IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 200 Poster Session / Soil Organic Matter, Caustobiolites, and Biochar Hall A P2.7 Pushing the Limits of Low-Field NMR: Improving DDIF Method Resolution to Study Biochar Internal Structures José María García de Castro Barragán1, Etelvino Henrique Novotny2, Heike Knicker1* 1 Instituto de la Grasa (IG-CSIC), Carretera de Utrera, km. 1, 41013, Seville, Spain. 2 EMBRAPA Solos, Rua Jardim Botânico, 1024, Rio de Janeiro, Brazil. * corresponding author: heknicker@ig.csic.es Biochar Pore Size Distribution (PSD) is essential for understanding and optimising its use as a sustainable horticultural substrate. However, the characterisation of its smallest pores remains analytically challenging. The Low-Field Nuclear Magnetic Resonance (NMR) is a promising non-destructive technique to study porous systems. For that, the Decay Due to Diffusion in Internal Field (DDIF) method, which estimates the PSD from the exponential decay of the NMR signal of water molecules. The DDIF acquisition involves two complementary pulse sequences: one that provides signals encoding the diffusion of water molecules through the porous network, and another reference sequence, that reflects signals from only the spin-lattice relaxation contribution. The subtraction of both signals allows the isolation of the DDIF signal from the spin-lattice relaxation background. The encoding time (Te) was calibrated independently for each sample, and PSD was subsequently obtained via Inverse Laplace Transformation (ILT) of the resulting decay curves. The resolution of the standard DDIF is limited to approximately 10 µm, since the signals of large micropores often mask those of smaller ones. To improve the resolution, a novel T1-filter presaturation step was developed and implemented, selectively suppressing the signal from water in large pores and allowing an optimised Te. This improvement enhanced the detectable pore size range to below 1 µm, an expansion of approximately one order of magnitude. In the present work, we applied this technique to estimate how washing with distilled water affected PSD of biochars derived from tomato greens, vineyard pruning residues, and Acacia bushes before and after they were applied as planting substrate for tomato and lettuce cultivation for 4 weeks in the greenhouse. Before the measurement process, the samples were ground, pressed, and saturated with water in vacuum, ensuring a complete porous filling. Preliminary results, obtained from the combination of the standard and T1-filtered DDIF experiments reveal a full PSD range (from <1 µm to 1000 µm). We observed that washing of the biochars increased the relative contribution of
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