Water dynamics in different biochar fractions

• Published in: Magnetic resonance in chemistry Vol. 53; no. 9; pp. 726 - 734
• Main Authors: Conte, Pellegrino, Nestle, Nikolaus
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01-09-2015; Wiley Subscription Services, Inc
• Subjects: biochar; Charcoal - analysis; Chemical Fractionation; Diffusion; Dynamics; Exchange; Fertilizers - analysis; FFC NMR relaxometry; Imaging; Inhomogeneity; Magnetic Resonance Spectroscopy; micro-CT imaging; micro-tomography; Nuclear magnetic resonance; Populus - chemistry; Porosity; Soil - chemistry; Texture; texture size; Three dimensional; Tomography, X-Ray Computed; water; Water - analysis; micro-tomography; dynamics; biochar; FFC NMR relaxometry; micro-CT imaging; texture size; water
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.4204

Biochar is a carbonaceous porous material deliberately applied to soil to improve its fertility. The mechanisms through which biochar acts on fertility are still poorly understood. The effect of biochar texture size on water dynamics was investigated here in order to provide information to address future research on nutrient mobility towards plant roots as biochar is applied as soil amendment. A poplar biochar has been stainless steel fractionated in three different textured fractions (1.0–2.0 mm, 0.3–1.0 mm and <0.3 mm, respectively). Water‐saturated fractions were analyzed by fast field cycling (FFC) NMR relaxometry. Results proved that 3D exchange between bound and bulk water predominantly occurred in the coarsest fraction. However, as porosity decreased, water motion was mainly associated to a restricted 2D diffusion among the surface‐site pores and the bulk‐site ones. The X‐ray μ‐CT imaging analyses on the dry fractions revealed the lowest surface/volume ratio for the coarsest fraction, thereby corroborating the 3D water exchange mechanism hypothesized by FFC NMR relaxometry. However, multi‐micrometer porosity was evidenced in all the samples. The latter finding suggested that the 3D exchange mechanism cannot even be neglected in the finest fraction as previously excluded only on the basis of NMR relaxometry results. X‐ray μ‐CT imaging showed heterogeneous distribution of inorganic materials inside all the fractions. The mineral components may contribute to the water relaxation mechanisms by FFC NMR relaxometry. Further studies are needed to understand the role of the inorganic particles on water dynamics. Copyright © 2015 John Wiley & Sons, Ltd.
NMR relaxometry and m‐CT imaging revealed an inhomogeneous distribution of particle sizes in three different biochar fractions separated by using stainless steel sieves. Water motion is affected by particle size inhomogeneity in all the fractions studied here. Particle size inhomogeneity suggests that 3D exchange with bulk water predominates in the coarsest biochar sample, while the 3D motion mechanism co‐occurs with the 2D water diffusion as particle size reduces.