Reduced belowground allocation of freshly assimilated C contributes to negative plant-soil feedback in successive winter wheat rotations.

Aims: Successive winter wheat (WW) rotations are associated with yield reduction, often attributed to the unfavorable soil microbes that persist in the soil through plant residues. How rotational positions of WW affect the allocation of freshly assimilated carbon (C), an energy source for soil microbes, above and belowground remains largely unknown. Methods: A 13CO2 pulse labeling rhizotron experiment was conducted in the greenhouse to study freshly fixed C allocation patterns. WW was grown in soil after oilseed rape (W1), after one season of WW (W2), and after three successive seasons of WW (W4). We used an automatic manifold system to measure excess 13C of soil respiration at six depths and five different dates. Excess 13C was also measured in dissolved organic C (DOC), microbial and plant biomass pools. Results: There was a strong yield decline in successive WW rotations accompanied by distinct changes in root growth. Higher excess 13C of soil respiration was measured in W1 compared to W4, especially in the topsoil during at later growth stages. Higher excess 13C of the DOC and the microbial biomass was also traced in W1 and W4 compared to W2. Less 13C was taken up by successive WW rotations. Conclusions: Our study demonstrates a mechanism through which the rotational position of WW affects the allocation of freshly assimilated C above and belowground. WW after oilseed rape sustains belowground allocation of freshly assimilated C for a longer time than successively grown WW and incorporates more of this C to its biomass. [ABSTRACT FROM AUTHOR]