Table 1.

Characterization techniques for biominerals formed in the exterior and interior surface of biochar-treated root at micron- and sub-micron scale.

Techniques	Lateral resolution	Sample preparation	Characteristics of biominerals	Results
VSM magnetometry	n.a.	Critical point drying (CPD)	Magnetic nanoparticles with diameters of less than 50 nm in the root	Fig. S1
Raman microspectroscopy	~ 1 μm	CPD and longitudinal cutting	Micron-sized biochar particles on the root surface	Fig. S1
LA-ICP-MS	n.a.	Freeze drying and longitudinal cutting	The plant nutrients (i.e., Fe, Si, and P) significantly increased inside the biochar-treated root	Figure 1
ToF-SIMS	~ 100 nm	Freeze drying and longitudinal cutting	The iron oxides and hydroxides in the plaque layer	Figures 2, 3, and 4
SEM-EDX	~ 1 μm	CPD and longitudinal cutting and coating with chromium	Co-location of Fe- and Si-bearing ions at the root exterior and interior	Figures 3 and 4
STEM-EDX/EELS	< 0.14 nm	Embedding in Spurr’s resin and transverse cutting by ultramicrotomy (thickness ~ 60 nm)	(1) Nanoscale Fe–Si–P association on the root surface (2) ~ 5 nm iron core of ferritin inside the root (3) Intracellular needle-like clusters of iron nanoparticles in the aegirine phase	Figure 5 and Fig. S2
AFM	~ 100 nm	Embedding in Spurr’s resin and transverse cutting by microtomy (thickness ~ 400 nm)	Micron-sized electrically charged domains inside the root	Figure 6