The effects of biochar (BC) on methane (CH 4) emission may vary with nitrogen (N) fertilization rate, organic inputs, and water management. In this short review, we studied the changes in the direction and magnitude of CH 4 emission by BC under these different conditions. Literature data indicated that co-application of BC with N fertilizer may increase or decrease CH 4 emission depending on the N fertilization rates. Although the threshold N fertilization rate for the enhanced CH 4 emission by BC is unclear, it was suggested that N fertilization may enhance BC-induced CH 4 oxidation via the supply of N to methanotrophs. Although organic inputs stimulate CH 4 emission, co-application of BC with organic inputs efficiently decreases CH 4 emission by reducing C substrate availability for methanogens via sorption onto BC particles. The application of BC also enhances CH 4 oxidation under water-saving irrigation to stimulate the methanotroph activity under aerobic conditions. Our study suggests that application of BC may be a measure to mitigate CH 4 emission from paddy soils amended with organic inputs and it may further enhance CH 4 oxidation under water-saving irrigation.

This study investigated the benefits and potential risks of biochar application using literature data on CH 4 emission from paddy fields (n = 107) and N 2 O from upland fields (n = 174). Literature data indicated that biochar application decreases a substantial amount of CH 4 (by 14.4%) from paddy and N 2 O (by 22.9%) from upland fields. However, the regression analysis between biochar application and greenhouse gas (GHG) mitigation was not significant, suggesting that the performance of biochar on GHG reduction is not additive due to the diminished capacity of unit-weight biochar to mitigate GHG with increased biochar application rate. Besides, it is also warned that heavy application of biochar may cause soil and food contamination and ecological toxicity because of toxic compounds such as polycyclic aromatic hydrocarbon and heavy metals contained in biochar. An optimal biochar application rate by considering these potential risks of biochar needs to be established to take full advantage of biochar to mitigate GHG and enhance soil carbon sequestration.

Correlations of electrical conductivity (EC) and sodium adsorption