Carbon transfer, partitioning and residence time in the plant-soil system: a comparison of two <sup>13</sup>CO<sub>2</sub> labelling techniques

Abstract

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.

Functional differences in the microbial processing of recent assimilates under two contrasting perennial bioenergy plantations

“…The exponential decay was taken to represent the labile C pool (A) whilst the non -zero asymptote represented the fraction fixed either into structural biomass or long term storage pools (B) Shibistova et al, 2012;Studer et al, 2014;Subke et al, 2012). These functions were used to estimate the mean residence time (MRT) of C in pool A and the size of pool B relative to maximum enrichment here defined as CUE.…”

“…This supports recent work by Rousk and Frey (2015) which found a positive relationship between fungal dominance and ecosystem C losses in temperate forests. However, continuous 13 C labelling or the use of 14 C is a more sensitive technique better suited to studying the dynamics of larger C pools, due to the significant dilution of the 13 C pulse signal in large C pools such as SOM (Kuzyakov and Domanski, 2000;Studer et al, 2014;Trumbore, 2009).To further our mechanistic understanding of the links between microbial community composition, activity and soil C sequestration in perennial bioenergy systems, further study is required tracing assimilated C belowground into root exudates, microbial biomass, soil aggregates and fractions. However, our study does elucidate the important roles that soil fungi and bacteria play in the turnover of recently assimilated C under SRC willow and miscanthus respectively.…”

Temporal Dynamism of Resource Capture: A Missing Factor in Ecology?

“…Stable isotope labelling ( 15 N/ 13 C) can allow plant impacts on soil nutrient cycles to be quantified [46]. This can be done non-destructively and dynamically through isotopic partitioning of soil CO2 efflux into plant and SOM-derived components [47], or tracing 15 N fluxes (derived from labelled organic matter) in soil solution [48][49][50]. This approach allows the timing and magnitude of soil community priming to be measured over time, and compared relative to other temporally dynamic measurements including RNA expression (see above) and resource capture.…”

Multi-isotope labelling (<sup>13</sup>C, <sup>18</sup>O, <sup>2</sup>H) of fresh assimilates to trace organic matter dynamics in the plant-soil system

“…for the distinct values reported for the 13 C in leaves and petioles in this study and in 133! Studer et al (2014), since we analysed in the latter not only a sub-sample, but the total 134! leaf and petiole bulk material (including freshly produced leaves) to assess the 13 C 135!…”