Carbon transfer, partitioning and residence time in the plant-soil system: a comparison of two ¹³CO₂ labelling techniques

Abstract. Various ¹³CO₂ labelling approaches exist to trace carbon (C) dynamics in plant-soil systems. However, it is not clear if the different approaches yield the same results. Moreover, there is no consistent way of data analysis to date. In this study we compare with the same experimental setup the two main techniques: pulse and continuous labelling. We evaluate how these techniques perform to estimate the C transfer time, the C partitioning along time and the C residence time in different plant-soil compartments.

We used identical plant-soil systems (Populus deltoides × nigra, Cambisol soil) to compare the pulse labelling approach (exposure to 99 atom % ¹³CO₂ for three hours, traced for eight days) with a continuous labelling (exposure to 10 atom % ¹³CO₂, traced for 14 days). The experiments were conducted in climate chambers under controlled environmental conditions. Before label addition and at four successive sampling dates, the plant-soil systems were destructively harvested, separated into leaves, petioles, stems, cuttings, roots and soil and soil microbial biomass was extracted. The soil CO₂ efflux was sampled throughout the experiment. To model the C dynamics we used an exponential function to describe the ¹³C signal decline after pulse labelling. For the evaluation of the ¹³C distribution during the continuous labelling we applied a logistic function.

Pulse labelling is best suited to assess the minimum C transfer time from the leaves to other compartments, while continuous labelling can be used to estimate the mean transfer time through a compartment, including short-term storage pools. The C partitioning between the plant-soil compartments obtained was similar for both techniques, but the time of sampling had a large effect: shortly after labelling the allocation into leaves was overestimated and the soil ¹³CO₂ efflux underestimated. The results of belowground C partitioning were consistent for the two techniques only after eight days of labelling, when the ¹³C import and export was at equilibrium. The C mean residence times estimated by the rate constant of the exponential and logistic function were not valid here (non-steady state). However, the duration of the accumulation phase (continuous labelling) could be used to estimate the C residence time.

Pulse and continuous labelling techniques are both well suited to assess C cycling. With pulse labelling, the dynamics of fresh assimilates can be traced, whereas the continuous labelling gives a more integrated result of C cycling, due to the homogeneous labelling of C pools and fluxes. The logistic model applied here, has the potential to assess different parameters of C cycling independent on the sampling date and with no disputable assumptions.