Impact of biochar on soil N 2 O emissions under different biochar-carbon/fertilizer-nitrogen ratios at a constant moisture condition on a silt loam soil.

Biochar amendment has been proposed as a potential solution for improving soil quality and suppressing greenhouse gas emission. Considering the serious nitrogen fertilizer overuse problem in China, it is important to investigate the effect of biochar on soil with excess nitrogen fertilizer. Therefore, two sets of soil column experiments were conducted to explore the effect of biochar on N2 O emission from nitrogen fertilizer-overused soil. Three types of biochar (biochars pyrolzed at 300, 500 and 700°C, respectively) and one type of nitrogen fertilizer (ammonium sulfate) were investigated at varying application rates. It was found that N2 O emission was related to both biochar and N-fertilizer application rates, and increased N2 O emission was negatively correlated with the TC/IN ratio (the ratio of total carbon to inorganic nitrogen) after biochar application. The soil TC/IN ratio determined the ammonium utilization pathway, affecting the intensity of nitrification and N2 O emission. When the TC/IN ratio was relatively high (>60), suppressed nitrification led to the suppression of N2 O emission. Conversely, enhanced nitrification when the TC/IN ratio was relatively low (<45) caused the promotion of N2 O emission. In conclusion, biochar's suppression of soil N2 O emission was conditional and biochar should be applied in a proper ratio to nitrogen fertilizer to avoid excessive N2 O emission.