N2O Emission From a Subtropical Forest Is Dominantly Regulated by Soil Denitrifiers Under Exogenous N Enrichment and Seasonal Precipitation Distribution Change

Abstract

Nitrogen-rich tropical/subtropical forest soil acts as a terrestrial source of nitrous oxide (N₂O), a greenhouse gas commonly affected by soil nitrogen availability and soil moisture. However, in tropical and subtropical regions experiencing both elevated nitrogen deposition and altered precipitation regimes, it is unclear whether nitrogen deposition and precipitation regimes have interactive effects on forest soil N₂O emissions and what roles N₂O-associated nitrifiers/denitrifiers play in these interactions. We conducted a 2 year field study in a subtropical evergreen broadleaf forest in southern China by applying four treatments: nitrogen addition (N), seasonal precipitation distribution change (PC), both nitrogen addition and seasonal precipitation distribution change (NPC) and a control (C). We found that N₂O efflux from the forest soil was significantly greater in the wet season than in the dry season, but was promoted by 77.4% by the NPC treatment only in the dry season. Soil moisture and pH decreased in the PC and N treatments, respectively. The abundance of nitrifying gene AOA-amoA and denitrifying gene nosZ in the wet season and the abundance of denitrifying gene nirK in the dry season differed significantly among the four treatments. A structural equation model showed that precipitation change was more important than nitrogen addition in affecting soil properties (e.g., moisture and pH) and N₂O-associated nitrifiers/denitrifiers, while soil nirK- and nosZ-denitrifiers were the dominant functional microbes in regulating N₂O emissions. The results support predictions of future nitrogen losses (N₂O) in subtropical forests in the context of interactions between elevated nitrogen deposition and altered precipitation regimes.