Long-term ecosystem nitrogen limitation from foliar δ¹⁵ N data and a land surface model

Area of Science:

• Terrestrial Ecosystem Ecology
• Biogeochemical Cycles
• Climate Change Science

Background:

• Nutrient availability significantly impacts plant growth and the terrestrial carbon sink, especially under climate change and elevated CO2.
• Assessing large-scale, long-term nutrient limitation is challenging, leading to uncertainties in the terrestrial carbon cycle.
• Observed declines in leaf nitrogen (N) content and leaf δ15N suggest increasing N limitation, potentially driven by rising atmospheric CO2.

Purpose of the Study:

• To evaluate the utility of foliar δ15N isotopic data for inferring large-scale nitrogen limitation.
• To investigate whether observed trends in foliar N and δ15N are driven by increasing atmospheric CO2, climate change, or altered anthropogenic N deposition.
• To assess the role of nitrogen isotopic processes in land surface models.

Main Methods:

• Utilized the land surface model (LSM) QUINCY, which incorporates N isotopic processes.
• Integrated QUINCY with two extensive datasets of foliar N and N isotope content.
• Ran model simulations under various scenarios, including altered N deposition isotopic composition.

Main Results:

• The QUINCY model successfully captured observed changes in leaf N content and predicted widespread increases in N limitation.
• The model failed to replicate the observed pronounced, spatially heterogeneous decrease in foliar δ15N globally.
• Incorporating an observation-based temporal trend in N deposition isotopic composition enhanced the simulated decrease in leaf δ15N.

Conclusions:

• Foliar δ15N observations, by themselves, are insufficient for assessing global-scale N limitation.
• Combining foliar isotopic data with land surface models is crucial for understanding the drivers behind observed patterns in terrestrial ecosystems.
• Further research is needed to refine models and data integration for accurate assessment of nutrient limitation under global change.