A comparison of carbon flow from pre-labelled and pulse-labelled plants

Perennial rye-grass was subjected to two different14C labelling regimes to enable a partitioning of the carbon sources contributing to rhizosphere carbon-flow. Plant/soil microcosms were designed which enabled rye-grass plants to either receive a single pulse of14C−CO2 or to be pre-labelled using a series of14C−CO2 pulses, allowing the fate of newly photoassimilated carbon and carbon lost by root decomposition to be followed into the soil.
For young rye-grass plants grown over a short period, rhizosphere carbon flow was found to be dominated by newly photoassimilated carbon. Evidence for this came from the observed percentage of the total14C budget (i.e. total14C−CO2 fixed by the plants) lost from the root/soil system, which was 30 times greater for the pulse labelled compared to pre-labelled plants. Root decomposition was found to be less at 10°C compared to 20–25°C, though input of14C into the soil was the same at both temperatures.