Reduced sulfur trace gas exchange between a seasonally dry grassland and the atmosphere

Reduced sulfur gases are precursors to sulfate aerosols that act as cloud condensation nuclei and affect Earth’s radiative balance. The diminishing anthropogenic atmospheric sulfur inputs due to long-standing acid rain abatement activities increase the influence of natural emissions on the remaining sulfur budget. Most previous terrestrial observations of reduced sulfur gas fluxes focus on wetland ecosystems where sulfur gas emissions are highest. The diffuse natural production of sulfur gas from more widespread oxic ecosystems needs to be characterized. Here we report in situ fluxes of sulfur gases and CO2 from grasslands outside of Santa Cruz, CA, USA (36.96°N, 122.08°W). Monthly measurements were made using static flux chambers from March 2012 to March 2014. A large net emission of dimethyl sulfide (DMS) was observed during the growing season. During the non-growing dry season, small but quantifiable carbonyl sulfide (COS) exchange rates were correlated with soil temperature. When soil moisture was artificially increased in senescent grassland plots, the relative exchange of COS:CO2 increased and then returned to the original ratio within 2 h. Sulfur gas fluxes during wet season soil moisture transition events (i.e. after precipitation) indicate that understudied aerobic environments may provide an important contribution to atmospheric COS consumption and DMS production.