X-ray scans reveal biochar clogs soil pores rather than boosting water storage in sandy fields

Biochar has been heralded as a near-miraculous soil amendment: a charcoal-like material that locks away carbon for centuries, curbs greenhouse gas emissions, traps nutrients that would otherwise leach into waterways, and, according to a large body of literature, makes soils hold more water. But a new field experiment in the peanut-growing heart of Alabama suggests that, at least in coarse sandy soils, the reality is far more complicated. Researchers at Auburn University used medical-grade X-ray computed tomography to peer inside soil cores taken from plots treated with four different rates of southern yellow pine biochar, and what they found challenges the simple narrative that more biochar automatically means better soil.

The study, conducted at the Wiregrass Research and Extension Center in Headland, Alabama, was set up on a Dothan sandy loam in the Coastal Plain region, where more than 85 percent of the soil is sand and organic matter levels are low. The team, led by Ranveer Singh and Jasmeet Lamba of Auburn’s Department of Biosystems Engineering, together with crop scientist Charles Y. Chen and Sushil Adhikari, spread biochar produced by pyrolysis at roughly 980 degrees Celsius at rates of 0, 7.5, 15 and 30 megagrams per hectare, equivalent to 0, 1.1, 2.2 and 4.4 percent by weight of the top soil layer. The biochar was incorporated into the top 50 millimeters of soil after peanut emergence in May 2024, and undisturbed soil cores were collected just before harvest in October 2024.

The centerpiece of the investigation was the X-ray CT scanning, performed with a GE EVO Revolution 128-slice medical scanner at Auburn’s College of Veterinary Medicine. Operating at 140 kilovolts and 210 milliamperes, the scanner produced three-dimensional image stacks of the top 100 millimeters of soil at a resolution of about 0.195 millimeters. The raw grayscale data were then processed in ImageJ, where a local thresholding algorithm was carefully calibrated using artificial pores of known diameter embedded in the soil columns, ensuring that the segmentation of soil matrix and pore space was accurate to within 1.5 percent. From these images the researchers extracted macroporosity, macropore number density and pore circularity, the geometric fingerprints of the soil’s plumbing system.

The results were striking in their subtlety. In the top 50 millimeters, where the biochar had been incorporated, macroporosity showed no significant change at any application rate. But in the layer just below, between 50 and 100 millimeters, the macropore number density dropped dramatically: by 57 percent at the 15-megagram rate and 64 percent at the 30-megagram rate compared with untreated control plots. Pore circularity, a measure of how close pores are to a perfect circle, fell by about 5 percent at the moderate 15-megagram rate in the surface layer, indicating that biochar particles had disrupted the geometry of existing pores, merging them into elongated, irregular cavities rather than leaving them untouched.

The researchers interpret these patterns through two competing mechanisms they call the occupying effect and the expansion effect. Under the occupying effect, biochar particles physically fill existing soil pores, reducing the volume of large, freely draining channels. Under the expansion effect, the biochar’s own inherited internal porosity adds new pore space to the bulk soil. The data suggest that at rates up to about 15 megagrams per hectare, the occupying effect dominates in this sandy soil, while at 30 megagrams per hectare the pore indices began converging back toward control values, hinting that beyond that threshold the expansion effect may take over. The response, the authors argue, is likely a function of the soil-to-biochar ratio.

Perhaps the most intriguing finding concerns where the biochar actually went. Differences in pore indices in the 50-to-100-millimeter layer, below the incorporation depth, point to the preferential downward movement of biochar particles through the coarse pores of the sandy soil. This migration may explain why the amendment failed to deliver the expected benefits: the particles simply did not stay put. Previous studies have documented vertical transport of biochar in coarse-textured soils, and the Auburn team speculates that if biochar particles remained immobile at their depth of incorporation, improvements in bulk density and total porosity might have materialized, as they have in controlled pot experiments where such losses are negligible.

The hydro-physical measurements told a similarly sobering story. Soil water retention curves, generated by combining high-resolution evaporation-method data from HYPROP2 tensiometers in the wet range with WP4C dewpoint measurements extending to oven dryness, showed no significant effect of biochar at any rate or depth. Bulk density, total porosity, wet aggregate stability and plant-available water were all statistically unchanged four months after application. Even the highest dose of 30 megagrams per hectare, a substantial and costly amount, left the soil’s capacity to hold water against gravity and to supply it to plants essentially untouched.

Why did biochar fail here when it has succeeded elsewhere? The authors point to the soil itself. Clay particles, with their high surface area and reactive surfaces, provide the binding sites where organic matter and biochar can knit stable aggregates; sandy soils with low clay and low organic matter offer few such opportunities. Pyrolysis temperature matters too: biochar produced at high temperatures, as in this study, can be hydrophobic, repelling water rather than absorbing it, at least until environmental exposure gradually wears that hydrophobicity down. Other researchers have found that water retention in sandy soils improves only at much higher application rates, in the range of 40 to 65 megagrams per hectare, doses that would be economically prohibitive for most farms.

The implications reach beyond agronomy into environmental policy. Biochar’s ability to reduce nutrient leaching and greenhouse gas emissions depends in part on how it reshapes the soil’s pore network. If biochar occupies large pores and shifts flow from rapid preferential channels to slower matrix flow, solutes spend more time in contact with biochar surfaces and can be adsorbed. But if the expansion effect creates abundant new preferential pathways, contaminants could bypass the amendment entirely. Knowing which regime prevails, at which dose, in which soil, is therefore essential for anyone counting on biochar to deliver its promised environmental services.

The Auburn team is candid that their hypothesis, that biochar would improve the hydro-physical properties of a poorly structured sandy soil within a single growing season, was rejected by the data. They argue that the field needs more refined prescriptions: biochar types and rates matched to specific soil textures, targeted application methods such as banding near the root zone or subsurface placement to reduce runoff losses, and longer observation windows to capture slow structural changes. They also call for micro-CT imaging at micrometer resolution to resolve the pores inside biochar particles themselves, which current medical scanners cannot detect. For now, the message for farmers on sandy ground is one of tempered expectations: biochar remains a promising tool for carbon sequestration, but transforming it into a reliable water-management strategy will require far more precision than simply spreading charcoal and hoping for the best.

Subject of Research: Short-term effects of pine biochar application rates on soil pore networks and hydro-physical properties in sandy soil

Article Title: Elucidating short-term impact of biochar addition on soil pore networks using X-ray computed tomography and soil water retention characteristics

Article References: Singh, R., Lamba, J., Chen, C. Y., & Adhikari, S. (2026). Elucidating short-term impact of biochar addition on soil pore networks using X-ray computed tomography and soil water retention characteristics. Discover Soil, 3(1), Article 102. https://doi.org/10.1007/s44378-026-00252-7

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00252-7

Keywords: biochar, soil pore networks, X-ray computed tomography, macroporosity, soil water retention, sandy soil, pyrolysis, soil physics, peanut production, soil amendment, hydro-physical properties, pore circularity