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

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: the pulse and the continuous labelling. We evaluate how these techniques perform to estimate the C transfer velocity, the C partitioning along time and the C residence time in different plant-soil compartments. <br><br> We used identical plant-soil systems (<i>Populus deltoides x nigra</i>, 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 the microbial biomass was extracted from the soil. 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 suggest to use a logistic function. <br><br> Pulse labelling is best suited to assess the maximum C transfer velocity from the leaves to other compartments. With continuous labelling, the mean transfer velocity through a compartment, including short-term storage pools, can be observed. The C partitioning between the plant-soil compartments 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 time estimated by the rate constant of the exponential and logistic function was not valid here. However, the duration of the accumulation phase (continuous labelling) could be used to estimate the C residence time. <br><br> 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 on C cycling, due to the homogeneous labelling of C pools and fluxes. The logistic model suggested here, has the potential to assess different parameters of C cycling independent on the sampling date and with no disputable assumptions

Impact of interspecific competition and drought on the allocation of new assimilates in trees

“…For both species, we did not observe a clear time lag in label transport as a consequence of drought, neither when considering WSOM nor soil CO 2 . While the first appearance of the label in soil CO 2 in the controls was comparable to results for beech and other tree seedlings (Barthel et al 2011;Epron et al 2012;Studer et al 2014;Blessing et al 2015), the absence of transport retardation upon drought was in contrast to previous findings (Ruehr et al 2009;Barthel et al 2011;Zang et al 2014). Delays in assimilate transport from the leaves to roots for beech exposed to drought have been documented to range up to approximately six-fold compared to controls (Ruehr et al 2009).…”

Biological mechanisms may contribute to soil carbon saturation patterns

“…For example, as organic matter inputs increase causing a concomitant increase in microbial populations, nitrogen‐poor plant inputs might induce nutrient limitation with a negative feedback to microbial reproduction (Schimel & Weintraub, 2003), quorum‐sensing bacteria might produce antimicrobial compounds (Hibbing et al, 2010), or predation could constrain populations of microbial prey species (Thakur & Geisen, 2019). Whatever the mechanism, it is clear that microbial populations tend to exhibit a constrained response to resources in culture (Kingsland, 2002) and, in agreement with previous observations (Geyer et al, 2019; Studer et al, 2014), we hypothesize that this phenomenon could be extended to the bulk soil microbial biomass, thus leading to constraints on microbial biomass as C inputs increase. However, while previous meta‐analyses show a clear, but variable, positive effect of C inputs on microbial biomass (Kallenbach & Grandy, 2011; Li et al, 2018), we are not aware of a study which has generally evaluated the shape of this relationship.…”

Laser spectroscopy steered 13C‐labelling of plant material in a walk‐in growth chamber

“…different plant organs or compounds are labelled with specific δ 13 C‐CO 2 values, making it possible to distinguish their fates in subsequent processes. Pulse labelling is low‐cost, does not require environmental controls, and can be highly effective in homogeneously labelling fresh assimilates in order to trace their dynamics in qualitative tracing . However, the method is not suitable to obtain homogeneously labelled material, due to inherently varying atmospheric δ 13 C‐CO 2 values.…”

“…Pulse labelling is low-cost, does not require environmental controls, and can be highly effective in homogeneously labelling fresh assimilates in order to trace their dynamics in qualitative tracing. 9,10 However, the method is not suitable to obtain homogeneously labelled material, due to inherently varying atmospheric δ 13 C-CO 2 values. For example, Bromand et al 11 took into account the changing photosynthetic rate of wheat throughout plant growth and obtained δ 13 C values of 2717‰ for leaves and 2158‰ for chaff and grain.…”