Spatial Microanalysis of Natural ¹³C/¹²C Abundance in Environmental Samples Using Laser Ablation-Isotope Ratio Mass Spectrometry

Abstract

The stable 13 C/ 12 C 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-isotope ratio mass spectrometry (LA-IRMS) methodology 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 δ 13 C 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 13 C/ 12 C abundance can now be mapped at a spatial resolution down to 10 μm. The respective δ 13 C 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.

Linking rhizosphere processes across scales: Opinion

“…The distribution of "dynamic C" (i.e., root-exuded C) follows a gradient into the soil and can be traced using stable C isotope labelling. Labelled C exuded by the plant root can be followed and quantified on the millimetre-scale using an adequate sampling technique and can be visualized in 2D at the micro-scale when using resin embedded root-soil sections and nano-scale secondary ion mass spectrometry (NanoSIMS) (Vidal et al 2018) or laser ablation-isotope ratio mass spectrometry (LA-IRMS) (Rodionov et al, 2019). This information can be used following two different approaches.…”

“…The second fraction of C in the rhizosphere is a rather static, discontinuous fraction made of dead plant residues and microbial necromass and mineral-associated organic matter (MAOM) (e.g., Liang et al, 2019) that can be mapped to the POM considered by Portell et al (2018). This fraction, which we can expect to be located more irregularly in areas with decreased accessibility by the microorganisms (Rodionov et al, 2019;Totsche et al, 2018) can also be estimated using NanoSIMS and LA-IRMS.…”

“…The first challenge is to simulate the distribution of water and air within the pore space of the subdomains as described in the example 2 above. Root derived C has been shown to be distributed at least 100 µm distant from the root plane (Rodionov et al, 2019), but more likely extends in the mm range (Jones et al, 2004). Our first working assumption will be to assimilate 13-C labelled C to diffusable dissolved organic matter (DOC, i.e., readily available C for microorganisms) and all non-labelled C within a 100-1000 µm-rhizosphere-range as particulate organic matter (POM, that can be mineralized under the action of microbial enzymes).…”

Linking rhizosphere processes across scales: Opinion

“…The distribution of "dynamic C" (i.e., root-exuded C) follows a gradient into the soil and can be traced using stable C isotope labelling. Labelled C exuded by the plant root can be followed and quantified on the millimetrescale using an adequate sampling technique and can be visualized in 2D at the micro-scale when using resin embedded root-soil sections and nano-scale secondary ion mass spectrometry (NanoSIMS) (Vidal et al 2018) or laser ablation-isotope ratio mass spectrometry (LA-IRMS) (Rodionov et al 2019). This information can be used following two different approaches.…”

Permafrost soil complexity evaluated by laboratory imaging Vis‐NIR spectroscopy

“…The authors used the method on dried intact soil profiles and were able to differentiate soil elemental and structural features in mineral and organic soils. Because a variety of techniques that aim to analyse intact soil structures at the microscale make use of resin‐embedded soil sections (Eickhorst & Tippkoetter, 2008; Herrmann, Ritz, et al, 2007; Juyal et al, 2019; Mueller et al, 2013; Nunan et al, 2001; Rodionov et al, 2019; Schlueter et al, 2019; Vidal et al, 2018), the use of imVisIR on these sections would enable upscaling and correlation with bulk soil features (Manß, Hilgers, Buddenbaum, & Stanjek, 2017). However, because, up to now, imVisIR has been used in soil science exclusively to analyse natural non‐embedded soil, it was not clear if the use of embedding resin would hamper the classification and thus the quantification of microscale soil structures.…”