A method for (13)C-labeling of metabolic carbohydrates within French bean leaves (Phaseolus vulgaris L.) for decomposition studies in soils

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.

Variation of 13C and 15N enrichments in different plant components of labeled winter wheat (Triticum aestivum L.)

Information on the homogeneity and distribution of ¹³carbon (¹³C) and nitrogen (¹⁵N) labeling in winter wheat (Triticum aestivum L.) is limited. We conducted a dual labeling experiment to evaluate the variability of ¹³C and ¹⁵N enrichment in aboveground parts of labeled winter wheat plants. Labeling with ¹³C and ¹⁵N was performed on non-nitrogen fertilized (-N) and nitrogen fertilized (+N, 250 kg N ha^-1) plants at the elongation and grain filling stages. Aboveground parts of wheat were destructively sampled at 28 days after labeling. As winter wheat growth progressed, δ ¹³C values of wheat ears increased significantly, whereas those of leaves and stems decreased significantly. At the elongation stage, N addition tended to reduce the aboveground δ ¹³C values through dilution of C uptake. At the two stages, upper (newly developed) leaves were more highly enriched with ¹³C compared with that of lower (aged) leaves. Variability between individual wheat plants and among pots at the grain filling stage was smaller than that at the elongation stage, especially for the -N treatment. Compared with those of ¹³C labeling, differences in ¹⁵N excess between aboveground components (leaves and stems) under ¹⁵N labeling conditions were much smaller. We conclude that non-N fertilization and labeling at the grain filling stage may produce more uniformly ¹³C-labeled wheat materials, whereas the materials were more highly ¹³C-enriched at the elongation stage, although the δ ¹³C values were more variable. The ¹⁵N-enriched straw tissues via urea fertilization were more uniformly labeled at the grain filling stage compared with that at the elongation stage.

Different effects of plant-derived dissolved organic matter (DOM) and urea on the priming of soil organic carbon

Soil organic carbon (SOC) mineralization is important for the regulation of the global climate and soil fertility. Decomposition of SOC may be significantly affected by the supply of plant-derived labile carbon (C). To investigate the impact of plant-derived dissolved organic matter (DOM) and urea (N) additions on the decomposition of native SOC as well as to elucidate the underlying mechanisms of priming effects (PEs), a batch of incubation experiments was conducted for 250 days by application of (13)C-labeled plant-derived DOM and urea to soils. The direction of PE induced by the addition of DOM was different from the addition of N, i.e. it switched from negative to positive in DOM-amended soils, whereas in the N-treated soil it switched from positive to negative. Adding DOM alone was favorable for soil C sequestration (59 ± 5 mg C per kg soil), whereas adding N alone or together with DOM accelerated the decomposition of native SOC, causing net C losses (-62 ± 4 and -34 ± 31 mg C per kg soil, respectively). These findings indicate that N addition and its interaction with DOM are not favorable for soil C sequestration. Adding DOM alone increased the level of dissolved organic carbon (DOC), but it did not increase the level of soil mineral N. Changes in the ratio of microbial biomass carbon (MBC) to microbial biomass nitrogen (MBN) and microbial metabolic quotient (qCO2) after the addition of DOM and N suggest that a possible shift in the microbial community composition may occur in the present study. Adding DOM with or without N increased the activities of β-glucosidase and urease. Changes in the direction and magnitude of PE were closely related to changes in soil C and N availability. Soil C and N availability might influence the PE through affecting the microbial biomass and extracellular enzyme activity as well as causing a possible shift in the microbial community composition.

Variability of 13C-labeling in plant leaves

RATIONALE

Plant tissues artificially labeled with (13)C are increasingly used in environmental studies to unravel biogeochemical and ecophysiological processes. However, the variability of (13)C-content in labeled tissues has never been carefully investigated. Hence, this study aimed at documenting the variability of (13)C-content in artificially labeled leaves.

METHODS

European beech and Italian ryegrass were subjected to long-term (13)C-labeling in a controlled-environment growth chamber. The (13)C-content of the leaves obtained after several months labeling was determined by isotope ratio mass spectrometry.

RESULTS

The (13)C-content of the labeled leaves exhibited inter- and intra-leaf variability much higher than those naturally occurring in unlabeled plants, which do not exceed a few per mil. This variability was correlated with labeling intensity: the isotope composition of leaves varied in ranges of ca 60‰ and 90‰ for experiments that led to average leaf (13)C-content of ca +15‰ and +450‰, respectively.

CONCLUSIONS

The reported variability of isotope composition in (13)C-enriched leaves is critical, and should be taken into account in subsequent experimental investigations of environmental processes using (13)C-labeled plant tissues.