Meta-analysis on farmland soil N₂O emissions under biochar application in China

In this study, a Meta-analysis was conducted to make clear the effects of biochar application on soil N₂O emissions and the main controlling factors in China. Based on observation data from published papers in the public domain, the effects of biochar application on soil N₂O emissions were quantitatively analyzed under different climatic conditions, soil properties, field management practices and biochar properties. Then path analysis was used to determine the dominant factors influencing soil N₂O emissions. The results showed that biochar application significantly decreased soil N₂O emissions (P < 0.05) under annual mean precipitation more than 600 mm, and the reduction effect was strengthened with increasing annual mean precipitation. Also when annual mean sunshine hours was greater than 1 000 h, the N₂O emission-reduction effect of biochar application decreased in trend following an initial increase with increasing annual mean sunshine hours. With increasing soil pH above 6.5, soil N₂O emission reduction first increased and then decreased under biochar application. Biochar application significantly decreased soil N₂O emissions in loam soils (P < 0.05), whereas the effects in sandy and clay soils were not significant (P>0.05). Moreover, the reduction in soil N₂O emissions under film mulching was higher than that without film mulching. The effect of biochar on soil emission decreased with increasing nitrogen application rate, but increased with increasing specific surface area of biochar. When C:N ratio of biochar was 30-500, the effects of biochar application on soil N₂O emissions was significant at P < 0.05. When the amount of biochar was 20-160 t·hm^-2, soil N₂O emissions significantly decreased with increasing biochar application rate. Furthermore, significant differences in N₂O emissions were observed for different regions under biochar application. For example, there was a significant reduction in N₂O in South, East, Central and Northeast regions of China (P < 0.05), except Northwest China (P>0.05). Generally, N fertilizer input, biochar application amount, annual mean temperature and annual mean precipitation were the dominant factors affecting the N₂O emission-mitigation effects of biochar, which was controlled by the interactions among these factors. The study provided a reference base for the promotion of biochar technology in farming in China and emission reduction of soil N₂O.