Biochar-induced reduction of N2O emission from East Asian soils under aerobic conditions: Review and data analysis

Global meta-analyses showed that biochar application can reduce N2O emission. However, no relevant review study is available for East Asian countries which are responsible for 70% of gaseous N losses from croplands globally. This review analyzed data of the biochar-induced N2O mitigation affected by experimental conditions, including experimental types, biochar types and application rates, soil properties, and chemical forms and application rates of N fertilizer for East Asian countries. The magnitude of biochar-induced N2O mitigation was evaluated by calculating N2O reduction index (Rindex, percentage reduction of N2O by biochar relative to control). The Rindex was further standardized against biochar application rate by calculating Rindex per unit of biochar application rate (ton ha−1) (Unit Rindex). The Rindex averaged across different experimental types (n = 196) was −21.1 ± 2.4%. Incubation and pot experiments showed greater Rindex than column and field experiments due to higher biochar application rate and shorter experiment duration. Feedstock type and pyrolysis temperature also affected Rindex; either bamboo feedstock or pyrolysis at > 400 °C resulted in a greater Rindex. The magnitude of Rindex also increased with increasing biochar rate. Soil properties did not affect Rindex when evaluated across all experimental types, but there was an indication that biochar decreased N2O emission more at a lower soil moisture level in field experiments. The magnitude of Rindex increased with increasing N fertilizer rate up to 500–600 kg N ha−1, but it decreased thereafter. The Unit Rindex averaged across experimental types was −1.2 ± 0.9%, and it was rarely affected by experimental type and conditions but diminished with increasing biochar rate. Our results highlight that since N2O mitigation by biochar is affected by biochar application rate, Rindex needs to be carefully evaluated by standardizing against biochar application rate to suggest the best conditions for biochar usage in East Asia.