Defoliation and the availability of currently assimilated carbon in the Phleum pratense rhizosphere

It has been hypothesised that defoliation and aboveground herbivory increase the availability of currently assimilated C to organisms living in plant rhizospheres. We established a growth chamber experiment consisting of Phleum pratense individuals growing in sand culture to examine the short- and long-term effects of defoliation on the availability of current C assimilates in the P. pratense rhizosphere. Using 14CO2 pulse labelling, we followed partitioning of currently assimilated C between shoots, roots and rhizosphere-derived organic matter (RDOM). The experiment constituted of two treatments, defoliation history and recent defoliation, in a fully factorial design. Defoliation history had three levels: (1) no past defoliation, (2) defoliation 1 week before labelling, and (3) defoliation 1, 2 and 3 weeks before labelling, while recent defoliation had two levels: (1) no defoliation and (2) defoliation immediately before labelling. Recent defoliation reduced the amount of 14C radioactivity in shoots, roots and RDOM, while defoliation history did not have a significant effect. Neither treatment affected the proportions of shoot, root and RDOM radioactivity-to-total recovered radioactivity or the ratio of RDOM radioactivity-to-root radioactivity. The results suggest that the amount of current C assimilates found in the P. pratense rhizosphere decreases shortly after defoliation, but is not affected by defoliations that have occurred at least 1 week earlier. The results further suggest that the defoliation treatments, despite affecting the quantity of assimilated current C, do not affect the allocation pattern of assimilated C within P. pratense individuals or the ratio of current assimilates found in P. pratense roots-to-those available in its rhizosphere. On the whole, our results do not support the hypothesis that defoliation increases the availability of current photosynthate to soil decomposer food webs.