impact on the environment. Its atmospheric transport and subsequent deposition has been shown to alter nutrient-poor ecosystems thereby reducing biodiversity. As the most abundant base in the atmosphere, NH<sub>3</sub> plays a key role in secondary aerosol formation impacting air quality and climate. Due to the lack of long term observations and challenges in performing NH<sub>3</sub> flux measurements, large uncertainties exist in both emission quantification from fertilized crop fields and in the bi-directional exchange of NH<sub>3</sub> with agroecosystems. We measured NH<sub>3</sub> fluxes above a corn field using the eddy covariance technique together with a quantum cascade laser spectroscopy analyzer over two consecutive growing seasons in 2017 and 2018. We found that after initial NH<sub>3</sub> emissions following fertilizer application, periods of both NH<sub>3</sub> emission and deposition with similar flux magnitudes prevailed throughout the growing seasons (ranging approximately between ±300 ng m<sup>-2</sup> s<sup>-1</sup>), highlighting the importance of the corn crop canopy for regulating the net NH<sub>3</sub> exchange. To evaluate the underlying processes of the NH<sub>3</sub> bi-directional exchange, a two-layer compensation point model was used. Based on the large range of environmental conditions encountered during the extensive flux measurements periods, the validity of different parameterizations could be assessed. In particular, processes regulating stomatal and non-stomatal flux pathways will be discussed.</p>

Economic impacts of ambient ozone pollution on wood production in Italy

Worldwide, tropospheric ozone (O3) is a potential threat to wood production, but our understanding of O3 economic impacts on forests is still limited. To overcome this issue, we developed an approach for integrating O3 risk modelling and economic estimates, by using the Italian forests as a case study. Results suggested a significant impact of O3 expressed in terms of stomatal flux with an hourly threshold of uptake (Y = 1 nmol O3 m−2 leaf area s−1 to represent the detoxification capacity of trees), i.e. POD1. In 2005, the annual POD1 averaged over Italy was 20.4 mmol m−2 and the consequent potential damage ranged from 790.90 M€ to 2.85 B€ of capital value (i.e. 255–869 € ha−1, on average) depending on the interest rate. The annual damage ranged from 31.6 to 57.1 M€ (i.e. 10–17 € ha−1 per year, on average). There was also a 1.1% reduction in the profitable forest areas, i.e. with a positive Forest Expectation Value (FEV), with significant declines of the annual national wood production of firewood (− 7.5%), timber pole (− 7.4%), roundwood (− 5.0%) and paper mill (− 4.8%). Results were significantly different in the different Italian regions. We recommend our combined approach for further studies under different economic and phytoclimatic conditions.

Trends in tropospheric ozone concentrations and forest impact metrics in Europe over the time period 2000–2014

In Europe, tropospheric ozone pollution appears as a major air quality issue, and ozone concentrations remain potentially harmful to vegetation. In this study we compared the trends of two ozone metrics widely used for forests protection in Europe, the AOT40 (Accumulated Ozone over Threshold of 40 ppb) which only depends on surface air ozone concentrations, and the Phytotoxic Ozone Dose which is the accumulated ozone uptake through stomata over the growing season, and above a threshold Y of uptake (PODY). By using a chemistry transport model, we found that European-averaged ground-level ozone concentrations (− 2%) and AOT40 metric (− 26.5%) significantly declined from 2000 to 2014, due to successful control strategies to reduce the emission of ozone precursors in Europe since the early 1990s. In contrast, the stomatal ozone uptake by forests increased from 17.5 to 26.6 mmol O3 m−2 despite the reduction in ozone concentrations, leading to an increase of potential ozone damage on plants in Europe. In a climate change context, a biologically-sound stomatal flux-based standard (PODY) as new European legislative standard is needed.

Can Reduced Irrigation Mitigate Ozone Impacts on an Ozone-Sensitive African Wheat Variety?

