Determination of microbial versus root-produced CO2 in an agricultural ecosystem by means of delta(CO2)-C-13 measurements in soil air

The amounts of microbial and root-respired CO2 in a maize/winter wheat agri cultural system in south western Germany were investigated by measurements of the CO(2)mixing ratio and the C-13/C-12 ratio in soil air. CO2 fluxes at the soil surface for the period of investigation (1993-1995) were also det ermined. Root respired CO2 shows a strong correlation with the plant mass a bove ground surface of the respective vegetation (R-2 greater than or equal to 0.88); the maximum CO, release from roots was in August For the maize ( 2.0 +/- 0.5 mmol m(-2) h(-1)) and in June for winter wheat (1.5 +/- 0.5 mmo l m(-2) h(-1)). Maximum CO2 production by roots correlate well with the max imum amount of plant root matter. integrating the CO2 production over the w hole growing season and normalizing to the dry root matter yields, the CO2 production per gram dry organic root matter (DORM) of maize was found to be 0.14 +/- 0.03 gC (g DORM)(-1). At the sites investigated, root-produced CO 2 contributed (16 +/- 4)% for maize, and (24 +/- 4)% for winter wheat, resp ectively, to the total annual CO, production in the soil (450 +/- 50 gC m(- 2) for maize, 210 +/- 30 gC m(-2) for winter wheat).