Tea plantation affects soil nitrogen transformations in subtropical China

This study proposed to investigate soil N transformation rates under different tea plantation ages. These plantations receive a great deal of nitrogen fertilizer each year. It was hypothesized that the inhibition of nitrification by increasing soil acidification would be counteracted by the N application. We aimed to explore whether this relationship would influence soil nitrogen transformations within tea plantations. Tea fields with different establishment periods (8 years, 36 years, and 105 years) and adjacent forest soils were collected from Hangzhou city, China. The 15N dilution technique and a 15N tracing model were used to investigate changes in soil gross N transformation rates, additionally combining the 15N dilution technique with acetylene inhibition to distinguish heterotrophic and autotrophic nitrification rates. Both 15N labeling methods demonstrated that land use conversion and fertilization significantly change the soil N transformation rates. The gross N mineralization rate was much higher in the tea soils compared with that in the adjacent forest soil and increased with planting age in the tea soils. Interestingly, appreciable NO3− production was detected in the forest soil caused by heterotrophic nitrification, whereas only a small amount of nitrate was formed by heterotrophs in the tea soils. Gross nitrification rates increased with the planting age from 8 to 36 years but decreased at 105 years. Nitrification was the main ammonium consuming process in the tea soils and a positive relationship was observed between ammonia oxidizing archaea (AOA) amoA abundance and nitrification rate, suggesting AOA as the dominant nitrification drivers in these tea soils. Overall, the conversion from forest to tea soil enhanced the gross rate of nitrification, N mineralization, and NH4+ immobilization, but N/I (nitrification/ammonium immobilization) also increased significantly in the tea plantations, indicating a high nitrate leaching and runoff risk.