Sunlight Pre-Treatment Turns Hydrochar Into a Stronger Soil Amendment

Hydrochar, the carbon-rich material left behind when wet biomass is cooked under pressure at relatively low temperatures, has long been viewed as a promising soil amendment with one nagging drawback: it is chemically unstable and releases large amounts of dissolved organic matter into the ground. A new study published in Carbon Research reports that simply exposing hydrochar to simulated sunlight before it reaches the soil can resolve much of that instability while simultaneously making the material more useful to soil microbes. The finding, drawn from pig manure and mandarin peel hydrochars produced at 180 and 260 degrees Celsius, suggests that a low-energy aging step could become a standard part of preparing carbon-based materials for sustainable agriculture.

The research team, led by Lili He and Yahui Ji with colleagues at several Chinese institutions, prepared four hydrochars by heating ground pig manure or citrus peel in a sealed stainless-steel vessel with water at a solid-to-liquid ratio of 1:10. The two carbonization temperatures, 180 and 260 degrees Celsius, generated internal pressures of 4 and 8 megapascals respectively over a one-hour retention time. The resulting materials were labeled PHC180, PHC260, MHC180, and MHC260, where P denotes pig manure and M denotes mandarin peel. Half of each batch was then placed in a xenon arc weathering chamber and irradiated according to the ASTM G155 protocol, receiving ultraviolet and visible light at 0.35 watts per square meter per nanometer at a wavelength of 340 nanometers for a total of 200 hours, with deionized water sprayed every 120 minutes to mimic wetting events. The aged counterparts were designated PAC180, PAC260, MAC180, and MAC260.

Characterization by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and nitrogen adsorption revealed that the higher preparation temperature of 260 degrees Celsius produced hydrochars with greater specific surface area and more heterogeneous surface morphology than the 180-degree versions. Solar aging then pushed these surfaces further along a photochemical trajectory. The irradiated materials developed additional micropores, and XPS showed a consistent enrichment of covalently bonded oxygen-containing groups such as C–O and C=O on the outermost surface, an effect the authors attribute to photo-oxidation of aromatic carbon. Thermogravimetric analysis added an important nuance: thermal stability was governed almost entirely by the original carbonization temperature rather than by sunlight exposure. The main decomposition peak shifted from around 350 degrees Celsius in the 180-degree hydrochars to 420 to 450 degrees Celsius in the 260-degree materials, reflecting more thermally stable aromatic structures formed during intensified carbonization, and solar aging did not alter the fundamental shape or peak positions of these curves.

The dissolved organic matter fraction told a subtler story. Three-dimensional excitation-emission matrix fluorescence coupled with parallel factor analysis identified three components in the hydrochar leachates: fulvic acid-like, soluble microbial byproduct-like, and humic acid-like substances. Total dissolved organic carbon declined after irradiation, dropping by 1.07 to 8.71 milligrams per gram relative to the pristine materials, and fluorescence intensity diminished accordingly. Yet the molecular composition shifted in ways that could matter for soil biology. High-resolution mass spectrometry using a 15-tesla Fourier transform ion cyclotron resonance instrument showed that the number of assigned molecular formulas actually rose with aging, from 5610 in MHC260 to 6468 in MAC260 and from 6226 in PHC260 to 6394 in PAC260. The proportion of protein-like substances increased by 1.2 percent in the aged mandarin peel hydrochar and by 5.5 percent in the aged pig manure hydrochar, while lipid-like substances declined. The authors interpret this as photolysis breaking macromolecules into smaller, more bioavailable fragments.

Other molecular indicators reinforced the picture of selective transformation. Intensity-weighted hydrogen-to-carbon and oxygen-to-carbon ratios fell sharply, by 53.9 to 71.0 percent and 85.3 to 88.7 percent respectively, in the aged samples, indicating dehydration during the aging process. The modified aromaticity index climbed by 30.1 and 65.6 percent, signaling higher aromaticity in the remaining dissolved organic matter. Van Krevelen diagrams confirmed that lignin-like compounds dominated both pristine and aged hydrochars, but the relative abundance of individual compound classes shifted measurably after irradiation. Double-bond equivalence distributions also changed in a feedstock-dependent manner: pig manure-derived aged hydrochar showed a higher degree of unsaturation than its pristine counterpart, while the mandarin peel material was largely unchanged in that respect. Taken together, the spectroscopic and mass-spectrometric evidence indicates that sunlight does not simply strip organic matter from hydrochar; it reshuffles what remains into a more diverse and, in parts, more microbially accessible mixture.

