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Disturbance and topography shape nitrogen availability and δ15N over long-term forest succession
Citation:
Perakis, S., A. Tepley, AND J. Compton. Disturbance and topography shape nitrogen availability and δ15N over long-term forest succession. ECOSYSTEMS. Springer, New York, NY, , 16, (2015).
Impact/Purpose:
Stable isotope ratios can be useful indicators of the balance between nitrogen (N) inputs, internal cycling and outputs in ecosystems. Researchers from USGS, University of Colorado and USEPA used stable nitrogen isotopes of soil and foliage to examine forests at the HJ Andrews Long-Term Ecosystem Research site in western Oregon. They addressed the question fo whether disturbance and succession led to increased soil δ15N values as expected from fractionating N loss, or decreased soil δ15N values reflecting N inputs to soil. These forests tended to have different patterns with stand age over 800+ years of forest succession, reflecting tight N cycling in unpolluted forests when compared to δ15N trends in more N-polluted forests. They concluded that disturbance and soil erosion were the most important factors affecting N balances in these N-limited, montane forests. These findings show the differences between forests without human N inputs and also provides insights for modeling of N cycling across large scales.
Description:
Forest disturbance and long-term succession can promote open N cycling that increases N loss and soil δ15N values. We examined soil and foliar patterns in N and δ15N, and soil N mineralization, across a topographically complex montane forest landscape influenced by human logging and burning, and by natural wildfire disturbance. We were particularly interested in whether disturbance and succession led to increased soil δ15N values as expected from fractionating N loss, or decreased soil δ15N values reflecting N inputs to soil. Disturbance caused consistent declines in soil δ15N values, both in logged and burned old-growth forests measured 40 to 50 years after disturbance, and in forests where intense wildfire had opened the canopy within the last 200 years. Mass balance calculations show that lower soil δ15N values after disturbance reflects N inputs to soil from disturbed vegetation or symbiotic N fixation, rather than increased fractionating N loss. Forests exhibiting slow succession from shrubs to conifers displayed lower soil δ15N values and larger declines in soil δ15N after disturbance, suggesting slow canopy closure permits greater N fixation in early succession. Soil δ15N values increased very slowly over 800+ years of forest succession, reflecting tight N cycling in unpolluted forests when compared to δ15N trends in more N-polluted forests. Soils on steep slopes displayed lower δ15N values, consistent with more rapid soil turnover due to erosion. We conclude that disturbance and soil erosion increase the role of N inputs in shaping N cycling and soil δ15N in N-limited, montane forests
