Contribution of aboveground plants, the rhizosphere and root-free-soils to total COS and DMS fluxes at three key growth stages in rice paddies

Carbonyl sulfide (COS) and dimethyl sulfide (DMS) are trace sulfur gases that contribute substantially to sulfate aerosols or cloud condensation nuclei in the upper and lower atmosphere and therefore play great roles in the earth's radiative balance. COS and DMS fluxes in rice (Oryza sativa L.) have been measured previously in rice paddies, and this study was designed to further investigate the role of aboveground plants, the rhizosphere and the root-free-soils in the exchange of COS and DMS by comparing COS and DMS fluxes among planted plots (PP), non-planted plots (NPP), plots with aboveground plants just cut before test (plants-cut plots, PCP) at three key growth stages (tillering, jointing-booting and mature stage). The average COS fluxes in NPP, PCP, and PP were 10.5 ± 3.8, 6.1 ± 4.6, and −12.3 ± 5.0 pmol m−2 s−1, suggesting that COS emission from root-free-soils (10.5 pmol m−2 s−1) was surpassed by uptake of the rhizosphere (−4.4 pmol m−2 s−1) and aboveground plants (−18.4 pmol m−2 s−1). DMS emission rates were 60.5 ± 25.2, 20.2 ± 7.3 and 30.8 ± 10.8 pmol m−2 s−1 in PP, NPP and PCP, respectively. Calculated DMS emission from the aboveground plants, rhizosphere and the root-free-soils were 29.7, 10.6 and 20.2 pmol m−2 s−1, accounting respectively for 49.0%, 17.6% and 33.4% of total DMS emission. Directly measured COS and DMS fluxes from aboveground plants averaged −14.2 ± 7.2 and 22.3 ± 10.6 pmol m−2 s−1, approximating that of −18.4 and 29.7 pmol m−2 s−1 calculated by subtracting fluxes in PP and PCP, respectively. Preliminary diurnal flux observation at jointing-booting stage revealed significant linear correlation between COS uptake rates by above ground plants (MFCOS) and photosynthesis rates (Pn) with a MFCOS/Pn slope of 0.48 pmol μmol−1. DMS emission from the aboveground plants was also found to be significantly increased with temperature.