Nutrient availability increases whole plant growth, but not leaf photosynthesis, in a closed canopy temperate forest

Background/Question/Methods

Terrestrial biosphere models are sensitive to the representation of photosynthetic processes. Many terrestrial biosphere models predict leaf photosynthesis indirectly from soil nitrogen based on empirical relationships between soil nitrogen, leaf nitrogen, and photosynthetic capacity. Recent studies call the generality of these relationships into question, suggesting that aboveground climate dictates nitrogen investment toward leaf photosynthesis, while soil nitrogen dictates investment toward whole plant processes such as leaf production or whole plant growth. However, few manipulative experiments quantitatively assess impacts of climate or soil nitrogen on leaf and whole plant processes. Here, we examined the effects of soil nitrogen on leaf and whole plant processes by measuring leaf and whole plant traits of upper canopy trees in a decadal nitrogen-by-sulfur field manipulation experiment. We expected that treatment combinations would create a soil nitrogen availability gradient, where nitrogen and sulfur additions would increase and decrease nitrogen availability, respectively. We hypothesized that soil nitrogen would not affect leaf photosynthesis, but would increase whole plant growth. We predicted that, because aboveground climate was similar between plots, that demand to allocate nitrogen toward leaf photosynthesis would be set by climatic conditions, with excess nitrogen allocated toward leaf production or whole plant growth.

Results/Conclusions

Soil nitrogen generally did not influence measures of leaf photosynthesis. Specifically, there was no effect of nitrogen addition on leaf-level net photosynthesis rate, maximum Rubisco carboxylation rate, RuBP regeneration rate, or nitrogen mass per unit leaf area. There was also no effect of nitrogen addition on stomatal conductance, stomatal limitation, or the ratio of intercellular CO2 to extracellular CO2. However, there was a negative effect of nitrogen addition on δ13C, indicating that nitrogen addition may have decreased water-use efficiency throughout the leaf lifespan. In contrast, soil nitrogen generally increased measures of whole plant growth. Specifically, nitrogen addition increased relative growth rate and the marginal increase in diameter at breast height between 2011 and 2019. These results suggest that soil nitrogen may not influence leaf-level processes without a concurrent increase in demand to allocate nitrogen toward leaf tissue. Further, these results indicate that, while soil nitrogen may positively affect whole plant processes, the fate of nitrogen allocation toward whole plant photosynthesis compared to other whole plant processes (e.g., growth, woody tissue production) may be context dependent on growing conditions. Overall, these results provide empirical evidence that support the need to update the representation of photosynthesis in terrestrial biosphere models.