Continuous biochar application results in higher greenhouse gas emissions than a single biochar application in an upland agroecosystem.

Applying biochar to soil enhances carbon sequestration and mitigates greenhouse gas (GHG) emissions. However, the impact of biochar application frequency on soil GHG emissions is unclear. In the present study, a field experiment was performed to investigate the characteristics of GHG emissions and their influencing factors in upland yellow soil after three years application of biochar (single and continuous application). The results indicated that biochar application significantly increased soil organic carbon, pH, electrical conductivity (EC), and water content and significantly decreased soil bulk density. Soil microbial biomass carbon (MBC) and nitrogen initially increased but then decreased with increasing biochar application rates. The soil pH, EC, and MBC were significantly higher with continuous biochar application over three years than with a single application. Biochar application at higher rates notably enhanced cumulative soil CH4 uptake while significantly increasing cumulative CO2 emissions. Structural equation modelling indicated that a single biochar application had an overall inhibitory effect on soil CO2 and N2O emissions, while promoted CH4 uptake. In contrast, continuous biochar application over three years suppressed N2O emissions and promoted CH4 uptake, while promoted CO2 emissions. Continuous biochar application over three years significantly increased the soil GWP value by 27.8 % compared to single biochar application. These results indicate that biochar application in upland agricultural ecosystems can reduce soil GHG emissions by modulating soil physical, chemical, and biological properties. In conclusion, compared to continuous application over three years, a single application effectively reduces GHG emissions.