Production of CO2 in Soil Profiles of a California Annual Grassland

Noah Fierer,Susan E Trumbore,Oliver A Chadwick

doi:10.1007/s10021-003-0151-y

Abstract

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO2 to the atmosphere. Although a large number of studies have focused on the CO2 flux at the soil–air interface, relatively few studies have examined the rates of CO2 production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO2 in the soil air, but the sources of this CO2 and the spatiotemporal dynamics of CO2 production throughout the soil profile are poorly understood. We studied CO2 dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO2 concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO2 production with soil profile depth. The CO2 diffusion constants were checked for accuracy using measured soil air 222Rn activities. The modeled net CO2 production rates were compared with CO2 fluxes measured at the soil surface. In general, the modeled and measured net CO2 fluxes were very similar although the model consistently underestimated CO2 production rates in the surficial soil horizons when the soils were moist. Profile CO2 production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO2 production in both the surface and subsurface soil horizons. Whole profile CO2 production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO2 flux from the soil profiles are relatively low (4–11 mg C– CO2 m−2 h−1), 20%–50% of the CO2 diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance 14C signatures of the CO2 and soil organic C suggest that the subsurface CO2 is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.

Highlights

Worldwide, soils store approximately 1600 Pg of organic carbon (C) (Eswaran and others 1993), an amount of C more than two times greater than that stored in the atmosphere
Because a large portion of the organic C stored in soil resides in deeper soil horizons (Batjes 1996), even small changes in the rate of organic C mineralization within subsurface horizons could have significant effects on atmospheric CO2 concentrations and global C dynamics
A strong El Nino event occurred during the winter of 1997–1998, producing one of wettest years on record for the study area and bringing the soil profiles to some of the highest soil moisture levels we measured over the course of the study period (Figure 3)

Summary

Introduction

Soils store approximately 1600 Pg of organic carbon (C) (Eswaran and others 1993), an amount of C more than two times greater than that stored in the atmosphere. This explanation is not likely; the organic C contents and aboveground net productivity measured at Profile 4 are near-average for all of the studied profiles (Table 1), so we would not expect Profile 4 to have unusually high rates of CO2 production in the near-surface horizons.

Results

Conclusion