Microbial assimilation of plant-derived carbon in soil traced by isotope analysis

The flow of new and native plant-derived C in the rhizosphere of an agricultural field during one growing season was tracked, the ratios in different soil C pools were quantified, and the residence times (τs) were estimated. For this the natural differences in 13C abundances of: (1) C4 soil (with a history of C4 plant, Miscanthus sinensis, cultivation), (2) C3 soil (history of C3 plant cultivation), and (3) C4/3 soil (C4 soil, planted with a C3 plant, Triticum aestivum) were used. Total amounts and δ13C values of total soil C, non-hydrolysable C, light fraction C, water-soluble C, microbial biomass C, and phospholipid fatty acids (PLFA) were determined. Using the δ13C values of soil C in a mixing and a 1-box model enabled the quantification of relative contributions of C3 plant and C4 plant C to the total amount of the respective C pools in the C4/3 soil and their τs. Compared to early spring (March), the percentage of C3 plant C increased in all pools in June and August, showing the addition of new C to the different soil C fractions. In August the contribution of new C to microbial biomass C and water-soluble C reached 64 and 89%, respectively. The τs of these pools were 115 and 147 days. The δ13C values of the dominant soil PLFA, 18:1ω7c, cy19:0, 18:1ω9c, 16:0, and 10Me16:0, showed wide ranges (−35.1 to −13.0‰) suggesting that the microbial community utilized different pools as C sources during the season. The δ13C values of PLFA, therefore, enabled the analysis of the metabolically active populations. The majority of δ13C values of PLFA from the C4/3 soil were closely related to those of PLFA from the C3 soil when T. aestivum biomass contributions to the soil were high in June and August. Specific populations reacted differently to changes in environmental conditions and supplies of C sources, which reflect the high functional diversity of soil microorganism