Carbon budget study using CO₂ flux measurements from a no till system in central Ohio

Enhancing carbon sequestration in soil is an important means to reduce net emissions of carbon dioxide (CO₂) to the atmosphere. The soil organic carbon (SOC) pool is the net result of carbon (C) input in the form of crop residue and biomass, and output including CO₂ flux and other losses. The objectives of this study were to: (1) determine the influence of known additions of crop residue in a no till system on CO₂ flux measured with the static chamber alkali-absorption method, and (2) calculate the C budget from the CO₂ flux and C added in crop residue. The experiment was started on a Crosby silt loam (Stagnic Luvisol) in 1989. Annually 0, 2, 4, 8 and 16 Mg ha^-1 wheat straw (Triticum aestivum L.) was applied. Noon soil temperature and daily CO₂ flux were determined in 1997. Residue application rate had a significant impact on soil temperature as measured at noon, especially early in the growing season. Noon soil temperature was up to 14°C higher under unmulched compared with that of mulched treatments. Measured CO₂ flux ranged from 0.4 to 4.2 g C m^-2 per day. Differences in CO₂ flux between crop residue treatments were not significant on most sampling dates, probably due to the presence of undecomposed residue in the soil which did not contribute to the CO₂ flux, and to variability in sampling. Average daily soil temperature at 5 cm depth determined on a nearby weather station explained 60% of the variation in CO₂ flux from bare plots. The C budget calculated from CO₂ flux measurements indicated a net depletion of SOC in all treatments. However, measured SOC contents indicated an increase of SOC over time in some treatments. It is likely that the annual CO₂ flux measured with the static chamber alkali-absorption method is overestimated due to omission of CO₂ measurements when soil water content is high, and perhaps other, as yet unknown factors. A comparison of the static chamber technique and improved dynamic chamber techniques and micrometeorological methods is recommended. (C) 2000 Elsevier Science B.V.