In a study published in the journal Biochar, researchers Kaiyue Song, Zhiwei Liu, Ruiling Ma, Qi Yi, Jufeng Zheng, Rongjun Bian, Kun Cheng, Shaopan Xia, Xiaoyu Liu, Xuhui Zhang, and Lianqing Li examined how long-term biochar applications influence soil organic carbon stability through depth-dependent responses. Soil microbial necromass carbon, which consists of the cellular remnants of dead bacteria and fungi, represents a major and highly stable component of soil organic carbon pools. Understanding how this microbially mediated carbon pool reacts to management practices over extended periods is vital for designing effective climate change mitigation strategies in global agriculture. The research team established a twelve-year field experiment across two highly contrasting cropland soils to evaluate these dynamics at different depths.

The experimental findings demonstrated a clear divergence in how microbial necromass accumulates in the upper and lower layers of the soil profile. In the topsoil layer, which spans from zero to twenty centimeters, long-term biochar amendment significantly enhanced total microbial necromass carbon concentrations. Specifically, the biochar treatment expanded these stable carbon reserves by twenty-three point three percent in a carbon-rich Entisol and by thirty-nine point zero percent in a carbon-poor Ultisol. This accumulation was primarily driven by a robust positive response in fungal necromass, which increased more intensely than bacterial necromass across both soil types. The increase in topsoil microbial biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More and the shift toward fungal dominance directly favored the formation of stable soil aggregates, which physically shelter organic carbon from rapid breakdown.

In stark contrast to the topsoil results, the subsoil layer between twenty and forty centimeters deep exhibited a significant reduction in microbial necromass carbon following the twelve-year amendment. Subsoil necromass carbon values decreased by eighteen point seven percent in the Entisol and by thirty point four percent in the Ultisol. This unexpected decline suggests that applying biochar to the surface can disrupt deeper soil carbon dynamics over time. The reduction in the deeper layer was accompanied by a noticeable decrease in the overall contribution of microbial necromass to the total soil organic carbon pool, illustrating a strong depth dependency in microbially mediated sequestration pathways.

The underlying mechanisms driving these opposing depth responses are tied to biochar-induced changes in nutrient availability and microbial physiological traits. In the topsoil, biochar increased available nitrogen and phosphorus while reducing the microbial metabolic quotient and biomass-specific enzyme activities. This combination indicates higher microbial efficiency, allowing a larger portion of carbon to be assimilated into living biomass and eventually stored as dead cell remnants rather than being lost to the atmosphere as respiratory carbon dioxide. Furthermore, a reduction in the activity of specific enzymes responsible for breaking down dead cell components helped preserve the newly formed topsoil necromass.

In the subsoil, the opposite conditions developed because biochar restricted the downward translocation of essential nutrients from the upper layer. The highly porous structure of surface biochar strongly adsorbed nutrients, leading to a significant drop in available nitrogen within the deeper soil profile. To cope with this severe nutrient scarcity, subsoil microorganisms altered their metabolic strategies, elevating their metabolic quotient and increasing their investment in specific nutrient-acquiring enzymes. This shift forced the microbial community to actively decompose or mine the existing stable necromass to extract necessary nutrients, which ultimately depleted the subsoil carbon reserves.

To test the global application of these field site observations, the authors conducted a comprehensive meta-analysis compiling eighty-five pairs of topsoil data from twenty-three peer-reviewed studies. The statistical synthesis confirmed that biochar application generated a positive response in topsoil total necromass carbon in eighty-three point five percent of the recorded cases, resulting in an overall average increase of ten point two percent. The meta-analysis also verified that fungal necromass values grew by twelve point one percent globally, reinforcing the field finding that fungi respond more favorably to biochar than bacteria.

The global data synthesis further revealed that the magnitude of carbon accumulation is heavily regulated by initial soil properties, climate conditions, and application parameters. Biochar stimulated the highest relative gains in total microbial necromass carbon when applied to warm, humid regions and within soils characterized by low initial organic carbon levels and coarse, sandy textures. Additionally, the positive effects on necromass storage intensified progressively over time, reaching a clear peak ten years after the initial soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More. This temporal pattern emphasizes that short-term experiments often underestimate the true capacity of biochar to support microbially mediated soil carbon stabilization.

Source: Song, K., Liu, Z., Ma, R., Yi, Q., Zheng, J., Bian, R., Cheng, K., Xia, S., Liu, X., Zhang, X., & Li, L. (2026). Depth-dependent microbial necromass carbon accumulation responses to long-term biochar amendment in croplands. Biochar, 8(1), 78.