Soil carbon dioxide flux in agricultural ecosystems

Although present in the atmosphere in very small proportions, CO{dollar}sb2{dollar} plays an important role in the global carbon cycle. Soil CO{dollar}sb2{dollar} flux was studied in two agricultural ecosystems: maize (an annual) and alfalfa (a perennial). In maize, soil CO{dollar}sb2{dollar} flux was significantly higher at the within row (WR) position compared to the between row (BR) position. Weighted flux in maize ranged from 0.11 to 0.4 mg CO{dollar}rmsb2 msp{lcub}-2{rcub} ssp{lcub}-1{rcub}{dollar} (25.9 to 94.3 kg CO{dollar}sb2{dollar}-C ha{dollar}rmsp{lcub}-1{rcub} daysp{lcub}-1{rcub}{dollar}) in 1992 and from 0.05 to 0.28 mg CO{dollar}rmsb2 msp{lcub}-2{rcub}ssp{lcub}-1{rcub}{dollar} (11.8 to 65.98 kg CO{dollar}sb2{dollar}-C {dollar}rm hasp{lcub}-1{rcub} daysp{lcub}-1{rcub}{dollar}) in 1993. Maize flux was higher in 1992 compared to 1993. Lower soil temperatures and higher soil moisture (precipitation was 55% above normal in 1993) may have influenced the low flux rates in 1993.; Maize soil CO{dollar}sb2{dollar} flux was significantly different than alfalfa flux in both years. In 1992, maize flux was higher than alfalfa flux at mid-season. Alfalfa flux was higher in 1993 compared to 1992. Flux in alfalfa ranged from 0.10 to 0.3 mg CO{dollar}rmsb2 msp{lcub}-2{rcub}ssp{lcub}-1{rcub}{dollar} in 1992 (23.6 to 70.7 kg CO{dollar}sb2{dollar}-C {dollar}rm hasp{lcub}-1{rcub} daysp{lcub}-1{rcub}{dollar}) and from 0.25 to 0.39 mg CO{dollar}rmsb2 msp{lcub}-2{rcub}ssp{lcub}-1{rcub}{dollar} (58.9 to 91.9 kg CO{dollar}sb2{dollar}-C {dollar}rm hasp{lcub}-1{rcub} daysp{lcub}-1{rcub}{dollar}) in 1993.; We examined diel (i.e. 24 hour) soil CO{dollar}sb2{dollar} flux on selected days in 1992. The trend in CO{dollar}sb2{dollar} flux showed a minimum flux in the morning and a maximum flux in the late afternoon or evening which appeared to be related to the diel variation in soil temperature. A multiple regression of flux against soil moisture, soil temperature, and LAI explained 47% of variability in within row maize, 32% in between row maize, 45% in weighted maize, and 48% in alfalfa (using the soil temperature and moisture terms only) and a Q{dollar}sb{lcub}10{rcub}{dollar} of about 2 using daily averaged flux.; In maize, the relative contribution of the root plus rhizosphere towards surface CO{dollar}sb2{dollar} flux was estimated to average 54% over a growing season. Seasonal soil flux in maize was estimated to be 5500 kg C ha{dollar}sp{lcub}-1{rcub}{dollar} (June through September) in 1992 and 4600 kg C ha{dollar}sp{lcub}-1{rcub}{dollar} (June through October) in 1993. In alfalfa, seasonal flux was estimated to be 4400 kg C ha{dollar}sp{lcub}-1{rcub}{dollar} in 1992 and 5600 kg C ha{dollar}sp{lcub}-1{rcub}{dollar} in 1993 (June through September). Mean concentrations of CO{dollar}sb2{dollar} in the soil air at different depths under maize ranged from less than 1% (10000 ppmv) to 8.5% (85000 ppmv). Soil air CO{dollar}sb2{dollar} varied over both depth and diel and seasonal time.