Synergistic effects of leaf nitrogen and phosphorus on photosynthetic capacity in subtropical forest

The photosynthetic capacity is contingent upon the balance between nitrogen (N) and phosphorus (P) concentration, as well as environmental factors. Ensuring a balanced and timely supply of nitrogen and phosphorus facilitated healthy leaf growth and sustained efficient photosynthetic activity during trees active growth phases. However, the effects of the interactions between these factors on photosynthesis, particularly in the unique context of karst ecosystems, remain unclear. To address this, we conducted an assessment of photosynthetic parameters, including the 25 ℃ maximum carboxylation rate (V cmax,25 ) and the 25 ℃ maximum electron transport rate (J max,25 ), and chemical traits of leaves (leaf N, leaf P, and N:P ratio) in nine locally dominant species across both subtropical non-karst and karst areas in southwestern China. Our findings revealed that concentrations of leaf phosphorus and soil phosphorus were significantly higher in karst areas compared to non-karst areas. Additionally, the V cmax,25 of both karst and non-karst species were synergistically affected by leaf N and P concentrations, rather than being constrained by least available nutrient. Specifically, V cmax,25 of karst species was strongly related to leaf P, and increasing leaf N substantially increased the sensitivity of V cmax,25 to leaf P, highlighting the importance of maintaining a balance between N and P concentrations. These insights substantially enhance the understanding of photosynthetic dynamics and resource management in diverse ecosystems, providing a solid foundation for further research and conservation strategies.