Soil N2O Emission and the N-Cycling Microbial Communities Are Mediated By the Easily Mineralizable C of Pyrogenic C Under Contrasting Moisture Status and N Sources Applications.

N₂O is a potent greenhouse gas and produced as a by-product of soil nitrification and denitrification processes. Most studies have reported the positive role of pyrogenic carbon (PyC) in decreasing soil N₂O emission reduction, while others have shown the opposite results. These conflicting results are probably attributed to PyCs heterogeneity. Thus, it is important to determine the key mechanisms whereby PyCs impact N₂O emission and related nitrifier and denitrifier communities. Here, two PyCs, i.e. aliphatic C dominated PyC (Py300) and aromatic C dominated PyC (Py700) were produced at the pyrolysis temperatures of 300 and 700 ^oC, respectively. The dissolved organic carbon (DOC300) and acetone extractable C (AeC300) extracted from Py300 were added to Py700, resulting in Py700+DOC300 and Py700+AeC300, respectively. We added four PyCs to a red soil under contrasting moisture status (i.e. 60% and 130% field capacity) and with the application of urine or sodium nitrate. Denitrification was the dominant process for N₂O emission rather than nitrification. PyCs with easily mineralizable carbon (EMC) (i.e. Py300, Py700+AeC300 and Py300+DOC300) significantly increased N₂O emission, particularly in the treatment with high soil moisture (130% FC) and sodium nitrate application. However, the PyC with little easily mineralizable carbon (i.e. Py700) had no effects. The effects of PyCs on N₂O emission increase were: Py300 > Py700+AeC300 > Py300+DOC300 > Py700. In addition, PyCs with EMC significantly increased the abundance of AOB amoA, nirK, nirS and nosZ, and had no effects on AOA amoA abundance, while Py700 did not significantly affect any nitrifier/denitrifier genes. The aromatic structure and quinonyl of Py700 can act as electron acceptors and compete with NO₃^- for electrons under anaerobic condition, inhibiting N₂O emission. However, the PyCs with abundant microbial EMC can provide the electrons for NO₃^- reduction during denitrifier metabolism and thus increase N₂O emission.