Long-term manure amendment reduces nitrous oxide emissions through decreasing the abundance ratio of amoA and nosZ genes in an Ultisol

Organic fertilizers are frequently applied to soils to improve soil multifunctionality and maintain crop produc-tivity in agricultural practices. However, how organic fertilizations influence soil nitrous oxide (N2O) emissions remains ambiguous, especially considering their linkage with the abundance of nitrogen (N)-cycling microor-ganisms. Here, we conducted an incubation experiment to examine the effect of partial replacement of mineral fertilizers with organic fertilizers on the abundance of N-cycling functional genes and N2O emissions at 60 % and 100 % water-filled pore space (WFPS) in an acidic Ultisol. Soil samples were collected from 32-year fertilization trials, including four treatments of control (no fertilizer), mineral NPK fertilizer (NPK), NPK + Peanut Straw (NPKS), and NPK + Pig Manure (NPKM). The N2O emissions from soils incubated at 100 % WFPS were signif-icantly higher than those at 60 % WFPS in all treatments. Compared with NPK, NPKS and NPKM reduced N2O emissions, with a more pronounced effect in NPKM. The N2O flux was positively associated with the abundance of AOA and AOB amoA genes, and the abundance ratio of amoA/nosZ (i.e., the ratio of AOA amoA plus AOB amoA and nosZ CYRILLIC CAPITAL LETTER BYELORUSSIAN-UKRAINIAN I plus nosZ CYRILLIC CAPITAL LETTER BYELORUSSIAN-UKRAINIAN ICYRILLIC CAPITAL LETTER BYELORUSSIAN-UKRAINIAN I) when the peak N2O flux appeared. Random forest analysis showed that the abundance ratio of amoA/nosZ was the best predictor for N2O emissions. Compared with NPK, NPKM increased soil pH but reduced the abundance ratio of amoA/nosZ, which might be the key factor in reducing N2O emissions. Together, our findings suggested that long-term manure application reduced N2O emissions by decreasing the abundance ratio of amoA/nosZ and provided novel insights into the impact of organic fertilizers on N2O emissions in acidic agricultural soils.