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.