Repeated ¹⁴CO ₂pulse-labelling reveals an additional net gain of soil carbon during growth of spring wheat under free air carbon dioxide enrichment ( enfree air carbon dioxide enrichmentFACE )

Rising levels of atmospheric CO2 have often been found to increase above and belowground biomass production of C3 plants. The additional translocation of organic matter into soils by increased root mass and exudates are supposed to possibly increase C pools in terrestrial ecosystems. Corresponding investigations were mostly conducted under more or less artificial indoor conditions with disturbed soils. To overcome these limitations, we conducted a 14CO2 pulse-labelling experiment within the German FACE project to elucidate the role of an arable crop system in carbon sequestration under elevated CO2. We cultivated spring wheat cv. "Minaret" with usual fertilisation and ample water supply in stainless steel cylinders forced into the soil of a control and a FACE plot. Between stem elongation and beginning of ripening the plants were repeatedly pulse-labelled with 14CO2 in the field. Soil born total CO2 and 14CO2 was monitored daily till harvest. Thereafter, the distribution of 14C was analysed in all plant parts, soil, soil mineral fractions and soil microbial biomass. Due to the small number of grown wheat plants (40) in each ring and the inherent low statistical power, no significant above and belowground growth effect of elevated CO2 was detected at harvest. But in comparison to ambient conditions, 28% more 14CO2 and 12% more total CO2 was evolved from soil under elevated CO2 (550 mmol CO2 mol-1). In the root-free soil 27% more residual 14C was found in the FACE soil than in the soil from the ambient ring. In soil samples from both treatments about 80% of residual 14C was found in the clay fraction and 7% in the silt fraction. Very low 14C contents in the CFE extracts of microbial biomass in the soil from both CO2 treatments did not allow assessing their influence on this parameter. Since the calculated specific radioactivity of soil born 14CO2 gave no indication of an accelerated priming effect in the FACE soil, we conclude that wheat plants grown under elevated CO2 can contribute to an additional net carbon gain in soils.