DMS pulse and COS valley: the effect of simulated rainfall on sulfur gas exchange in dry soils of uncultivated marine terraces

Atmospheric sulfur compounds regulate climate by affecting cloud dynamics and reducing the amount of solar radiation that reaches the Earth's surface. Quantifying the terrestrial-atmosphere exchanges of sulfur has been challenging as only some of the controlling factors are known. In general, oxic soils are observed to act as a sink of reduced sulfur compounds (RSCs), while anoxic soils tend to act a source. Changes in soil moisture are therefore expected to greatly influence the direction of net gas fluxes of RSCs. Here we report the effect of simulated rainfall on soil samples from uncultivated marine terraces near Santa Cruz, CA, U.S.A (37.0°N, 122°W). Soils were collected in the dry season of a Mediterranean-type climate and air dried before the analysis. The rate of production of dimethyl sulfide (DMS), a compound known to be produced by phytoplankton and bacteria, increased dramatically in the first hours after water addition, tapering off over a few days. A concurrent pulse in microbial respiration (as CO_2) was observed. Soils that experience lengthy dry periods, such as those from arid and Mediterranean climates, have been shown to exhibit increases of carbon mineralization after rain events due to a combination of released soil organic matter and increased microbial activity. Conversely, production of carbonyl sulfide (COS), the most abundant reduced sulfur compound in the atmosphere, decreased immediately upon wetting the soil, perhaps due to isolation of the soil atmosphere from the headspace by water. These simultaneous processes after the addition of water can transform a soil in the bulk oxic state into a net source of RSCs in a relatively short span of time.