Tea plantation affects soil nitrogen transformations in subtropical China

PURPOSE: 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. MATERIALS AND METHODS: 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. RESULTS AND DISCUSSION: 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. CONCLUSION: 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.