Increasing nitrogen availability increases water-use efficiency and decreases nitrogen-use efficiency in Acer...

Area of Science:

• Plant Physiology
• Ecology
• Biogeochemical Cycles

Background:

• Photosynthesis is crucial for terrestrial carbon, water, and nutrient cycles.
• Photosynthetic least-cost theory posits plants minimize nutrient and water investment for optimal photosynthesis.
• The theory predicts increased nutrient availability leads to higher photosynthetic enzyme allocation and reduced stomatal conductance, enhancing water-use efficiency.

Purpose of the Study:

• To experimentally test photosynthetic least-cost theory in Acer saccharum (sugar maple) trees.
• To investigate the effects of nitrogen availability and soil pH on photosynthetic traits and water-use efficiency.

Main Methods:

• A 9-year field experiment manipulated soil nitrogen availability and pH in Acer saccharum stands.
• Measured key photosynthetic traits including net photosynthesis (Anet), stomatal conductance (gs), leaf nitrogen content (Narea), maximum carboxylation capacity (Vcmax), and light-saturated electron transport rate (Jmax).
• Analyzed the ratio of leaf intercellular to atmospheric CO2 concentration (χ) to assess water-use efficiency.

Main Results:

• Increased soil nitrogen availability enhanced leaf nitrogen content and photosynthetic capacity (Vcmax, Jmax) but did not alter net photosynthesis or stomatal conductance.
• Higher nitrogen availability led to a decrease in χ, indicating improved water-use efficiency and a strengthened nitrogen-water use tradeoff.
• Soil pH had no direct effect on photosynthetic traits, but indirectly influenced nitrogen-water tradeoffs in the absence of nitrogen additions.

Conclusions:

• Acer saccharum maintained net photosynthesis across nitrogen gradients by optimizing nitrogen and water use tradeoffs, supporting the photosynthetic least-cost theory.
• Elevated nitrogen deposition may increase water-use efficiency more than photosynthetic rates.
• Soil pH effects on plant water and nutrient use are likely mediated by soil nitrogen availability.