Abstract:

Aims: Assessing the impact of nitrogen-phosphorus imbalance inputs on the carbon sink capacity of subtropical forests is currently a hot topic. The maximum carboxylation rate (V_cmax) and maximum electron transport rate (J_max) are crucial parameters in the C₃ plant photosynthetic model (FvCB model), providing insights into the mechanisms underlying photosynthetic biochemical limitations in response to N and P deposition. 

Methods: We examined the effects of six years N and P addition on the FvCB model parameters of saplings and adult trees among six dominant tree species in an evergreen broad-leaved forest of Jiulianshan, Jiangxi Province. 

Results: We found that addition of either N or P significantly improved the photosynthetic performance of saplings. Specifically, N addition significantly increased the J_max 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. 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. 

Conclusions: 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, small trees, large trees