Biochar reduces N2O emission from fertilized cropland soils: a meta-analysis

Nitrous oxide (N 2 O) emissions from soil are an important contributor to global warming, particularly from intensively fertilized croplands. Biochar is commonly applied to reduce N 2 O emissions and raise soil fertility by regulating soil structure, microbial processes, and crop nitrogen use efficiency. However, the effects of biochar on N 2 O emissions from fertilized croplands depend on its sources and production conditions, including feedstocks, pyrolysis temperatures, properties and application rates. To generalize findings from individual studies, we synthesized 550 observations that simultaneously measured N 2 O emissions, nitrification enzyme activity (NEA), denitrification enzyme activity (DEA), and relevant functional genes ( AOA , AOB , narG , nirK , nirS, and nosZ ) to assess their responses to biochar production conditions, properties and application rates across cropland ecosystems. Wheat straw biochar increased the abundances of all functional genes related to N 2 O emissions and DEA, while pyrolysis temperatures exceeding 450 ℃ decreased DEA. Low-temperature pyrolysis biochar was particularly effective in reducing N 2 O emissions. The abundance of denitrifiers and DEA-related genes increased with the pH, ash content, and cation exchange capacity (CEC) of biochar. As biochar application rates increased, N 2 O emissions were reduced, largely due to an increase in nosZ gene abundance and soil pH. A common biochar application rate of 20 t ha −1 decreased N 2 O emissions by 19%, primarily through reduced denitrification, while 50 t ha −1 reduced N 2 O emissions by 48%. Biochar preparation conditions, and property changes had no significant effects on N 2 O emissions at application rates below 20 t ha −1 . When application rates exceeded 20 t ha −1 , biochar pyrolysis temperature and properties influenced N 2 O emissions, indicating a threshold application rate, beyond which biochar affects N 2 O emissions. Biochar regulates the soil N cycle and N 2 O emissions primarily through denitrification, with effects dependent on the biochar application rate. These findings underscore the crucial potential of increased biochar application to reduce N 2 O emissions from fertilized soils globally, thereby contributing to climate change mitigation. Graphical Abstract