The mechanisms governing low denitrification capacity and high nitrogen oxide gas emissions in subtropical forest soils in China

Previous studies have demonstrated that denitrification rates are low in subtropical forest soils. However, the mechanisms governing this process are not well known. This study seeks to identify the mechanisms responsible for the low denitrification capacity and high nitrogen oxide gas ratio in subtropical forest soils in China. The denitrification capacity and nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2) emission rates were measured using the acetylene inhibition method under conditions of added nitrate and anoxia. The abundance of nitrate reductase (narG), nitrite reductase (nirK), nitric oxide reductase (cnorB), and nitrous oxide reductase (nosZ) was measured using real-time, quantitative polymerase chain reaction, and sequencing of the nirK and norB products was performed to analyze the population structure of denitrifying bacteria. These results showed that the denitrification capacity in subtropical forest soils was lower than in temperate forest soils (p < 0.05). Multiple regression analysis showed that redox potential at the start of incubation (Ehi), rather than soil pH or soil organic C, was the key soil variable influencing denitrification, and Ehi alone could explain 68% of the variations in denitrification capacity. The high Ehi in subtropical soils led to a low abundance of nirK and significant differences in the population structure of denitrifying bacteria between subtropical and temperate soils. Therefore, Ehi was responsible for the low denitrification capacity in subtropical forest soils. The ratio of NO to total denitrification gas products (p < 0.01) and the ratio of NO and N2O to total denitrification gas products (p < 0.05) were significantly higher in subtropical forest soils than in temperate forest soils, while the reverse trend was observed for the ratio of N2 to total denitrification gas products (p < 0.05). A high Ehi reduced the specific reduction activity of each nosZ copy and, in turn, resulted in a large ratio of NO and N2O to total denitrification gas products in subtropical soils. Thus, NO and N2O, but not N2, were the dominant denitrification gas products, accounting for 80%, even under the highly anaerobic conditions in subtropical forest soils and despite low denitrification capacity. These results were significant for understanding the “Hole in the Pipe” model and NO and N2O gases emission in subtropical forest soils. Despite the fact that the nitrogen flowing through the pipe (denitrification capacity) was low, the large holes in the pipe resulted in a large quantity of NO and N2O gases leaking out. This leakage may be a potential mechanism for the high levels of NO and N2O gas emission in subtropical forest soils and could partly explain why NO and N2O emissions are generally high in subtropical and tropical soils.