Response of photosynthetic physiological characteristics to nitrogen addition by seedlings of two dominant tree species in a broadleaved-Korean pine mixed forest on Changbai Mountain
Photosynthesis is an important factor when evaluating the carbon cycle under global climate change scenario. Available nitrogen is a key nutritional factor for plant growth,which affects plant photosynthesis in case of high nitrogen deposition. The ecophysiology parameters of two dominant species (Pinus koraiensis and Tilia amurensis) were studied in a broadleaved-Korean pine mixed forest on Changbai Mountain through nitrogen deposition experiments. The nitrogen input levels were 0,23,46,and 69 kg N hm-2 a-1,respectively. The maximum net photosynthetic rate (A_(max)), stomatal conductance (G_(smax)), and water use efficiency (WUE, mumol /mmol) were measured using a Li- 6400 portable photosynthesis system under different nitrogen input levels. Additionally,leaf nitrogen content,leaf chlorophyll content, specific leaf area,and photosynthetic nitrogen use efficiency (PNUE,mumol mol-1 s-1) were also measured. In this paper, the variation in Amax with increasing nitrogen input was analyzed,and the reason was explored combined with variations in other leaf traits parameters (G_(smax),leaf chlorophyll content,leaf nitrogen content,PNUE,WUE,and specific leaf area) under the same treatments. The results showed that nitrogen addition (046 kg N hm~(-2) a~(-1)) resulted in a significant increase in A_(max),and when the added nitrogen was further increased to 69 kg N hm~(-2) a~(-1),the A_(max) decreased. The variations of G_(smax),leaf chlorophyll content,PNUE,and specific leaf area was similar to that of A_(max) under different nitrogen input levels. Significantly positive relationships were noted between A_(max) and leaf traits such as Gsmax,leaf chlorophyll content, PNUE,and specific leaf area under the different nitrogen input levels. A positive relationship was noted between A_(max) and leaf nitrogen content only for nitrogen addition of 046 kg N hm~(-2) a~(-1). No significant correlation was noted between A_(max) and WUE. The increase of eco-physiological factors of Tilia amurensis (A_(max),G_(smax),leaf nitrogen content,specific leaf area, PNUE,and WUE) were higher than that of Pinus koraiensis under each nitrogen addition treatment. Our study results will provide the basis for assessing the effects of nitrogen deposition on carbon cycle in the northeast of China.