Soil organic matter mineralization and residue decomposition of spring wheat grown under elevated CO2 atmosphere

The influence of elevated atmospheric CO2 concentrations ([CO2]) on the decomposition of spring wheat (Triticum aestivum L. cv. Triso) residues remaining in the soil after harvest was simulated in a microcosm incubation experiment in the lab. Undisturbed soil cores with and without visible wheat residues were taken in the third year after establishment from a Mini-free-air carbon dioxide enrichment (FACE) system, in which we used 13C-depleted CO2 to determine the contribution of plant-derived carbon to overall carbon mineralization. The Mini-FACE system is located on a Gleyic Cambisol near Hohenheim (Baden-Württemberg, Germany). Carbon dioxide production and leaching of nitrogen and inorganic and organic carbon were measured during 191 days of incubation. Rates of CO2 production were generally highest in all treatments during the first two weeks of the incubation and this was followed by a steady decrease until day 58. After this day mineralization rates declined only weakly until the end of the incubation. Cumulative carbon mineralization was similar in the two treatments without visible wheat residues, but significantly lower in the elevated (−19.0%) versus ambient [CO2] treatment with visible plant residues (significant [CO2] × residue interaction; F1,13 = 7.17; P = 0.019). This result demonstrated reduced decomposition of wheat residues grown under elevated [CO2]. The contribution of plant-derived carbon to soil respiration was highest in the beginning, followed by a steady decrease until the end of the incubation. Irrespective of incubation time, the amount of mineralized, plant-derived carbon was higher in the treatment with visible wheat residues. Leaching of inorganic carbon (DIC) tended to be affected by [CO2] (F6,8 = 4.50; P = 0.057), with more DIC leached in the elevated [CO2] treatment without (+47.2%) and with visible plant residues (+29.5%) than in the respective ambient CO2 treatments. The amount of carbon potentially sequestered as DIC in the wheat cropping system was small compared to the effects of elevated [CO2] on the amounts and decomposition of plant residues. Increased input of plant residues and reduced decomposition of plant-derived carbon are discussed as possible mechanisms for enhanced carbon sequestration under elevated atmospheric CO2 concentration.