Effects of simulated nitrogen deposition on nutrient balance of Chinese fir (Cunninghamia lanceolata) seedlings

Human activities such as fossil fuel combustion, production and use of chemical fertilizers, and livestock ranching have dramatically altered the global nitrogen cycle. A potential concern of chronic anthropogenic N emitted to atmosphere is altered nutrient cycling in forest ecosystems. Most studies reporting effects of N additions on forest ecosystems have been primarily carried out in temperate regions of Europe and North America. Importantly, however, there is a strong need to understand how nitrogen interacts with forests in the tropical and subtropical regions because rates of nitrogen deposition in these areas are projected to increase rapidly in upcoming decades. To assess the impact of simulated nitrogen deposition on plant nutrient balance, one-year-old Chinese fir seedlings (Cunninghamia lanceolata (Lamb.) Hook.) from a nursery were transplanted at the beginning of March 2007 before budbreak to plastic pots containing 17.5 kg of forest soil (red earth). The seedlings were kept in natural daylight in a greenhouse and irrigated by hand with distilled water before treatment. The experiment started in April 2007 and lasted for 12 months. Ammonium nitrate solutions were sprayed on the seedlings every three days at four doses, N1 (6 g N m super(-2) times a super(-1)), N2 (12 g N m super(-2) times a super(-1)), N3 (24 g N m super(-2) times a super(-1)) and N4 (g N m super(-2) times a super(-1)), with N0 (0 g N m super(-2) times a super(-1)) as the control and six replicates in each treatment. Totally 30 Chinese fir seedlings were available for destructive sampling to monitor biomass and nutrient concentration at the end of experiment. The amount of water solution sprayed to the seedlings each month was equivalent to the average monthly precipitation recorded over the last 30 years. The results indicated that nitrogen addition increased N, K and Mg contents in leaves, shoots (including branches and stems), coarse roots and fine roots of the seedlings, but reduced Ca concentration. Phosphorus contents in the seedlings responded positively to low-to-medium N treatments (N1, N2), but negatively to high N treatments (N3, N4). Nitrogen loads generally resulted in increased ratios of N relative to other elements in the seedlings, but decreased N/K and N/Mg in coarse roots. Linear regression correlations between nitrogen contents in leaves or fine roots (independent variables) and other element contents in leaves or fine roots (dependent variables) were develop