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Original article (peer-reviewed)

Journal Biogeosciences
Volume (Issue) 11(6)
Page(s) 1637 - 1648
Title of proceedings Biogeosciences

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 resi- dence time in different plant-soil compartments. We used identical plant-soil systems (Populus del- toides×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 en- vironmental conditions. Before label addition and at four successive sampling dates, the plant-soil systems were de- structively harvested, separated into leaves, petioles, stems, cuttings, roots and soil and soil microbial biomass was ex- tracted. The soil CO2 efflux was sampled throughout the ex- periment. To model the C dynamics we used an exponen- tial function to describe the 13C signal decline after pulse la- belling. 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 trans- fer time through a compartment, including short-term stor- age pools. The C partitioning between the plant-soil com- partments 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 par- titioning 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 dynam- ics 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 logis- tic model applied here, has the potential to assess different parameters of C cycling independent on the sampling date and with no disputable assumptions.
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