Abstract

Biochar, as an effective strategy to enhance soil carbon storage and reduce atmospheric carbon dioxide emissions, is increasingly recognized as a key approach to addressing climate change and optimizing soil carbon cycling (SCC). Previous reviews have examined the effects of biochar on greenhouse gas emissions, but gaps remain in understanding its influence on SCC. This study aims to fill this gap by analyzing the responses of three major SCC processes—total carbon dioxide emissions, total microbial respiration, and carbon storage—to biochar application.
We conducted a global meta-analysis incorporating 75 studies and 250 observations. The results showed that biochar application increased total soil carbon dioxide emissions by an average of 11%, though this was not statistically significant within the confidence interval (CI [0%, 23%]). Total microbial respiration also showed no significant change (change rate: 10%, CI [−2%, 23%]). However, biochar application significantly enhanced soil carbon storage by 61% (CI [36%, 90%]).
This study further explored the key factors affecting SCC, including experimental design, geographic location, biochar application rate, feedstock type, and pyrolysis temperature. Under laboratory incubation conditions, biochar had a positive impact on all SCC processes. Geographically, biochar application showed great potential for increasing soil carbon storage in the Middle East, Europe, and Asia. Increasing biochar application rates promoted both carbon storage and total microbial respiration. Regarding feedstock types, manure-derived biochar significantly increased microbial respiration, while woody biochar mainly affected total carbon dioxide emissions. Notably, lower pyrolysis temperatures showed greater potential for improving carbon storage and microbial respiration.
In summary, while biochar holds promise for carbon sequestration, its effects on total microbial respiration and total carbon dioxide emissions remain inconclusive.

Highlights
1.This meta-analysis reveals the effects of biochar on three key soil carbon cycling processes: carbon storage, total carbon dioxide flux, and total microbial respiration.
2.Biochar application can significantly enhance soil carbon storage.
3.The effects of biochar on total microbial respiration and total carbon dioxide flux are not significant.

Conclusions
1.Biochar significantly increases soil carbon storage, providing an effective strategy for environmental pollution control and climate change mitigation.
2.Although biochar has a notable positive effect on carbon storage, its impacts on total microbial respiration and total carbon dioxide flux are not obvious; the mechanisms and effects require further in-depth study.
3.Factors such as experimental design, geographic region, and biochar application rate significantly influence biochar’s effects on soil carbon cycling.
4.Increasing the application rate of biochar can enhance carbon storage, while manure-derived biochar positively promotes soil microbial respiration.
5.This study highlights the complexity of biochar’s influence on soil carbon cycling and points to future research directions, emphasizing the need for more comprehensive studies to fully reveal biochar’s potential applications and its specific environmental impacts.