Seedling leaves allocate lower fractions of nitrogen to photosynthetic apparatus in nitrogen fixing trees than in non-nitrogen fixing trees in subtropical China

Photosynthetic-nitrogen use efficiency (PNUE) is a useful trait to characterize leaf physiology and survival strategy. PNUE can also be considered as part of ‘leaf economics spectrum’ interrelated with leaf nutrient concentrations, photosynthesis and respiration, leaf life-span and dry-mass investment. However, few studies have paid attention to PNUE of N-fixing tree seedlings in subtropical China. In this study, we investigated the differences in PNUE, leaf nitrogen (N) allocation, and mesophyll conductance ( g m ) in Dalbergia odorifera and Erythrophleum fordii (N-fixing trees), and Betula alnoides and Castanopsis hystrix (non-N-fixing trees). PNUE of D . odorifera and E . fordii were significantly lower than those of B . alnoides and C . hystrix mainly because of their allocation of a lower fraction of leaf N to Rubisco ( P R ) and bioenergetics ( P B ). Mesophyll conductance had a significant positive correlation with PNUE in D . odorifera , E . fordii , and B . alnoides , but the effect of g m on PNUE was different between species. The fraction of leaf N to cell wall ( P CW ) had a significant negative correlation with P R in B . alnoides and C . hystrix seedling leaves, but no correlation in D . odorifera and E . fordii seedling leaves, which may indicate that B . alnoides and C . hystrix seedling leaves did not have enough N to satisfy the demand from both the cell wall and Rubisco. Our results indicate that B . alnoides and C . hystrix may have a higher competitive ability in natural ecosystems with fertile soil, and D . odorifera and E . fordii may grow well in N-poor soil. Mixing these non-N-fixing and N-fixing trees for afforestation is useful for improving soil N utilization efficiency in the tropical forests.