Separated pathways for biochar to affect soil N2O emission under different moisture contents

Highlights: • Effect of biochar on soil N₂O emission was moisture-content-dependent. • B300 promoted N₂O emission when moisture content was low but reduced when high. • Biochar promoted N₂O emission via increasing the concentration of DOC. • Biochar reduced N₂O emission by increasing the abundance ratio of nosZ to nirS. Dry land is a massive contributor to global nitrous oxide (N₂O) production and biochar is a potential material for soil amendment that can impact soil N₂O emission. Considering that the moisture content of dry land is usually changeable, it is essential to investigate the effect of biochar on soil N₂O emission under different moisture contents. Therefore, column experiments were conducted with two biochars (B300 and B500, biochars pyrolyzed at 300 and 500 °C, respectively) under five moisture contents (18%, 21%, 24%, 27% and 30%, w/w). The results showed that B300 promoted N₂O emission under the moisture contents of 18%, 21% and 24% by increasing the content of dissolved organic carbon and thus enhancing the microbial processes related to N₂O production. However, when the moisture contents were 27% and 30%, the promotion of N₂O production was overwhelmed by the improvement in N₂O reduction due to the B300 induced increase in the abundance ratio of nosZ to nirS, leading to the decrease in N₂O emission. Moreover, B500 did not alter the content of dissolved organic matter significantly and thus caused no significant change in N₂O emission when the moisture contents were 18%, 21% and 24%. But it was able to increase the abundance ratio of nosZ to nirS and thus decrease N₂O emission when the moisture contents were 27% and 30%. The results further clarified the effect of biochar on soil N₂O emission and helped to evaluate the N₂O-suppressing-potential of biochar.