Influence of soil moisture on isoprene emissions from a northern mixed hardwood forest

The objective of this study was to identify a relationship between soil moisture and isoprene emissions and treat the variables in a manner that could be compared with the Model of Emissions for Gases and Aerosols in Nature model’s (MEGAN) treatment of changes in soil moisture as a factor affecting emissions of isoprene. Soil moisture data and isoprene flux measurements from a northern mixed hardwood forest in northern Michigan were studied for the 2000-2005 growing seasons. The two variables were examined using the parameters from the MEGAN model where soil moisture emission activity factors, calculated from observed soil moisture measurement data, were compared to observed isoprene fluxes normalized for photosynthetic photon flux density, temperature, and light. Variability in the trends was observed between the individual years showing positive, negative and no relationships. The results demonstrate that there is no relationship between soil moisture and isoprene emissions. INTRODUCTION Considered among one of the most significant atmosphere-biosphere interactions (Sharkey et al. 2001), the production and emission of isoprene (2-methyl-1,3-butadiene) from vegetation has a profound influence on atmospheric chemistry (Guenther et al. 2000). For North America isoprene annual emissions are estimated to be 29 Tg C/yr, which is 35% of total biogenic volatile organic compounds (BVOC) emissions, and which is comparable to anthropogenic VOC emissions (Guenther et al. 2000). In NOxrich environments, such as the eastern U.S., photochemical oxidation of VOCs (including isoprene) can increase ozone (O3) concentrations by 20-80% (Pierce et al. 1998). On a global scale oxidation of isoprene with a hydroxyl radical modifies the atmosphere’s oxidative capacity resulting in longer atmospheric residence times for methane (CH4), which is an important greenhouse gas. Isoprene can also produce small amounts of secondary organic aerosols, which have implications for climate change (ref.). As a result, the development and evaluation of biogenic emission models is a critical aspect for managing air quality and for understanding global climate change. Accurate measurements of BVOC emissions are needed for regional atmospheric chemistry and climate models. Biogenic isoprene is a hemiterpene made from dimethylallyl pyrophosphate (DMAPP) by the enzyme isoprene synthase (Silver and Fall 1991). Isoprene emission is influenced by both environmental and physiological processes. And in contrast to many other VOCs produced by plants, isoprene is not stored within specialized storage organelles (Monson et al. 1995); but rather, isoprene upon synthesis diffuses from within the chloroplast and leave out through the stomata of the leaf epidermis (Lerdau 2007). Through a combination of laboratory and field studies the patterns and magnitudes of isoprene emission have been determined for a wide variety of plant species (Guenther et al. 2000). It is well known that isoprene emissions only occur during the day and the emission rate is strongly dependent on light and temperature (Sanadze 1969;Guenther et al. 1993). At 30oC isoprene emission rates can be as high as 100 μg/m 2 /h for deciduous species such as oak and aspen (ref.). Isoprene is emitted from a number of deciduous species including oak, aspen, eucalyptus, and for conifers only from spruce (ref.). While research has established an important role for isoprene in biosphereatmosphere interactions, it is unclear why isoprene is produced and emitted by plants. There are currently three competing hypotheses for why plants emit isoprene. The first postulates that isoprene allows for thermotolerance protecting the plant against short high-temperature episodes (Sharkey and Singsaas 1995). The second hypothesis proposes that isoprene serves as an antioxidant protecting the plant against detrimental exposure to ozone (Sharkey 2001), and the third suggests that isoprene emission serves as a route for releasing excess carbon and energy (Logan et al. 2000 as cited in Sharkey 2001). In addition to that critical question, there remains a gap in knowledge regarding our understanding of the relationships affecting isoprene behavior. Recent temperature history, shaded and sunlit leaves, time within the growing season, and soil moisture are among a number of environmental factors for which the influences on isoprene emission remain unclear. A greater understanding of isoprene emissions and its relationship to a number of environmental factors is critical for generating models with improved ability to predict isoprene emissions and impacts on photochemistry. Measurements of isoprene emissions have been made using leaf or branch enclosure techniques (ref.), and at the canopy scale, using eddy covariance methods (Guenther and Hills 1998; Westberg et al. 1998; Pressley et al. 2005). The canopy scale eddy flux measurements are particularly valuable for linking leaf level dynamics to regional scale emissions. Pressley et al. made measurements over a northern hardwood forest using eddy flux techniques covering four growing seasons (1999-2002). This flux record now extends into 2007. Our understanding of leaf level emissions has been incorporated into emission inventory models as a basis for treating isoprene within regional and global scale photochemical modeling systems. Two current emission inventory models include the USEPA Biogenic Emission Inventory System 3 (BEIS3) (Pierce et al. 2002) and the Model of Emissions of Gases and Aerosols from Nature (MEGAN) (Guenther et al. 2006). Pressley et al. (2005) employed long-term above canopy isoprene flux measurements to evaluate the performance of BEIS3. The results showed that BEIS3 estimates agreed well with observations during the midsummer period but BEIS3 overestimates observations during the spring onset and fall decline of emissions (Pressley et al. 2005). Predictions with the BEIS3 model yielded fractional errors ranging from 31%-65% (Pressley) – showing that current models do not correctly capture all of the variability associated with observed isoprene fluxes. In the MEGAN model Guenther and colleagues have developed a framework to treat a large number of environmental factors that can affect emissions. For example, the effects of soil moisture are incorporated through relationships between stomatal conductance and soil moisture and also through an empirical emission activity factor that is a function of soil moisture. However, for soil moisture and other environmental parameters there have been no comprehensive evaluations of the MEGAN model. Predicted isoprene fluxes (MEGAN) are plotted versus observed isoprene fluxes in Figure 1. In some years (2001, 2002, 2005) predicted and observed fluxes are in fairly to very good agreement. However, in other years (2000, 2003, 2004) there significant disagreement and it is not clear why MEGAN’s ability to predict isoprene emissions is so variable. Fig. 1. Year to year comparison of MEGAN predictions of isoprene flux with observed isoprene fluxes for a mixed northern hardwood forest located in northern Michigan. Goals and Hypotheses The overall goal for this work is to improve our understanding of isoprene emissions and improve our ability to accurately model isoprene emissions for regional and global chemical modeling. Specifically, we investigate the relationship of soil moisture and isoprene emissions; and investigate the relative importance of stomatal and non-stomatal emissions of isoprene. We used the available continuous isoprene flux data from the PROPHET Tower site and UMBS Flux data to evaluate the relationship of soil moisture and isoprene emissions in a manner that can be compared with the MEGAN’s treatment of soil moisture. Based on MEGAN’s treatment of soil moisture we expect decreased soil moisture will correspond to decreased isoprene emissions.