How Stand Age Affects Soil Nitrification and Nitrogen Gas Emissions in Tropical and Subtropical Tea Plantations

Tea plants prefer NH4+-N to NO3−-N, and thus nitrification would be detrimental to the N uptake of tea. However, the effects of different stand ages on nitrification and nitrogen oxide (NO and N2O) emissions in tropical and subtropical regions remain unclear. We performed an incubation experiment with tea field soils from different stand ages (5, 15, and 30 years) under different water contents in subtropical (Changsha, Hunan; C5L, C15L, C30L, C5H, C15H, C30H) and tropical regions (Baisha, Hainan; B5L, B15L, B30L, B5H, B15H, B30H). The results showed that the highest net nitrification rate was in C15L and B15. The results indicated that there was more NO3−-N loss in the 15-y tea field soil in both regions. The highest nitrogen oxide emissions from the subtropical and tropical plots were in C15H and B30H. Available K was the key variable for NO and N2O emissions in Changsha county, whereas SOM, pH, and available P were the key factors affecting NO and N2O emissions in Baisha county. Our findings suggest that more attention should be paid to NO3−-N loss in middle-aged (10–30 years) tea fields. Similarly, the focus should be given to nitrogen oxide emissions from middle-aged tea plantations in subtropical regions and old tea plantations (≥30 stand years) in tropical regions.