Nitrogen-Induced Changes in Soil Environmental Factors Are More Important Than Nitrification and Denitrification Gene Abundance in Regulating N(2)O Emissions in Subtropical Forest Soils

Subtropical regions are currently experiencing a dramatic increase in nitrogen (N) deposition; however, the contributions of nitrification and denitrification processes to soil N(2)O emissions and the underlying mechanisms under increasing N deposition remain unclear. Therefore, a (15)N-tracing laboratory experiment with four N application rates (0, 12.5, 25, and 50 μg (15)N g(–1) soil) was conducted to investigate the response of nitrification- and denitrification-derived N(2)O to N additions in an evergreen broad-leaved forest (BF) and a Pinus forest (PF) in the Wuyi Mountains in southeastern China. Moreover, the abundance of functional genes related to nitrification (amoA), denitrification (nirK, nirS, and nosZ), and soil properties were measured to clarify the underlying mechanisms. Results showed that nitrification-derived N(2)O emissions were generally decreased with increasing N input. However, denitrification-derived N(2)O emissions were a non-linear response to N additions, with maximum N(2)O emissions at the middle N application rate. Denitrification-derived N(2)O was the dominant pathway of N(2)O production, accounting for 64 to 100% of the total N(2)O fluxes. Soil NH(4)(+)-N content and pH were the predominant factors in regulating nitrification-derived N(2)O emissions in BF and PF, respectively. Soil pH and the nirS abundance contributed the most to the variations of denitrification-derived N(2)O emissions in BF and PF, respectively. Our results suggest that N application has the potential to increase the contribution of denitrification to N(2)O production in subtropical forest soils. Changes in soil chemical properties induced by N addition are more important than the abundance of nitrification and denitrification functional genes in regulating soil nitrification- and denitrification-derived N(2)O emissions.