Ground-level ozone (O3) pollution is known to adversely affect the production of O3-sensitive crops such as wheat. The magnitude of impact is dependent on the accumulated stomatal flux of O3 into the leaves. In well-irrigated plants, the leaf pores (stomata) tend to be wide open, which stimulates the stomatal flux and therefore the adverse impact of O3 on yield. To test whether reduced irrigation might mitigate O3 impacts on flag leaf photosynthesis and yield parameters, we exposed an O3-sensitive Kenyan wheat variety to peak concentrations of 30 and 80 ppb O3 for four weeks in solardomes and applied three irrigation regimes (well-watered, frequent deficit, and infrequent deficit irrigation) during the flowering and grain filling stage. Reduced irrigation stimulated 1000-grain weight and harvest index by 33% and 13%, respectively (when O3 treatments were pooled), which compensated for the O3-induced reductions observed in well-watered plants. Whilst full irrigation accelerated the O3-induced reduction in photosynthesis by a week, such an effect was not observed for the chlorophyll content index of the flag leaf. Further studies under field conditions are required to test whether reduced irrigation can be applied as a management tool to mitigate adverse impacts of O3 on wheat yield.

Ozone phytotoxicity in the Western Carpathian Mountains in Slovakia

In this work, the response of temperate coniferous forests to ozone air pollution (O3) in the mountain environment of the High Tatra Mts. (Western Carpathians) was analyzed. The modelling of stomatal O3 flux is a complex method for the estimation of phytotoxicity of O3 pollution to forest vegetation. Stomatal flux-based critical levels (CLef) for effects of O3 on radial growth take into account the varying influences of O3 concentration, meteorological variables, soil properties, and phenology. The application of the model DO3SE (Deposition of Ozone for Stomatal Exchange) at five experimental plots with altitudes varying from 810 to 1,778 m a.s.l. along vertical and spatial profile in the High Tatra Mts. revealed the high phytotoxic potential of O3 on spruce forests during the growing season 2014. The accumulated stomatal O3 flux above a threshold of Y (1 nmol m−2 s−1), i.e. POD1 (Phytotoxic Ozone Dose) ranged from 13.6 mmol m−2 at the Kolové pleso site (1,570 m a.s.l.) to 16.2 mmol m−2 at Skalnaté Pleso site (1,778 m a.s.l.). CLef for POD1 (8 mmol m−2) recommended for the protection of spruce forests were exceeded at all experimental plots from early July. Similarly, AOT40 index suggests vulnerability of mountain forests to O3 pollution. AOT40 values increased with altitude and reached values varying from 6.2 ppm h in Stará Lesná (810 m a.s.l.) to 10.7 ppm h at Skalnaté Pleso close to the timber line (1,778 m a.s.l.). Concentration-based critical level (CLec) of 5,000 ppb h was exceeded from June to August and was different for each experimental site.

Predicting and partitioning ozone fluxes to maize crops from sowing to harvest: the Surfatm-O₃ model

Terrestrial ecosystems represent a major sink for ozone (O3) and also a critical control of tropospheric O3 budget. However, due to its deleterious effects, plant functioning is affected by the ozone absorbed. It is thus necessary to both predict total ozone deposition to ecosystems and partition the fluxes in stomatal and non-stomatal pathways. The Surfatm-O3 model was developed to predict ozone deposition to agroecosystems from sowing to harvest, taking into account each deposition pathways during bare soil, growth, maturity, and senescence periods. An additional sink was added during senescence: stomatal deposition for yellow leaves, not able to photosynthesise but transpiring. The model was confronted to measurements performed over three maize crops in different regions of France. Modelled and measured fluxes agreed well for one dataset for any phenological stage, with only 4% difference over the whole cropping season. A larger discrepancy was found for the two other sites, 15% and 18% over the entire study period, especially during bare soil, early growth and senescence. This was attributed to site-specific soil resistance to ozone and possible chemical reactions between ozone and volatile organic compounds emitted during late senescence. Considering both night-time and daytime conditions, non-stomatal deposition was the major ozone sink, from 100% during bare soil period to 70–80% on average during maturity. However, considering only daytime conditions, especially under optimal climatic conditions for plant functioning, stomatal flux could represent 75% of total ozone flux. This model could improve estimates of crop yield losses and projections of tropospheric ozone budget.