Effects of simulated N deposition on foliar nutrient status, N metabolism and photosynthetic capacity of three dominant understory plant species in a mature tropical forest.

Anthropogenic increase of nitrogen (N) deposition has threatened forest ecosystem health at both regional and global scales. In N-limited ecosystems, atmospheric N input is regarded as an important nutrient source for plant growth. However, it remains an open question on how elevated N deposition affects plant growth in N-rich forest ecosystems. To address this question, we used a simulated N deposition experiment in an N-rich mature tropical forest of southern China, with N addition levels as 0kgNhayr (Control), 50kgNhayr (Low-N), 100kgNhayr (Middle-N) and 150kgNhayr (High-N), respectively. We measured foliar nutrient element status (e.g., N, P, K, Ca and Mg), N metabolism and photosynthesis capacity of three dominant understory plant species (Cryptocarya concinna and Cryptocarya chinensis as medium-light species; and Randia canthioides as shade tolerant species) in this forest. Results showed that two years of N addition greatly increased foliar N content, but decreased the content of nutrient cations (e.g., K, Ca and Mg). Nitrogen addition also increased N accumulation as organic forms as soluble protein and/or free amino acid (FAA), but not as chlorophyll in all three species. We further found that the photosynthesis capacity (Pmax) of C. concinna and C. chinensis decreased significantly with elevated N addition, with no effects on R. canthioides. However, photosynthetic nitrogen use efficiency (PNUE) significantly declined with N addition for all three species, with significantly negative relationships between PNUE/Pmax and foliar N content. These findings suggest that excess N inputs can accelerate nutrient imbalance, and inhibit photosynthetic capacity of understory plant species, indicating continuous high N deposition can threat understory plant growth in N-rich tropical forests in the future. Meanwhile, PNUE can be used as a sensitive indicator to assess ecosystem N status under chronic N deposition.