Biochar is often described as a climate-smart soil amendment because it can improve soil quality while storing carbon for long periods. But once biochar is added to soil, it does not stay unchanged. Rain, drying, minerals, oxygen, and microorganisms gradually alter its surface, chemistry, and environmental functions. For saline soils, where salt stress already limits crop production across many agricultural regions, scientists have had limited information about how biochar changes over time.
A new study published in Biochar provides a closer look at that process. Researchers found that higher soil salinity can slow the aging of biochar, helping it retain more carbon-rich, aromatic structures while reducing microbial colonization, especially by fungi.
“Biochar is widely used to improve saline-alkali soils, but its long-term performance depends on how it ages after entering the soil environment,” said corresponding author Rongjiang Yao of the Institute of Soil Science, Chinese Academy of Sciences. “Our study shows that salinity is not just a stress factor for plants and microbes. It also reshapes the aging pathway of biochar itself.”
To simulate long-term field aging, the team collected agricultural soils with low, moderate, and high salinity from coastal farmland in Jiangsu Province, China. Wheat-straw biochar was mixed into these soils and exposed to repeated wetting and drying cycles. The experiment simulated approximately eight years of natural aging and allowed the researchers to track changes in biochar chemistry, surface structure, mineral composition, and microbial communities.
The results showed a clear pattern. Biochar aged in high-salinity soil retained higher total carbon, stronger aromaticity, and more surface C-C/C=C carbon structures than biochar aged in low-salinity soil. At the same time, it showed lower oxygen content, lower oxidation degree, and less surface C-O bonding, all indicators of slower aging. By the final aging cycle, the O/C ratio of biochar aged in high-salinity soil was 9.82% lower than that in low-salinity soil. Across the aging process, total carbon content declined by about 20%, mainly due to the loss of labile carbon and mineralization of organic matter.
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