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 (tau s) were estimated. For this the natural differences in C-13 abundances of: (1) C-4 soil (with a history of C-4 plant, Miscanthus sinensis, cultivation), (2) C-3 soil (history of C-3 plant cultivation), and (3) C-4/3 soil (C-4 soil, planted with a C-3 plant, Triticum aestivum) were used. Total amounts and delta C-13 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 delta C-13 values of soil C in a mixing and a 1-box model enabled the quantification of relative contributions of C-3 plant and C-4 plant C to the total amount of the respective C pools in the C-4/3 soil and their ts. Compared to early spring ( March), the percentage of C-3 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 ts of these pools were 115 and 147 days. The delta C-13 values of the dominant soil PLFA, 18: 1 omega 7c, cy19: 0, 18:1 omega 9c, 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 delta C-13 values of PLFA, therefore, enabled the analysis of the metabolically active populations. The majority of delta C-13 values of PLFA from the C-4/3 soil were closely related to those of PLFA from the C-3 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.