Camellia oleifera Litter Interacts with Nitrogen and Biochar to Modulate N2O and CO2 Emissions: A Biphasic Acidification Mechanism

Excessive N application in Camellia oleifera plantations exacerbates soil acidification and N2O emissions, intensified by the input of Al-accumulating litter. Biochar is a promising amendment, yet how litter decomposition interacts with N and biochar to modulate acidification and greenhouse gas emissions remains unclear. To understand how decomposition of Al-accumulating litter interacts with N and biochar in the soil acidification process and gas emissions, a twelve-month laboratory incubation study was conducted using a fully factorial, three-factor completely randomized design to examine litter decomposition. The experimental factors included nitrogen fertilization, biochar amendment, and litter input level. The results showed that litter transiently activated biochar alkalinity, raising pH to 5.7–6.3, but subsequent organic acid release drove sustained re-acidification (ΔpH −0.4 to −0.5). This pH trajectory controlled denitrification: early high pH favored complete denitrification (nosZ > nirK), while later acidification inhibited N2O reductase, boosting N2O emissions under single litter and N. Litter-C primed native soil organic carbon, doubling cumulative CO2 emissions. Biochar further elevated CO2 emission rate by 7.6% under double litter input treatment via porous-microsite priming. These results demonstrated that litter quantity dictates a temporal switch from biochar alkali activation to organic acid overrun, creating an acid rebound that amplifies N2O while sustaining CO2 release. Optimizing litter retention and biochar application timing is essential to break the acid-N2O feedback in intensively managed C. oleifera systems.