Soil nitrogen level controls biochar's enhancement of microbial-derived carbon sequestration

Abstract

Biochar is a carbon (C)-rich product of thermochemical conversion of organic materials that can enhance soil C sequestration. Given the tight coupling of C and nitrogen (N) cycles and the constraints of C:N stoichiometry on microbial growth, soil N level can regulate microbial growth, with implications for microbial necromass formation and soil organic carbon (SOC) accumulation. Here, we conducted a comprehensive meta-analysis of 932 paired observations globally to assess biochar effects on microbial biomass C (MBC), microbial necromass C (MNC), and SOC. Biochar increased soil C pools, with larger gains under low-N conditions (MBC + 37.4%, MNC + 14.0%, SOC + 47.9%) than under high-N conditions (MBC + 22.7%, MNC + 6.15%, SOC + 29.4%). In high-N soils, MBC accumulation is optimized under higher biochar application rates, warmer climates, and lower initial SOC/TN ratios, whereas MNC accumulation responds more strongly to longer experimental duration, higher biochar total C content, and alkaline soils. In low-N soils, MBC responds to soil properties (initial SOC and soil depth) and experimental duration, while MNC increases are mainly driven by biochar properties (biochar application rate and total C content) and experimental duration. Overall, initial soil N level dictates distinct C sequestration pathways—high-N soils favor physical protection, while low-N soils rely on microbial-mineral association. This N-dependent mechanism enables precision biochar management for climate-smart agriculture.

Graphical Abstract

Keywords

Biochar / Soil N availability / Microbial biomass C / Microbial necromass C / Precision C management

Highlight

•	The global meta-analysis revealing that initial soil N level mediates the pathways of biochar-driven microbial C sequestration.
•	Biochar increased microbial-derived C pools to a greater extent in low-N soils than in high-N soils.
•	The physical protection dominates in high-N soils, while microbial-mineral association prevails in low-N soils.

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Funding

National Key R&D Program of China(2023YFD1900600)

the Science and Technology Major Project of Ordos(ZD20232320)

the Hebei Province Full-time Introduction of National High-Level Innovative Talents Program(2024HBQZYCXY037)

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