Soil Surface CO2 Flux in a Wheat Field: Measurements and Modeling

Soil surface CO₂ flux (Fs) is generated primarily by microbial and root respiration. Evaluation of which ecosystem parameters dominate production and transport of CO₂ in soil is necessary to determine those land management techniques that cause an agroecosystem to be a source or sink of carbon. The objective of this study was comparison of numerical model (UNSATCHEM) results to field measurements of Fs by a chamber flux measurement system. Soil temperature, moisture, microbial biomass C, microbial activity, leaf area index (LAI) and Fs were monitored at a winter wheat (Triticum aestivum L.) site in Oklahoma from 1998-2001. Fs rates were highest during December-May (~8 μmoles m^-2 s{-1}). Microbial respiration was 82% of the total annual flux (1100 g C m^-2) for this system. A Q₁₀ of 2.9 (r²=0.998) was determined from 48 h laboratory soil incubations at temperatures between 5 and 45{°}C. Initial UNSATCHEM modeling of CO₂ production and transport in the uppermost 1.5 m for 958 days starting August 21, 1997 was conducted using standard CO₂ production parameter values for microbial and root respiration. Model prediction generally conformed to measured soil surface CO₂ flux but during each springtime, the period of rapid growth of the wheat, the measured fluxes exceeded model predictions by a factor of approximately 2.5.