Changes in the carbon and nutrient status of Cryptomeria japonica needles and fine roots following 7 years of nitrogen addition

Abstract

Anthropogenically increased nitrogen (N) deposition may affect the nutrient dynamics of forested ecosystems. To investigate the potential effects of excessive N deposition on Japanese forests, we treated the soil in a 20-year-old Japanese cedar (Cryptomeria japonica) stand with 10 l m^-2 of 10 mM HNO₃ solution, 10 mM NH₄NO₃ solution, or tap water (as a control), monthly for 7 years. A total of 168 and 336 kg N ha^-1 year^-1 was added in the HNO₃ and NH₄NO₃ plots, respectively. Tree growth, the amount of nutrients and the carbon concentration of both current shoots and fine roots (<2 mm in diameter) in the surface soil (0-5 cm) were measured. The foliar N concentration increased in both N-fertilized plots during the first 3 years, particularly in the NH₄NO₃ plots. Similarly, the fine-root N concentration was greater in the N-fertilized plots than in the control plots. However, growth in both height and diameter at breast height of Japanese cedar trees were not significantly affected by N fertilization. The foliar K and P concentrations tended to decrease in treatment plots over time when compared with the control plots. Our study suggests that 7 years of excessive N fertilization had no positive or negative effect on the growth of young Japanese cedar trees, although the nutrient status of current shoots and fine roots was altered.