To test whether these surface and molecular changes translate into soil benefits, the team ran flooded paddy soil microcosms. Thirty grams of dry-equivalent paddy soil received one percent by weight of either pristine or aged 260-degree hydrochar, with three biological replicates per treatment, a shallow water layer to simulate paddy conditions, and standard fertilization of 220 kilograms of nitrogen, 60 kilograms of phosphorus, and 60 kilograms of potassium per hectare. After 45 days of incubation, hydrochar amendment had raised soil organic carbon by 53.3 to 110.0 percent, total nitrogen by 14.2 to 28.5 percent, and hydrogen content by 4.1 to 12.5 percent relative to a fertilized control without hydrochar. Dissolved organic carbon and dissolved organic nitrogen in the soil increased by 30.4 to 86.9 percent and 17.2 to 31.6 percent respectively, and the humic acid-like fraction of soil dissolved organic matter rose prominently, especially in the mandarin peel 260-degree treatment.

Not every change was uniformly positive, and the study is candid about the trade-offs. Soil pH shifted in treatment-specific ways, rising to 8.2 in the pristine mandarin peel hydrochar and 7.5 in the aged pig manure hydrochar, falling to 7.2 in the aged mandarin peel treatment, and holding at 7.1 in the pristine pig manure treatment against a control of 7.1. More notably, ammonium nitrogen diverged sharply: pristine hydrochars raised soil ammonium nitrogen by 59.1 percent in the mandarin peel treatment and 31.8 percent in the pig manure treatment, whereas the aged versions lowered it by 28.1 and 25.7 percent respectively. The authors link this decline to upregulated denitrification and dissimilatory nitrogen-reduction genes such as norB, napA, nrfA, narH, and narG, together with adsorption of ammonium onto the newly oxidized hydrochar surfaces, and they argue that the lost ammonium represents a conversion into other nitrogen pools within an accelerated cycle rather than a net depletion of soil nitrogen fertility. Floodwater analysis echoed the pattern, with hydrochar-derived dissolved organic carbon raising floodwater dissolved organic carbon 2.5- to 5.5-fold and floodwater dissolved organic nitrogen and ammonium nitrogen increasing by 76.2 to 409.1 percent and 28.5 to 50.0 percent respectively.

The microbial response was selective rather than wholesale. Alpha diversity metrics such as Chao1 and Shannon indices showed no significant differences among treatments, but community composition shifted at the phylum and genus levels. Proteobacteria and Firmicutes increased in several hydrochar treatments, and at the genus level the pristine pig manure hydrochar significantly boosted Geobacter, possibly because its conductive surface and redox-active sites facilitate extracellular electron transfer, while the pristine mandarin peel hydrochar significantly enriched Bacillus, a genus whose members decompose organic matter and enhance nutrient availability. Predicted functional profiling with PICRUSt2 indicated that hydrochar application increased the abundance of genes for glucosidase, galactosidase, cellulase, and urease activity, along with phosphorus mineralization genes including inorganic phosphorus solubilization, alkaline phosphatase, and acid phosphatase. A structural equation model with a goodness-of-fit index above 0.9 and a standardized root mean square residual below 0.08 further indicated that soil dissolved organic carbon effectively promoted bacterial proliferation, with a path coefficient of 0.71, while soil organic carbon enhanced carbon and nitrogen metabolism gene abundance.

The authors frame the work as a demonstration that solar radiation aging is a low-energy modification strategy for carbon-based soil amendments, one that avoids the high energy demands of pyrolysis while tailoring the material’s surface chemistry and dissolved organic matter profile. They are careful to note the boundaries of the evidence: the findings come from paddy soil microcosms characterized by pronounced redox fluctuations, the aging protocol represents an accelerated exposure broadly comparable to weeks to months of natural solar weathering rather than years, and the functional gene results are predictions from PICRUSt2 rather than direct measurements of expressed enzymes. Field-scale validation, optimization of aging parameters, and systematic evaluation of ecological and economic benefits remain future work. Within those limits, the study offers a concrete molecular account of how sunlight pre-treatment reshapes hydrochar, reducing its labile organic load while concentrating bioavailable, low-molecular-weight compounds that feed the soil microbes responsible for carbon, nitrogen, and phosphorus cycling, and it positions a simple, passive aging step as a practical lever for improving the performance of hydrochar in flooded rice systems and potentially beyond.

Subject of Research: Solar radiation aging of hydrochar and its effects on dissolved organic matter, soil properties, and microbial communities in paddy soil

Article Title: Solar radiation aging enhances hydrochar’s soil remediation potential by altering dissolved organic matter and microbial communities

Article References: He, L., Ji, Y., Li, D., Wang, Y., Lyu, H., Wang, B., Chen, B., Liu, X., Chen, C., Guo, B., Li, N., & Feng, Y. (2026). Solar radiation aging enhances hydrochar’s soil remediation potential by altering dissolved organic matter and microbial communities. Carbon Research, 5(1), Article 45. https://doi.org/10.1007/s44246-026-00277-1

Image Credits: AI Generated

DOI: 10.1007/s44246-026-00277-1

Keywords: hydrochar, solar aging, soil remediation, dissolved organic matter, paddy soil, soil microbiome, carbon sequestration, nitrogen cycling, phosphorus mineralization, photo-oxidation, soil amendment, hydrothermal carbonization