Chrome-free leather biodegrades faster in soil, reshaping microbial communities

Finished leather may look inert, but buried in soil it becomes food for an entire microbial ecosystem, and how quickly that happens depends heavily on how the leather was tanned. Chrome-free suede leather fell in between at 8.04%. The findings come from a team led by researchers including Xia Li, Yunhang Zeng, Bi Shi and Ya-nan Wang, who engineered an evaluation system combining soil burial degradation, metagenomic sequencing of the soil microbiome, and analysis of the biogeochemical cycles that leather decomposition sets in motion.

The three materials tested were representative finished leathers purchased from a tannery in Guangdong Province, China. Chrome grain leather, abbreviated CGL, was tanned with conventional chromium salts. The two chrome-free materials, grain leather (FGL) and suede leather (FSL), were both tanned with TWS, an amphoteric aldehyde tanning agent that the authors describe as one of the most widely employed chrome-free tanning agents in contemporary leather manufacturing. Chromium tanning has dominated the industry for more than 160 years because it gives leather outstanding hydrothermal stability, durability, flexibility and resistance to environmental attack, but the chromium-containing solid waste it generates poses significant post-treatment challenges, which is why chrome-free methods have become a key industry focus.

Chemical analysis first established why these leathers are inherently biodegradable. Carbon contents ranged from 44% to 48% across the three materials, and nitrogen contents were 9.8% for CGL, 10.1% for FGL and 11.4% for FSL. Using the standard nitrogen-to-protein conversion factor of 6.25, the researchers calculated that the protein content of all three leathers exceeded 60%. Lipid content, measured by Soxhlet extraction with dichloromethane, was highest in FGL at around 8.8%. Chromium analysis by inductively coupled plasma optical emission spectroscopy showed about 1.3% chromium in CGL, while chromium was not detected in the two chrome-free leathers. In other words, finished leather is rich in both the carbon and nitrogen sources that microorganisms need for proliferation.

The burial experiment followed the Chinese standard GB/T 32601.1–2016 with minor modifications. Rectangular strips measuring 12 by 2 centimeters were folded into a U-shape, placed in flowerpots and covered with soil whose moisture was held at 60 plus or minus 5% of water holding capacity. The pots sat in a biochemical incubator at 30 degrees Celsius and 95% to 100% relative humidity for nine months, with daily loosening of the surface soil and compensation of water loss. Biodegradation rate was expressed as the percentage of mass loss after cleaning, oven-drying at 105 degrees Celsius and precise weighing. The researchers also tracked shrinkage temperature, a measure of hydrothermal stability, with a TMA-YD4 tester.

The mass loss results tracked the tanning chemistry. The authors attribute CGL’s resistance to the crosslinking of collagen fibers by inorganic chromium complexes, which confers high resistance to microbial attack. FGL degraded the most, but surprisingly FSL, which lacks a coating, degraded relatively little; the team suggests this is because degradation-resistant retanning agents outweigh readily biodegradable fatliquors in that material. Shrinkage temperature told the same story: after nine months, Ts fell from 109.0 to 104.5 degrees Celsius for CGL, from 79.2 to 72.4 for FGL, and from 81.4 to 78.7 for FSL, mirroring the biodegradability ranking and indicating that microbial activity cleaves the crosslinks formed between collagen fibers and tanning agents.

The soil itself changed measurably as the leather broke down. Total inorganic carbon showed an initial rise followed by stabilization, which the authors link to microbial conversion of organic carbon into carbon dioxide and then into precipitated carbonates. In the CGL treatment, soil chromium climbed from 38.2 to 51.3 milligrams per kilogram over nine months. Total nitrogen dipped slightly in the first two months, possibly because microorganisms preferentially used soil nitrogen for growth while some nitrogen escaped as gas, then rose overall; at nine months the FSL and CGL soils held 2.45 and 1.07 times their initial nitrogen, respectively, evidence that collagen was being degraded into small nitrogen-containing compounds entering the soil nitrogen pool.

