Böden von Baden-Württemberg als Senke für klimarelevante Gase

Increasing atmospheric CO2-concentrations will alter plant growth and therefore also soil organic matter storage in the future. The aim of the present project was to investigate and quantify the effects of elevated atmospheric CO2 on the carbon storage potential of arable soils in Baden-Württemberg (Germany). Carbon turnover rates were investigated on the basis of soil samples taken from the first Free Air Carbon dioxide Enrichment experiment (Mini-FACE) in Baden-Württemberg. Carbon input into soil organic matter as well as into soil microbial biomass within the 5 year experimental period was quantified by fumigating the elevated-CO2 plots with 13C-labelled CO2. Soil samples from this spring wheat agroecosystem were taken in March and October 2004, 2005 and 2006. In addition stored samples from 2002; the first year of CO2-elevation were analysed. Neither the abundance of soil microorganisms nor their activity and soil enzyme activities were affected by elevated-CO2. Diversity of soil microbial community was influenced by elevated-CO2 only in March 2004, increased fungal biomass indicated a restricted short-term effect. Litter produced under elevated-CO2 showed a higher C/N ration that ambient litter. Incubation of wheat, cornflower, and mustard litter materials in microcosm experiments, additionally performed in the lab revealed that decomposition of elevated-CO2 litter materials was retarded in comparison to ambient litter. This was accompanied by lower CO2-production during winter months without vegetation on the field. During the growth period of the spring wheat plants soil CO2-efflux was therefore increased by elevated-CO2. Soil moisture was generally higher under elevated-CO2 in both, the field and the microcosm experiment.
New carbon inputs (Cnew) during the 5 year Mini-FACE experimental period were calculated by inverse modelling using the Roth-C (26.3) soil carbon model. Decomposition of pre-experimental carbon (Cold) was modelled with the same parameters. The right distribution of C into the different Roth-C model pools was verified by comparison the modelled and measured Cnew-inputs into the soil microbial biomass. Cnew-inputs under ambient-CO2 were calculated by assuming that effects of elevated CO2 on soil C fluxes were proportional to the effect on plant biomass. The sum of Cnew inputs and Cold decomposition revealed that under elevated-CO2 higher inputs had been levelled out by increased decomposition of Cold. Higher decomposition arises from the enhanced microbial activity due to higher soil moisture under elevated-CO2 during periods of low precipitation. For the first time it was possible to quantify the elevated-CO2 effect due to the high temporal resolution of the measured soil moisture data in the Mini-FACE experiment.
The results of the present study show, that the investigated agroecosystem will not function as a sink for CO2 under a future CO2-enriched atmosphere. By using other crops and management practices, associated with higher Cnew inputs the soil carbon balance under elevated-CO2 might become positive. However, the total effect of elevated-CO2 on C-sequestration will be minor against the potential occupied by changing land use management from conventional tillage to reduced tillage and by transforming arable land into grassland. Until now, the uncertainties about the slight CO2-effect in interaction with other factors arising from global climate change (temperature, amount and distribution of annual precipitation) and land use management impeded a precise and reliable prediction on the potential of agroecosystems from Baden-Württemberg to function as a sink for greenhouse gases in the future.