Ecosystem nutrient responses to chronic nitrogen inputs at Fernow Experimental Forest, West Virginia

Abstract

Among the current environmental concerns for forests of the eastern United States is nitrogen (N) saturation, a result of excessive inputs of N associated with acidic deposition. We studied nutrient responses on N-treated and untreated watersheds of the Fernow Experimental Forest, West Virginia, to test for evidence of N saturation on the treated watershed. The watersheds were WS7 (23-year-old even-aged control), WS4 (mature mixed-aged control), and WS3 (23-year-old even-aged treatment). WS3 has received aerial applications of (NH₄)₂SO₄ from 1989 to the present (a total of 4 years for the study period) at 3 × ambient inputs of N and S (54 and 61 kg•ha⁻¹•year⁻¹, respectively). Base-flow stream samples were collected weekly from each watershed and analyzed for NO₃⁻ and Ca²⁺. Mineral soil was incubated in situ, placed in bags, and buried about every 30 days during the growing season in each of seven sample plots within each watershed. Moist samples of soil from the bags were analyzed for extractable NH₄⁺ and NO₃⁻. In addition, forest floor material and leaves of an herbaceous species (Violarotundifolia Michx.) from each plot were analyzed for N and other nutrients. Violarotundifolia was present on all 21 plots and used as an additional indicator of N availability and soil fertility. Foliage tissue was sampled from overstory tree species (Liriodendrontulipifera L., Prunusserotina Ehrh., Betulalenta L., and Acerrubrum L.) from WS3 and WS7 and analyzed for nutrient content. Results from the 1993 growing season showed few, if any, differences among watersheds for (1) N content and C/N ratio of the mineral soil and forest floor and (2) relative proportion of NH₄⁺ and NO₃⁻ produced in the buried bags. Nitrification rates were equally high in soils of all watersheds; N concentrations were significantly higher in foliage tissue of overstory tree species and of V. rotundifolia in the treatment versus control watersheds; plant tissue Ca was significantly lower for the treatment watershed than for the control watersheds. Our results support the conclusions of earlier studies that high amounts of ambient N deposition have brought about N saturation on untreated watersheds at the Fernow Experimental Forest. This is suggested by minimal differences among watersheds in N mineralization and nitrification and soil and forest floor N. However, aggravated N saturation on our treated watershed can be seen in differences in plant tissue nutrients among watersheds and streamflow data, indicating increased losses of NO₃⁻ with accompanying losses of Ca²⁺ in response to further N additions to a N-saturated system.