Impact of plant species and atmospheric CO2 concentration on rhizodeposition and soil microbial activity and community composition

Both plant species and CO2 concentration can potentially affect rhizodeposition and consequently soil microbial activity and community composition. However, the effect differs based on plant developmental stage. We focused on the effect of three plant species (forbs, grasses, and N-2-fixers) at an early stage of development on root C deposition and fate, soil organic matter (SOM) mineralization and soil microbial community composition at ambient (aCO(2)) and elevated (eCO(2)) CO2 levels. Plants were grown from seed, under continuous C-13-labelling atmospheres (400 and 800 mu mol mol(-1) CO2), in grassland soil for three weeks. At the end of the growth period, soil respiration, dissolved organic C (DOC) and phospholipid fatty acid (PLFA) profiles were quantified and isotopically partitioned into root- and soil-derived components. Root-derived DOC (0.53 +/- 0.34 and 0.26 +/- 0.29 mu g mL soil solution(-1)) and soil-derived CO2 (6.14 +/- 0.55 and 5.04 +/- 0.44 mu g CO2-C h(-1)) were on average two times and 22% higher at eCO(2) than at aCO(2), respectively. Plant species differed in exudate production at aCO(2) (0.11 +/- 0.11, 0.10 +/- 0.18, and 0.58 +/- 0.58 mu g mL soil solution(-1) for Plantago, Festuca, and Lotus, respectively) but not at eCO(2) (0.20 +/- 0.28, 0.66 +/- 0.32, and 0.75 +/- 0.15 mu g mL soil solution(-1) for Plantago, Festuca, and Lotus, respectively). However, no differences among plant species or CO2 levels were apparent when DOC was expressed per gram of roots. Relative abundance of PLFAs did not differ between the two CO2 levels. A higher abundance of actinobacteria and G-positive bacteria occurred in unplanted (8.07 +/- 0.48 and 24.36 +/- 1.18 mol%) and Festuca-affected (7.63 +/- 0.31 and 23.62 +/- 0.69 mol%) soil than in Plantago- (7.04 +/- 0.36 and 23.41 +/- 1.13 mol%) and Lotus-affected (7.24 +/- 0.17 and 23.13 +/- 0.52 mol%) soil. In conclusion, the differences in root exudate production and soil respiration are mainly caused by differences in root biomass at an early stage of development. However, plant species evidently produce root exudates of varying quality affecting associated microbial community composition.