Connecting C and S biogeochemical cycles: mineralization in soil and rainfall events

Background/Question/Methods

The most abundant sulfur-containing gas, carbonyl sulfide (formula COS) can be used as a proxy for gross primary production (GPP), a crucial ecosystem parameter that cannot be directly measured.  Where CO₂ is both consumed and released by plants, COS is taken up in a predictable ratio to CO₂ then irreversibly destroyed.  Most of the underlying assumptions about using this GPP proxy have been investigated and verified; however, the existence of a COS source in soil still remains as an outstanding problem. This study describes a link between reduced sulfur and CO₂ production from terrestrial ecosystems to the atmosphere.  It is well-known that wet-up of dry soil can cause a large pulse of carbon mineralization, sometimes accounting for a substantial fraction of annual carbon exchange.  In laboratory and monthly field experiments, we wetted dry soil and litter from a Mediterranean grassland and a desert ecosystem to observe sulfur trace gas exchanges and their relationship to fluxes of CO₂.

Results/Conclusions

When soils were sufficiently dry (volumetric water content near 5%), exchanges of COS and CO₂ were near zero.  Upon adding enough artificial rainwater to change the soil moisture by 10%, a large flux of CO₂ and dimethyl sulfide (DMS) was observed.  DMS is a reduced sulfur gas, normally associated with oceanic ecosystems, which is oxidized in the atmosphere to form COS.  Subsequent measurements showed that the timing of peak DMS emission lagged approximately an hour behind the peak CO₂ flux.  In laboratory experiments, the soil system took several days to recover to pre-wet exchange rates, even though soil moisture did not return to original levels.  This large pulse of sulfur gas can account for a majority of the sulfur exchange from a dry or seasonally dry ecosystem.  The mechanisms controlling sulfur gas exchange in this case appear to be related to those affecting carbon mineralization, providing another important link between these two biogeochemical cycles.