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.