Positive correlation between wood δ ¹⁵N and stream nitrate concentrations in two temperate deciduous forests

Area of Science:

• Forest Ecology
• Biogeochemistry
• Hydrology

Background:

• Integrated analysis of terrestrial and aquatic nitrogen (N) cycling is limited.
• Long-term data on forest N availability and stream nitrate export are scarce.
• Understanding these drivers is crucial for forest and water resource management.

Purpose of the Study:

• To analyze the relationship between stream nitrate concentrations and terrestrial N availability.
• To investigate the influence of climate, acidic deposition, and forest disturbance on N cycling.
• To use tree-ring stable isotopes as a proxy for long-term N availability.

Main Methods:

• Analyzed stream nitrate concentrations and wood stable isotope ratios (δ¹⁵N) from 96 trees.
• Collected data across six headwater catchments in the eastern United States.
• Developed datasets for climate, acidic deposition, and forest disturbance.

Main Results:

• Stream nitrate concentration increased with catchment mean wood δ¹⁵N across space.
• Stream nitrate concentrations decreased with decreasing wood δ¹⁵N over time in most catchments.
• Wood δ¹⁵N showed negative relationships with disturbance and acidic deposition, indicating increased N limitation.

Conclusions:

• Spatial and temporal variations in terrestrial N availability influence stream nitrate concentrations.
• Forest disturbance and acidic deposition can temporarily alter N availability.
• Long-term declines in terrestrial N availability are a primary driver of reduced stream nitrate concentrations in the central Appalachians.