We used stable nitrogen isotopes from red pine trees to determine the history of nitrogen (N) cycling within the Deming Lake watershed at Itasca State Park. With nearly 250 years of data, results suggest a major decline in N availability in red pine beginning in the early 20th century based on detection of significant breakpoints. We believe this synchronous decline is the result of the fire suppression in the state park, though alternative biological and physical mechanisms cannot be ruled out.