Retention of Nitrate-N in Mineral Soil Organic Matter in Different Forest Age Classes
Conceptual models of nutrient retention in ecosystems suggest that mature forests receiving chronically elevated atmospheric nitrogen (N) deposition should experience increased nitrate (NO3−) losses to streams. However, at the Hubbard Brook Experimental Forest (New Hampshire, USA), recent stream NO3− concentrations have been unexpectedly low in mature watersheds. Poorly understood retention of NO3−-N in soil organic matter (SOM) may explain this discrepancy. The relative availability of C and N in SOM influences NO3−-N retention and may vary during succession due to processes of N mining and re-accumulation. To evaluate the strength of the SOM sink for NO3−-N, we applied a 15NO3− tracer to the mineral soil in eight stands spanning a forest chronosequence from about 20 years to old growth (≫ 200 years). We tracked 15N recovery in SOM fractions in the upper 10 cm of B horizon over 5 weeks. Overall, forest age did not directly control the 5-week recovery of 15N, but it had an indirect effect via its influence on SOM properties such as C/N. Old-growth forest soils had the lowest C/N, implying closer proximity to effective N saturation. Across sites, both the particulate- and mineral-associated SOM fractions rapidly incorporated 15N, but recovery in each fraction generally declined with time, reflecting the dynamic nature of SOM. These results indicate that mineral horizons can provide an important N sink through the short term in forests of all ages, but that SOM-N remains subject to active cycling and potential loss from the soil pool over the longer term.
Keywordsnitrogen 15N tracer chronosequence soil water immobilization Spodosol
We thank Nick Jakubek, Wade Simmons, Liza Tetley, Charlene Tarsa, and Mikayla Jacobs for assistance with field or laboratory work. We also thank two reviewers that provided helpful comments to improve the manuscript. The HBEF is administered by the US Department of Agriculture Forest Service, Northern Forest Research Station, Newtown Square, PA. Hubbard Brook is a National Science Foundation supported Long-Term Ecological Research site. Support for this project was provided by the National Science Foundation (DEB 1257956—Ecosystem Studies; and DEB 1633026 and DEB 1637685—Long-Term Ecological Research).
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