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).
- Craine JM, Elmore AJ, Wang L, Augusto L, Baisden WT, Brookshire ENJ, Cramer MD, Hasselquist NJ, Hobbie EA, Kahmen A, Koba K, Kranabetter JM, Mack MC, Marin-Spiotta E, Mayor JR, McLauchlan KK, Michelsen A, Nardoto GB, Oliveira RS, Perakis SS, Peri PL, Quesada CA, Richter A, Schipper LA, Stevenson BA, Turner BL, Viani RAG, Wanek W, Zeller B. 2015. Convergence of soil nitrogen isotopes across global climate gradients. Sci Rep 5:8280.PubMedPubMedCentralCrossRefGoogle Scholar
- Fahey TJ, Hughes JW, Pu M, Arthur MA. 1988. Root decomposition and nutrient flux following whole-tree harvest of northern hardwood forest. For Sci USA. http://agris.fao.org/agris-search/search.do?recordID=US8860983. Last accessed 08/02/2018.
- Kleber M, Mikutta R, Torn MS, Jahn R. 2005. Poorly crystalline mineral phases protect organic matter in acid subsoil horizons. Eur J Soil Sci 56:717–25.Google Scholar
- Kopáček J, Cosby BJ, Evans CD, Hruška J, Moldan F, Oulehle F, Šantrůčková H, Tahovská K, Wright RF. 2013. Nitrogen, organic carbon and sulphur cycling in terrestrial ecosystems: linking nitrogen saturation to carbon limitation of soil microbial processes. Biogeochemistry 115:33–51.CrossRefGoogle Scholar
- Templer PH, Mack MC, Iii FSC, Christenson LM, Compton JE, Crook HD, Currie WS, Curtis CJ, Dail DB, D’Antonio CM, Emmett BA, Epstein HE, Goodale CL, Gundersen P, Hobbie SE, Holland K, Hooper DU, Hungate BA, Lamontagne S, Nadelhoffer KJ, Osenberg CW, Perakis SS, Schleppi P, Schimel J, Schmidt IK, Sommerkorn M, Spoelstra J, Tietema A, Wessel WW, Zak DR. 2012. Sinks for nitrogen inputs in terrestrial ecosystems: a meta-analysis of 15N tracer field studies. Ecology 93:1816–29.PubMedCrossRefPubMedCentralGoogle Scholar