Carbon budget study using CO2 ̄ux 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 ̄ux and other losses. The objectives of this study were to: (1) determine the in ̄uence of known additions of crop residue in a no till system on CO2 ̄ux measured with the static chamber alkali-absorption method, and (2) calculate the C budget from the CO2 ̄ux 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 haÿ1 wheat straw (Triticum aestivum L.) was applied. Noon soil temperature and daily CO2 ̄ux were determined in 1997. Residue application rate had a signi®cant impact on soil temperature as measured at noon, especially early in the growing season. Noon soil temperature was up to 148C higher under unmulched compared with that of mulched treatments. Measured CO2 ̄ux ranged from 0.4 to 4.2 g C m ÿ2 per day. Differences in CO2 ̄ux between crop residue treatments were not signi®cant on most sampling dates, probably due to the presence of undecomposed residue in the soil which did not contribute to the CO2 ̄ux, 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 ̄ux from bare plots. The C budget calculated from CO2 ̄ux 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 ̄ux 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. # 2000 Elsevier Science B.V. All rights reserved.