PNNL

Abstract

Spatial and temporal heterogeneity of nutrient exchange within the rhizosphere is a topic of increasing interest, although challenging to study due to limits in existing analytical capabilities. Here, we developed and demonstrated a new approach applying laser ablation sample introduction to capillary absorption spectroscopy (LA-CAS) to characterize carbon isotopic distribution within plant tissues, rhizosphere, and soil. We exposed switchgrass plants to 13CO2 to allow tracing of 13C-labelled photosynthates within plant biomass and into the associated soil. The LA-CAS methods we describe leverage continuous measurements of a sample stream derived from laser ablation line scans (10-25 µm in width) over a sample surface which enables the user to produce an isotope map with a) higher data density or b) over larger spatial areas versus previously existing techniques. This versatility of LA-CAS is assessed through testing of a range of laser parameters (spot size, scan rates) on various materials (soil, plant biomass/tissues, and rhizosphere). We demonstrate the ability of LA-CAS to provide near instantaneous d13C measurements over isotopically distinct surfaces to enable high spatially resolved mapping of 13C-labelled material within the rhizosphere. Applying LA-CAS analysis to plant biomass, we observed higher d13C values concentrated within phloem structures, consistent with localized photosynthate transport. When mapping across the rhizosphere, 13C-enriched soil was typically present within 5-10 µm of root boundaries with a steep spatial increase in d13C when the scan approached the middle of the root. As with all LA approaches, care is required to ensure accurate results as phenomena linked to ablation, combustion, and isotopic measurement can impart artifacts if not carefully controlled. Still, taken as a whole, our demonstrations highlight the increased sample throughput, improved data density, and enhanced d13C capability of LA-CAS versus other LA techniques and emphasize the role this method can play in future plant and rhizosphere related studies.