Rice Rusk-Derived Biochar Suppressed N 2 O Emission from Acidic Arable Soil by Inhibiting Nitrate Reduction

The study aims to explore the potential mechanisms of rice husk-derived biochar’s effect on acidic arable soil N2O emission during denitrification. The microcosm experiment with four biochar addition rates (0%, 2%, 5%, and 10%, w/w) was carried out for 17 days consecutively under the condition of facilitating denitrification. N2O emission rate and the changes of soil pH, electrical conductivity (EC), inorganic nitrogen, and dissolved organic carbon (DOC) contents during incubation were explored. Moreover, N2O and N2 distribution pattern, as the end products of denitrification, was determined by acetylene inhibition method, and the effect of biochar on soil organic carbon mineralization was also studied by trapping CO2 in NaOH solution. The cumulative N2O emission of 0%, 2%, 5%, and 10% treatments was 100.2 ± 2.13, 40.7 ± 5.00, 4.25 ± 0.10, and 0.03 ± 0.002 µg N2O-N g−1 dry soil, respectively. The biochar significantly inhibited N2O emission from the N-fertilized acidic arable soil. The changes of soil pH, EC, inorganic nitrogen, and DOC contents and the distribution pattern of N2O and N2 indicated that the potential mechanism of biochar inhibiting N2O emission was that NO3− reduction was blocked by biochar during denitrification. Furtherly, the study showed that the biochar significantly promoted DOC mineralization of the acidic arable soil, and the effect was enhanced with biochar addition rate increasing from 2 to 10%. It was biochar’s promotion effect on DOC mineralization that resulted in C limitation during denitrification and then prevented NO3− reduction and suppressed N2O emission from the acidic arable soil.