ALBERT

Description: 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 (N 2 O), and dinitrogen (N 2 ) 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 ) were measured using real-time, quantitative PCR, 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 (Eh i ), rather than soil pH or soil organic C, was the key soil variable influencing denitrification, and Eh i alone could explain 68% of the variations in denitrification capacity. The high Eh i in subtropical soils led to a low abundance of nir K and significant differences in the population structure of denitrifying bacteria between subtropical and temperate soils. Therefore, Eh i 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 N 2 O 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 N 2 to total denitrification gas products (p 〈 0.05). A high Eh i reduced the specific reduction activity of each nosZ copy, and, in turn, resulted in a large ratio of NO and N 2 O to total denitrification gas products in subtropical soils. Thus, NO and N 2 O, but not N 2 , 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 N 2 O 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 N 2 O gases leaking out. This leakage may be a potential mechanism for the high levels of NO and N 2 O gase emission in subtropical forest soils and could partly explain why NO and N 2 O emissions are generally high in subtropical and tropical soils.