Articles | Volume 12, issue 13
Biogeosciences, 12, 3941–3952, 2015
https://doi.org/10.5194/bg-12-3941-2015
Biogeosciences, 12, 3941–3952, 2015
https://doi.org/10.5194/bg-12-3941-2015

Research article 01 Jul 2015

Research article | 01 Jul 2015

Spatiotemporal analysis of nitrogen cycling in a mixed coniferous forest of the northern United States

I. Howard1 and K. K. McLauchlan2 I. Howard and K. K. McLauchlan
  • 1Department of Geosciences, University of Arkansas, 216 Ozark Hall, Fayetteville, AR 72701, USA
  • 2Department of Geography, Kansas State University, 118 Seaton Hall, Manhattan, KS 66506, USA

Abstract. Nitrogen (N) is the limiting nutrient to primary productivity in a variety of temperate forests, and N cycling is undergoing a variety of anthropogenic changes, notably a doubling of reactive N (Nr) on a global scale. Yet, the magnitude of these changes to N cycling has been difficult to document in terrestrial ecosystems, especially in old-growth forests. To determine the trajectory of N cycling and the potential impacts of anthropogenic influences at local scales, we measured the composition of stable nitrogen isotopes (δ15N) in wood from living red pine trees (Pinus resinosa) at a single site in northern Minnesota, USA. A synchronous decline in wood δ15N values began approximately in the 1920s in 17 individual trees at different topographic positions, indicating a common driver. The decline in wood δ15N values corresponded with declines in sedimentary δ15N recorded in lacustrine sediments of the same catchment. Disturbance regime and species composition began to change at the turn of the 20th century with park establishment, providing a likely mechanism of decline in δ15N values toward present. While other mechanisms of this change are possible, we conclude that while there may be consequences of increased influxes of various forms of anthropogenic Nr into terrestrial ecosystems at the global level, these changes are not being expressed at a local level in this temperate forest ecosystem.

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Short summary
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.
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