Repeat-pulse 13CO2 labeling of canola and field pea: implications for soil organic matter studies

Tracing the synergistic migration of biochar and heavy metals based on 13C isotope signature technique: Effect of ionic strength and flow rate

Understanding the transport of biochar and heavy metals is important for evaluation of the long-term stability and ecotoxicity of heavy metals after biochar remediation. In this study, ¹³C-labelled biochar was prepared to investigate the synergistic down migration of biochar and heavy metals in the soil profile, and the effect of ionic strength (IS) and flow rate was examined. Results showed that the ¹³C-labelled biochar with high δ¹³C (249.3 ‰) was suitable for tracing the migration of biochar without influencing its adsorption for heavy metals (i.e., Cu²⁺ and Cd²⁺). Both higher IS and flow rate were favorable for the release of biochar, but higher IS inhibited the transport of biochar in soil profile, which was attributed to the enhanced primary- and secondary-minimum deposition based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) analysis. The transport of Cu²⁺ and Cd²⁺ was facilitated by high IS and flow rate. The release of Cd from biochar was mainly affected by IS, due to ion exchange and a weaker electrostatic attraction to biochar at higher IS, while that of Cu was mainly affected by flow rate related to co-migration of metal with biochar. Metal-biochar particle was the dominant form to migrate in upper soil layer, whereas, soluble Cd²⁺ and Cu²⁺ desorbed from biochar were the dominant forms that migrated to the deeper soil. The synergistic down migration of biochar and heavy metals might pose less risks than the sole migration of soluble metals. That is, high IS might cause higher risks than high flow rate even though biochar and metals might transport further with high flow rate. These findings will advance the current knowledge on the migration risk involved in the in-situ remediation of heavy metal-contaminated soils by biochar.

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

RATIONALE

Carbon-13 (¹³ C)-labelled plant material forms the basis for experiments elucidating soil organic carbon dynamics and greenhouse gas emissions. Quantitative field-scale tracing is only possible if plants are labelled homogeneously in large quantities. By using a laser spectrometer to automatically steer the isotopic ratio in the chamber, it is possible to obtain large amounts of homogeneously labelled plant material.

METHODS

Ninety-six maize plants were labelled for 25 days until tassel formation in a 15 m³ walk-in growth chamber with a continuous air δ¹³ C-CO₂ value of 400‰. A Los Gatos Research laser absorption spectrometer controlled the ambient δ¹³ C-CO₂ value in the chamber through steering of the mass flow controllers with ¹³ C-enriched and natural abundance CO₂ gas.

RESULTS

Laser absorption spectroscopy steering kept the δ¹³ C value of chamber air between 368 and 426‰. The resulting 1 kg dry matter of ¹³ C-labelled shoots showed an average δ¹³ C value of 384‰ and accuracy of 8‰ (half width of the 95% confidence interval). Only the oldest leaves showed larger heterogeneity. The growth chamber eliminated variability between plants. The δ¹³ C value of the stabile material did not differ significantly from that of bulk material.

CONCLUSIONS

Laser spectroscopy controlled ¹³ C labelling of plants in a walk-in growth chamber successfully kept the isotopic ratio of the CO₂ in the chamber air constant. Therefore, large quantities of material were labelled homogeneously at the inter- and intra-plant level, thus establishing a method to provide high-quality input for quantitative isotopic tracer studies.

Allocation of freshly assimilated carbon into primary and secondary metabolites after in situ ¹³C pulse labelling of Norway spruce (Picea abies)

Plants allocate carbon (C) to sink tissues depending on phenological, physiological or environmental factors. We still have little knowledge on C partitioning into various cellular compounds and metabolic pathways at various ecophysiological stages. We used compound-specific stable isotope analysis to investigate C partitioning of freshly assimilated C into tree compartments (needles, branches and stem) as well as into needle water-soluble organic C (WSOC), non-hydrolysable structural organic C (stOC) and individual chemical compound classes (amino acids, hemicellulose sugars, fatty acids and alkanes) of Norway spruce (Picea abies) following in situ (13)C pulse labelling 15 days after bud break. The (13)C allocation within the above-ground tree biomass demonstrated needles as a major C sink, accounting for 86% of the freshly assimilated C 6 h after labelling. In needles, the highest allocation occurred not only into the WSOC pool (44.1% of recovered needle (13)C) but also into stOC (33.9%). Needle growth, however, also caused high (13)C allocation into pathways not involved in the formation of structural compounds: (i) pathways in secondary metabolism, (ii) C-1 metabolism and (iii) amino acid synthesis from photorespiration. These pathways could be identified by a high (13)C enrichment of their key amino acids. In addition, (13)C was strongly allocated into the n-alkyl lipid fraction (0.3% of recovered (13)C), whereby (13)C allocation into cellular and cuticular exceeded that of epicuticular fatty acids. (13)C allocation decreased along the lipid transformation and translocation pathways: the allocation was highest for precursor fatty acids, lower for elongated fatty acids and lowest for the decarbonylated n-alkanes. The combination of (13)C pulse labelling with compound-specific (13)C analysis of key metabolites enabled tracing relevant C allocation pathways under field conditions. Besides the primary metabolism synthesizing structural cell compounds, a complex network of pathways consumed the assimilated (13)C and kept most of the assimilated C in the growing needles.