Retention of Nitrate-N in Mineral Soil Organic Matter in Different Forest Age Classes

• Published in: Ecosystems (New York) Vol. 22; no. 6; pp. 1280 - 1294
• Main Authors: Fuss, Colin B., Lovett, Gary M., Goodale, Christine L., Ollinger, Scott V., Lang, Ashley K., Ouimette, Andrew P.
• Format: Journal Article
• Language: English
• Published: New York Springer Science + Business Media 01.09.2019; Springer US; Springer; Springer Nature B.V
• Subjects: age structure; Atmospheric models; B horizons; Biomedical and Life Sciences; Carbon content; chronosequences; Ecology; Ecosystems; elevated atmospheric gases; Environment models; Environmental Management; Forest soils; Forests; Geoecology/Natural Processes; Hydrology/Water Resources; Life Sciences; Mineral industry; mineral soils; mining; Mining industry; New Hampshire; nitrate nitrogen; Nitrates; nitrogen; Nitrogen isotopes; Nutrient retention; Old growth; Old growth forests; Organic matter; Organic soils; Original Articles; Plant Sciences; Recovery; Retention; Soil erosion; Soil organic matter; Soils; stable isotopes; Streams; Tracers (Chemistry); Watersheds; Zoology; New Hampshire; Spodosol; soil water; immobilization; chronosequence; nitrogen; N tracer
• ISSN: 1432-9840; 1435-0629
• DOI: 10.1007/s10021-018-0328-z

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 reaccumulation. To evaluate the strength of the SOM sink for NO₃⁻-N, we applied a 15 NO₃⁻ 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 3 tracer to the mineral soil in eight stands spanning a forest chronosequence from about 20 years to old growth (200 years). We tracked 15 N 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.