Abstract:

Aims: Assessing the impact of nitrogen-phosphorus imbalance inputs on the carbon sink capacity of subtropical forests is currently a hot topic. Maximum carboxylation rate (V_cmax) and maximum electron transport rate (J_max) are crucial parameters in the C₃ plant photosynthetic model (FvCB model), exploring the mechanisms underlying photosynthetic biochemical limitations in response to N and P deposition, which will help to improve the understanding of biodiversity conservation and management in the context of global change. 

Methods: A six-year N and P addition experiment was conducted in an evergreen broad-leaved forest of Jiulianshan, Jiangxi Province. Six dominant tree species were selected to quantify V_cmax, J_max, leaf mass per area, leaf N and P content, and analyze correlations between photosynthetic parameters and leaf traits of saplings and adult trees under N and P addition. 

Results: We found that addition of either N or P significantly altered photosynthetic parameters of saplings. Specifically, N addition significantly increased the Jmax of saplings, while P addition significantly increased the V_cmax. However, combined N and P addition had no significant effects on V_cmax, J_max, and J_max/V_cmax, showing an antagonistic effect. In contrast, none of N and P treatments significantly affected V_cmax, J_max, and J_max/V_cmax in adult trees. During the transition from saplings to adult trees, V_cmax and J_max increased more than threefold. Furthermore, saplings exhibited species-specific responses to N and P addition. Schima superba and Castanopsis fargesii were more sensitive to N addition, Castanopsis carlesii was more sensitive to P addition, and Machilus breviflora was more sensitive to combined N and P addition. Combined N and P addition significantly decreased the leaf mass per area (LMA) of saplings while increasing leaf nitrogen concent, whereas P addition significantly increased leaf nitrogen content in adult trees. However, leaf traits did not explain the variation in photosynthetic parameters. 

Conclusions: These findings confirm the theoretical prediction that saplings were more responsive to nutrient inputs than adult trees, which may allocate more nutrient resources to reproduction and defense systems rather than altering photosynthetic biochemical limitations in response to N and P addition. The antagonistic effect of combined N and P addition on sapling photosynthetic parameters suggests that excessive nutrient supply led to nutrient allocation toward non-photosynthetic components, reducing the efficiency of the photosynthetic system. Clearly, N and P availability significantly alter the FvCB model parameters in an evergreen broad-leaved forest, with size-dependent and species-specific effects on photosynthetic biochemical limitations.

Key words: long-term nutrient additions, in-situ experiment, photosynthetic biochemical limitation, saplings, adult trees