Soil nitrification and denitrification in plantations with different stand ages.

The variations in soil nitrification and denitrification processes, together with the abundances of functional microbes were investigated in plantations with different stand ages of 5, 8, 21, 27, and 40 years old. The results showed that the net nitrification rate fluctuated with increasing forest ages, with that of 8-year- and 27-year-old plantations being significantly lower than other stand ages. The abundance of ammonia-oxidizing archaea (AOA) in the 27-year-old plantation was significantly lower than that of the 40-year-old plantation, while there was no significant difference among the other stand ages. There was no significant difference in the abundance of AOB gene, denitrifying functional genes or soil denitrification potential among different stand ages. The results of stepwise regression analysis showed that the abundance of AOA gene was not significantly affected by soil physical and chemical properties. In addition, the abundance of AOB was positively associated with soil total carbon content and soil pH. The abundance of denitrifying functional genes including , and increased with increasing soil pH. The abundance of and was influenced by soil total carbon. Stand age influenced soil net nitrification rate through the AOA abundance. Moreover, soil denitrification potential was directly affected by stand age, or indirectly affected by stand age through soil microbial biomass carbon, soil pH and denitrifying gene abundance of and . Compared with the denitrification process, soil nitrification and associated AOA gene abundance were more sensitive to the development of plantations. The rotation period sould be appropriately extended to reduce the risk of nitrogen losses resulting from soil nitrification.