Microbial activity indicators reinforced the pattern. Optical density at 600 nanometers, a rapid proxy for microbial cell density, was higher in the FGL and FSL soils than in the CGL soil, showing that chrome-free leather is more susceptible to microbial degradation and utilization. Soil pH declined from 8.74 in the control to 8.31, 8.11 and 8.12 in the CGL, FGL and FSL soils at nine months, an acidification the authors attribute to collagen breakdown and microbial metabolism. Because pH governs the transformation of chromium species, the team notes that chromium in the CGL soil may exist mainly as chromium hydroxide precipitates.

Metagenomic sequencing revealed how thoroughly the leather reshaped the buried-soil ecosystem. Samples were collected at six and nine months with three independent replicates and compared against an initial control soil. Heatmaps and principal component analysis showed samples clustering by leather type, with the chrome-free treatments pulling the community further from the control than chrome-tanned leather did. The number of microbial genera rose from 3318 in the control to 3854, 4080 and 3699 in the CGL, FGL and FSL soils after six months, and 90, 280 and 94 genera were unique to those treatments respectively. Ace and Shannon diversity indices both rose above control levels and continued climbing through nine months, with FGL at nine months showing the highest Ace index, indicating that leather burial considerably enhances both the richness and diversity of the soil microbial community.

Bacteria dominated throughout, accounting for 99.01% of the control community and remaining above 93% in all treated soils, while archaeal proportions increased 1.41-, 1.39- and 1.95-fold in the CGL, FGL and FSL six-month soils. The control community contained 197 phyla and 17,841 species, led by Actinomycetota at 39.59%, Pseudomonadota at 28.39%, Acidobacteriota, Chloroflexota and Gemmatimonadota. After six months, Chloroflexota expanded markedly in the CGL soil, a phylum frequently identified in chromium-containing environments and known to harbor heavy metal resistance genes. In the FGL soil, Pseudomonadota reached 36.91% and the fungal phylum Ascomycota appeared at 4.85%, with the genus Talaromyces at 4.01%; the authors suggest fungal hyphae and their extracellular hydrolytic enzymes may penetrate finished leather, making Ascomycota a critical initial colonizer. In the FSL soil, Pseudomonadota rose to 40.79% to become dominant, and the nitrifying phyla Nitrospirota and Nitrososphaerota increased to 2.22 and 2.06 times their control abundances, a signature of intense proteolysis releasing ammonia that stimulates the nitrification pathway.

Functional gene analysis tied these community shifts to collagen chemistry and the nitrogen cycle. Genes for enzymes of glycine, proline, hydroxyproline and arginine metabolism, including amino acid oxidase, proline iminopeptidase and ornithine aminotransferase, rose in abundance in all treated soils, with the amino acid oxidase gene in FGL at 2.28 times the control level. Genes for organic nitrogen metabolism such as gdhA, ureA and ureB increased, and Pseudomonadota, Actinomycetota and Chloroflexota together accounted for more than 80% of the microorganisms involved in that pathway. Nitrification genes amoC, amoB and amoA were more abundant in the chrome-free soils, and the nitrite-oxidizing genus Nitrospira made up 29.78%, 36.24% and 51.95% of nitrifying microorganisms in the CGL, FGL and FSL soils respectively. Genes for nitrate uptake and assimilatory nitrate reduction also rose. Since nitrate is the predominant form of nitrogen plants acquire from soil, the authors conclude that chrome-free finished leather may deliver nitrogen nutrition to plants more readily, continuously supplying organic nitrogen-rich substrates that support organic nitrogen metabolism, nitrification, assimilation and nitrate reduction, and helping stabilize soil fertility.

Subject of Research: Soil burial biodegradability of chrome-tanned and chrome-free finished leathers and its effects on soil microbial communities and biogeochemical cycles

Article Title: Soil burial evaluation of finished leather biodegradability: microbial community dynamics and biogeochemical cycles

Article References: Li, X., Zhang, Y., Yu, Y., Zeng, Y., Shi, B., & Wang, Y.-N. (2026). Soil burial evaluation of finished leather biodegradability: microbial community dynamics and biogeochemical cycles. Collagen and Leather, 8(1), Article 31. https://doi.org/10.1186/s42825-026-00268-3

Image Credits: AI Generated

DOI: 10.1186/s42825-026-00268-3

Keywords: finished leather, biodegradability, soil burial, chrome-free tanning, chromium tanning, metagenomics, soil microbiome, nitrogen cycle, collagen degradation, microbial diversity, sustainability, leather industry