Are BVOC exchanges in agricultural ecosystems overestimated? Insights from fluxes measured in a maize field over a whole growing season

Abstract

Abstract. Although maize is the second most important crop worldwide, and the most important C4 crop, no study on biogenic volatile organic compounds (BVOCs) has yet been conducted on this crop at ecosystem scale and over a whole growing season. This has led to large uncertainties in cropland BVOC emission estimations. This paper seeks to fill this gap by presenting, for the first time, BVOC fluxes measured in a maize field at ecosystem scale (using the disjunct eddy covariance by mass scanning technique) over a whole growing season in Belgium. The maize field emitted mainly methanol, although exchanges were bi-directional. The second most exchanged compound was acetic acid, which was taken up mainly in the growing season. Bi-directional exchanges of acetaldehyde, acetone and other oxygenated VOCs also occurred, whereas the terpenes, benzene and toluene exchanges were small, albeit significant. Surprisingly, BVOC exchanges were of the same order of magnitude on bare soil and on well developed vegetation, suggesting that soil is a major BVOC reservoir in agricultural ecosystems. Quantitatively, the maize BVOC emissions observed were lower than those reported in other maize, crops and grasses studies. The standard emission factors (SEFs) estimated in this study (231 ± 19 µg m −2 h −1 for methanol, 8 ± 5 µg m −2 h −1 for isoprene and 4 ± 6 µg m −2 h −1 for monoterpenes) were also much lower than those currently used by models for C4 crops, particularly for terpenes. These results suggest that maize fields are small BVOC exchangers in north-western Europe, with a lower BVOC emission impact than that modelled for growing C4 crops in this part of the world. They also reveal the high variability in BVOC exchanges across world regions for maize and suggest that SEFs should be estimated for each region separately.

Methanol emissions from maize: Ontogenetic dependence to varying light conditions and guttation as an additional factor constraining the flux

“…Although still smaller by a factor 2, our results for stage 2 and 3 maize leaves at maximum PPFD agree better with the recent eddy covariance methanol flux measurements obtained over a maize field (same variety as in this study) in Belgium (Bachy et al, 2016) at similar light and temperature and plant developmental conditions. Comparison of leaf scale and ecosystem scale measurements, however, is not straightforward and the difference in methanol flux values with Bachy et al (2016) could have several reasons such as the simultaneous contribution from growing and fully mature leaves, emissions from the soil and other parts of the plant (flowers, fruit), and the variation of light and temperature in the canopy. Because of these important differences amongst studies, more experiments are definitely required in order to c Taking into account a biomass dry weight of 100 g per m 2 of soil surface area (Das et al, 2003).…”

“…Because of these important differences amongst studies, more experiments are definitely required in order to c Taking into account a biomass dry weight of 100 g per m 2 of soil surface area (Das et al, 2003). d This study was carried out over an entire growth season, but only data from 73 to 96 days old maize plants (developmental stage R1 in Bachy et al, 2016) and within the indicated PPFD and temperature range were considered for averaging in order to allow a comparison with methanol emission rates from stage 2 and 3 leaves in this study.…”

“…Those studies, however, only covered a very limited period of the growing season and were conducted in very similar weather conditions. Recently, a field study was conducted to measure methanol exchanges from maize under natural environmental conditions for a whole growing season (Bachy et al, 2016). These flux measurements were performed at ecosystem-scale using the eddy covariance technique, thereby encompassing both soil and plant exchanges.…”

Dynamics and mechanisms of volatile organic compound exchanges in a winter wheat field

“…The instrumental set-up, flux calculation, filtering and error evaluation used for this campaign were identical to those used in two former BVOC flux measurement campaign performed on a maize field and on bare soil at the same site and were thoroughly described in Bachy et al (2016) and Bachy et al (2018). Time-lag determination, correction for low-pass filtering and uncertainty analysis are particularly important aspects, especially when dealing with disjunct eddy-covariance (Rinne and Ammann, 2012).…”

“…The new SEFs were based on a detailed literature survey, but, probably because of a lack of available and usable data for these ecosystems, the SEFs assigned for croplands still relied on default values or on flux values measured for minor crops (in terms of harvested area). From a BVOC flux measurement study conducted on a maize field, which is the second most important worldwide crop, it was also concluded that these SEFs may be highly over-estimated for C4 crops grown in North-West Europe (Bachy et al, 2016). As a consequence, there is a real need to quantify SEFs from flux measurements for major crops, including winter wheat.…”