Andrei Rodionov

Background and aims: Shrubs like Vaccinium vitis-idaea substantially contribute to forest carbon (C) sequestration. Therefore, understanding their allocation patterns under climate change is crucial. We conducted a translocation experiment with ¹³ CO ₂ pulse-labelling to test if (I) belowground pools accumulate total and recently assimilated C, (II) within fine roots, assimilated C is preferentially allocated to root tips and mycorrhized cells and (III), whether warming alters C allocation patterns. Methods: We translocated soil cores with V. vitis-idaea from North- to South-Finland (+ 3.7 °C mean temperature). After 2.5 years, we excavated and pulse-labelled them with ¹³ CO ₂ , tracing the ¹³ C in plant, soil, and respiration. We used laser-ablation isotope ratio mass spectrometry to examine ¹³ C distribution in fine roots. Results: Roots represented the largest plant C pool (north: 49.69 ± 9.73%, translocated: 43.92 ± 6.59% of plant C stock). A substantial amount of recently assimilated ¹³ C was recovered belowground (roots + soil, north: 15.59 ± 5.77%, translocated: 39.21 ± 29.32%) 8 days after labelling. Most assimilated C was respired (north: 63.3 ± 5.9%, translocated: 44.1 ± 3.2%) or recovered in leaves (north: 17.9 ± 5.92%, translocated: 19.2 ± 3.19%). Within fine roots, ¹³ C content tended to be higher at root tips, while mycorrhizal infection had no effect. Warming tended to increase C allocation to the shoot and significantly decreased ¹³ C in the microbial biomass in the mineral soil. Conclusion: V. vitis-idaea allocates a large portion of assimilates belowground, but 2.5 years of warming only marginally change C allocation patterns. Accelerated soil microbial processes after translocation may increase plant N availability, potentially affecting C allocation over longer time spans.

Quantification of microplastics in soil is needed to understand their impact and fate in agricultural areas. Often, low sample volume and removal of organic matter (OM) limit representative quantification. We present a method which allows simultaneous quantification of microplastics in homogenized, large environmental samples (>1 g) and tested polyethylene (PE), polyethylene terephthalate (PET), and polystyrene (PS) (200-400 μm) overestimation by fresh and diagenetically altered OM in agricultural soils using a new combination of large-volume pyrolysis adsorption with thermal desorption-gas chromatography-tandem mass spectrometry (TD-GC-MS/MS). Characteristic MS/MS profiles for PE, PET, and PS were derived from plastic pyrolysis and allowed for a new mass separation of PET. Volume-defined standard particles (125 × 125 × 20 μm³) were developed with the respective weight (PE: 0.48 ± 0.12, PET: 0.50 ± 0.10, PS: 0.31 ± 0.08 μg), which can be spiked into solid samples. Diagenetically altered OM contained compounds that could be incorrectly identified as PE and suggest a mathematical correction to account for OM contribution. With a standard addition method, we quantified PS, PET, and PE_corrected in two agricultural soils. This provides a base to simultaneously quantify a variety of microplastics in many environmental matrices and agricultural soil.

The stable 13C/12C isotope composition usually varies among different organic materials due to isotope fractionation during biochemical synthesis and degradation processes. Here we introduce a novel laser-ablation stable isotope-mass-spectrometry methodology (LA-IRMS) that allows highly resolved spatial analysis of carbon isotope signatures in solid samples down to a spatial resolution of 10 µm. The presented instrumental setup includes in-house-designed exchangeable ablation cells (3.8 and 0.4 mL, respectively) and an improved sample gas transfer, which allow accurate δ13C measurements of an acryl plate standard down to 0.6 and 0.4 ng of ablated carbon, respectively (standard deviation 0.25‰). Initial testing on plant and soil samples confirmed that microheterogeneity of their natural 13C/12C abundance can now be mapped at a spatial resolution down to 10 µm. The respective δ13C values in soils with C3/C4 crop sequence history varied by up to 14‰ across a distance of less than 100 µm in soil aggregates, while being partly sorted along rhizosphere gradients of < 300 µm from Miscanthus plant roots into the surrounding soil. These very first demonstrations point to the appearance of very small metabolic hotspots originating from different natural isotope discrimination processes, now traceable via LA-IRMS.

The challenges of Vis‐NIR spectroscopy are permanent soil surface variations of moisture and roughness. Both disturbance factors reduce the prediction accuracy of soil organic carbon (SOC) significantly. For improved SOC prediction, both disturbance effects have to be determined from Vis‐NIR spectra, which is especially challenging for roughness. Thus, an approach for roughness quantification under varying moisture and its impact on SOC assessment using Support Vector Machines is presented here.