Mechanisms Controlling CO2 Pulses upon Rewetting Dry Soils: Effects of Vegetation on Soil C Dynamics

Abstract:

Since 1958 it has been recognized that rewetting a dry soil produces a large pulse of respiration. However, the mechanisms controlling these pulses continue to be debated, with both physiological and physical mechanisms postulated. Recent studies suggest that a pool of water-extractable organic carbon (WEOC) increases as surface soils dry, concomitant with the increase in microbial biomass and the size of the rewetting CO₂ pulse, but that these patterns are weakened in soils below the rooting zone. Because physically protected soil C is made available by the rewetting event itself, it is unlikely that the WEOC was generated by physical processes. Thus, we asked: i) Does the microbial decomposition of ‘fresh’ plant detritus during the dry season generate a pool of bioavailable WEOC, and ii) does its rapid metabolism upon rewetting control the magnitude of CO₂ pulses? To answer these questions we manipulated plant inputs by thinning during the growing season, and measured CO₂ emissions and WEOC concentrations for two years at a seasonally-dry California grassland. We also estimated a rapidly bioavailable WEOC (BWEOC) pool by measuring headspace CO₂ after 3 hours of adding water. Opposite to our predictions, WEOC and BWEOC were most abundant in soils without plants. However, during the second year of treatment, soils with plants had higher BWEOC. Soil CO₂ emissions were greater during the dry season with plants than without plants during both years, as well as upon rewetting, especially during the second year of treatment when the BWEOC accumulated during the summer drought. Apparently, WEOC was not generated by the decomposition of ‘fresh’ plant C, but rather, by C in various stages of decomposition. Because CO₂ emissions during the dry season were higher with plants than without plants, interactions between plants and microbes appear to control the production of a BWEOC pool that influences the magnitude of CO₂ pulses upon rewetting.