Recent developments in optical systems (isotope-selective non-dispersive infrared spectrometry) for breath testing have provided a robust, low-cost option for undertaking (13)C analysis. Although these systems were initially developed for breath testing for Helicobacter pylori, they have an enormous potential as a soil science research tool. The relatively low cost of the equipment, US$15,000-25,000, is within the research budgets of most institutes or universities. The simplicity of the mechanisms and optical nature mean that the equipment requires relatively low maintenance and minimal training. Thus methods were developed to prepare soil and plant materials for analysis using the breath test analyser. Results that compare conventional mass spectrometric methods with the breath test analyser will be presented. In combination with simple (13)C-plant-labeling techniques it is possible to devise methods for estimating carbon sequestration under different agronomic management practices within a short time frame. This enables assessment of the carbon credit value of a particular agronomic practice, which can in turn be used by policy makers for decision-making purposes. For global understanding of the effect of agricultural practices on the carbon cycle, data are required from a range of cropping systems and agro-ecological zones. The method and the approach described will enable collection of hard data within a reasonable time.
Black carbon (BC) from incomplete combustion of organic materials is abundant in many soils. Its age is often higher than that of thermally unaltered soil organic carbon (SOC) owing to the presence of BC from fossil sources or to a high recalcitrance against microbial decomposition compared to that of plant residues. For a meaningful application of radiocarbon as an indicator for soil carbon age and turnover, the relative contribution of BC needs to be quantified, but BC is difficult to separate physically from soil. However, BC is thermally more stable than SOC, and hence thermal stability may provide a quantitative BC indicator. Here, we analyzed 30 light particulate organic carbon (POC) soil fractions for their thermal stability and for their 14 C signature. POC is particularly sensitive to "contamination" with BC, because it is obtained by combined size and density fractionation. A steady-state "bomb" 14 C model was used to derive mean POC ages. Soils from four sample sets, each consisting of six to eight individual POC samples and representing different field sites and POC types, were analyzed. Samples from one of the sets were virtually BC free, and their mean POC ages ranged from 60 to 100 yr. The 14 C signature of samples from the other three sets indicated the presence of very old carbon, with mean POC ages of several hundred and up to 3500 yr. Two indicators for thermal stability-(1) the amount of heat released at temperatures >450°C and (2) the amount of heat released at 500°C (the latter representing the peak temperature of heat released from charcoal isolated from soil)-correlated both significantly and nonlinearly with POC age, indicating that samples with high BC content were older than those with low BC content. It can be concluded that at an individual site with increasing abundance of BC, both the age and the thermal stability of POC increase. However, thermal stability proved to be a reliable predictor for BC in only one sample set, whereas thermal signals of the other two BC-containing sample sets were not significantly different from those of BC-free samples. Thermal stability thus gives no unequivocal indication for the presence of BC in POC across different sites.
Rationale
Carbon‐13 (13C)‐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 δ13C‐CO2 value of 400‰. A Los Gatos Research laser absorption spectrometer controlled the ambient δ13C‐CO2 value in the chamber through steering of the mass flow controllers with 13C‐enriched and natural abundance CO2 gas.
Results
Laser absorption spectroscopy steering kept the δ13C value of chamber air between 368 and 426‰. The resulting 1 kg dry matter of 13C‐labelled shoots showed an average δ13C 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 δ13C value of the stabile material did not differ significantly from that of bulk material.
Conclusions
Laser spectroscopy controlled 13C 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.
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