Response of photosynthetic physiological characteristics of rare broadleaved tree species Juglans mandshurica seedlings to simulated nitrogen deposition

Objective This paper aims to investigate the effects of simulated nitrogen deposition on the photosynthesis parameters and leaf traits of Juglans mandshurica on stub land in the growing season.

Method A nitrogen addition test was performed in Kaiyuan Tree Farm in Shulan City, Jilin Province of northeastern China. Saplings of Juglans mandshurica were taken artificially as experimental materials and urea solution was used as the nitrogen source and sprayed on canopy, with three levels of nitrogen solution as CK (0 kg/(year·ha)), LN (50 kg/(year·ha)), HN (100 kg/(year·ha)). The parameters of photosynthesis, fluorescence, leaf traits were determined, and the relevance among different photosynthetic parameters was analyzed.

Result (1) The photosynthetic capacity of Juglans mandshurica was enhanced by nitrogen treatment. The chlorophyll content, maximum carboxylation rate, instantaneous carboxylation rate and maximum electron transport rate of Juglans mandshurica increased significantly under both levels of nitrogen treatment. Meanwhile, the maximum net photosynthetic rate, net photosynthetic rate, total photosynthetic carbon assimilation per plant, light compensation point and light saturation point increased significantly under HN treatment. (2) Nitrogen treatment changed the stomal behavior of Juglans mandshurica. Under LN treatment, the stomatal conductance and transpiration rate decreased. Meanwhile, water use efficiency increased. (3) Nitrogen treatment expanded the leaf area of Juglans mandshurica. The leaflet area, total number of leaflets and total leaf area of Juglans mandshurica made positive response to HN treatment. (4) The fluorescence parameters increased at first and then decreased with increasing levels of nitrogen addition treatment. (5) The coefficient of correlation between total photosynthetic carbon assimilation per plant, and total leaf area was higher than that between total photosynthetic carbon assimilation per plant, and net photosynthetic rate per area.

Conclusion In general, the nitrogen addition treatment has a positive effect on the photosynthesis of Juglans mandshurica. However, the response of different photosynthetic physiological process varies through different levels of simulated nitrogen addition treatments. Furthermore, stimulated by nitrogen addition, increasing growth of total photosynthetic carbon assimilation per plant, was related more closely to expansion of total leaf area than augmentation of net photosynthetic rate per area.