Abstract
Abstract. During the GRAMINAE Integrated Experiment between 20 May and 15 June 2000, the ozone flux was measured by the eddy covariance method above intensively managed grassland in Braunschweig, northern Germany. Three different phases of vegetation were covered during the measuring campaign: tall grass canopy before cut (29 May 2000), short grass after cut, and re-growing vegetation after fertilization (5 June 2000). Results show that beside weather conditions, the agricultural activities significantly influenced the O3 fluxes. After the cut the daytime average of the deposition velocity (vd) decreased from 0.44 cm s−1 to 0.26 cm s−1 and increased again to 0.32 cm s−1 during the third period. Detailed model calculations were carried out to estimate deposition velocity and ozone flux. The model captures the general diurnal patter of deposition, with vd daytime values of 0.52, 0.24, and 0.35 cm s−1 in the first, second and third period, respectively. Thus the model predicts a stronger response to the cut than the measurements, which is nevertheless smaller than expected on the basis of change in leaf area. The results show that both cut and fertilization have complex impacts on fluxes. Reduction of vegetation by cutting decreased the stomatal flux initially greatly, but the stomatal flux recovered to 80% of its original value within a week. At the same time, the non-stomatal flux appears to have increased directly after the cut, which the model partially explains by an increase in the deposition to the soil. A missing sink after the cut may be the chemical interaction with biogenic volatile organic compounds released after the cut and exposed senescent plant parts, or the increase in soil NO emissions after fertilization. Increased canopy temperatures may also have promoted ozone destruction on leaf surfaces. These results demonstrate the importance of canopy structure and non-stomatal pathways on O3 fluxes.
Highlights
Tropospheric ozone (O3) has important effects on human health (Weschler, 2006) and plant functioning (Emberson, 2003)
Ozone can undergo chemical interactions with NO (Duyzer et al, 1997), biogenic volatile organic compounds (VOCs) and hydroxyl and nitrate radicals (Fuentes et al, 2007), and it can be destroyed on leaf surfaces and soils
While gas-phase chemistry has been suggested to dominate the non-stomatal in canopies emitting large amounts of VOCs (e.g. Blodgett Forest, CA; Kurpius and Goldstein, 2003), most studies have explained non-stomatal fluxes with surface processes, which is consistent with studies that show O3 to be destroyed at non-biological surfaces (e.g. Cape et al, 2009)
Summary
Tropospheric ozone (O3) has important effects on human health (Weschler, 2006) and plant functioning (Emberson, 2003). Significant progress has been made in the last decades in understanding the cycling of O3 in the troposphere (Crutzen et al, 1999), there are still gaps in our understanding of the deposition processes (Ashmore et al, 2007). This applies to the quantification of dry deposition, and in particular the fraction of the O3 that is absorbed through the stomata (and can cause plant damage) and the controls of the non-stomatal deposition. Even where canopy chemical reactions only account for a small fraction of the O3 flux they can substantially modify the NO and VOCs fluxes
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