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 (s) 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) C_4/3 soil (C₄ soil, planted with a C₃ plant, Triticum aestivum) were used. Total amounts and ¹³C 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 ¹³C 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 C_4/3 soil and their s. 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 s of these pools were 115 and 147 days. The ¹³C values of the dominant soil PLFA, 18:17c, cy19:0, 18:19c, 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 ¹³C values of PLFA, therefore, enabled the analysis of the metabolically active populations. The majority of ¹³C values of PLFA from the C_4/3 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 microorganisms.