EFFECTS OF HARVESTING ON SOIL NITROGEN (N) DYNAMICS IN A N-SATURATED HARDWOOD FOREST

Recent evidence suggests that soils of some central Appalachian hardwood forests have become nitrogen (N) saturated, a condition that develops when availability of soil N exceeds demand for N by plant roots and soil microbes. Among many environmental concerns associated with N saturation are the following: (1) greatly altered N cycle seen as a de-coupling of N dynamics from the high degree of biotic control that occurs in N-limited forest ecosystems and (2) greatly enhanced nitrification and leaching of NO3 along with calcium (Ca) and magnesium (Mg) from the soil and into streams, depleting base cation availability in the soil. Both of these have been demonstrated for hardwood forests of the Fernow Experimental Forest (FEF), West Virginia. Because increases in rates of nitrification and loss of soil cations often are associated with forest harvesting, it is important to determine whether the simultaneous effects of N saturation and harvesting may accelerate ecosystem degradation. Accordingly, we established a plot-based study to examine the effects of harvesting with and without addition of N and cations on soil N dynamics at FEF. Our treatments (each with four replicate 0.2-ha plots) were as follows: (1) Control (no treatment), (2) Harvest only (whole-tree harvesting [WTH] of entire plot), (3) Harvest+N (WTH plus 36 kg N/ha/yr), and (4) Harvest+N+cations (WTH plus 36 kg N/ha/yr plus 22.4 and 11.9 kg/ha/yr each of Ca and Mg, respectively). The cation addition treatment was done to simulate a mitigative treatment to replace loss of base cations to leaching. As expected at this N-saturated site, even our control plots exhibited high rates of N mineralization, mostly as net nitrification. However, these high rates were increased significantly by all harvest treatments. The addition of N in combination with harvesting did not increase nitrification significantly. By contrast, the addition of cations in combination with harvesting and N resulted in nitrification rates significantly higher than the other harvest treatments. These results illustrate the increasing complexities and challenges facing management of hardwood forests in the context of pollutant conditions. 1 Professor, Department of Biological Sciences, Marshall University, Huntington, WV 25755-2510; and Soil Scientist, Timber and Watershed Laboratory, USDA Forest Service, Parsons, WV 26287, respectively. Citation for proceedings: Stringer, Jeffrey W. ; Loftis, David L. , eds. 1999. Proceedings, 12th central hardwood forest conference; 1999 February 28-March 1-2; Lexington, KY. Gen. Tech. Rep. SRS-24. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 293 p. [Peer-reviewed paper]. percent higher in 1990 compared to 1970 (World Resources Institute 1994). In short, pollutant emissions of oxides of N are not expected to decline greatly in the near future. Thus, considering the likelihood of several eastern deciduous forests becoming N saturated in the future and the increased demand on these forests for the production of wood products, information on interactions between N saturation and forest harvesting will be of great importance for forest ecologists, forest and resource managers, and policy makers. Research suggests strongly that some sites have reached or are nearing N saturation (Stoddard 1994, Gilliam and others 1996, Fenn and others 1998, Aber and others 1998). Furthermore, studies of stream chemistry at other eastern forest sites have shown that elevated N additions resulted in elevated losses of N (as NO3) and base cations, especially calcium (Ca) and magnesium (Mg), suggesting a direct connection between N saturation (N additions) and the leaching of nutrient cations (Kahl and others 1993, Rustad and others 1993, Norton and others 1994). The effects of forest harvesting on ecosystem nutrient cycling have been studied for many forest types and harvesting practices. Indeed, much has been learned to indicate that response of the biota (recovering vegetation) is largely responsible for regulating nutrient change as the forest recovers from the disturbance of the harvest regime (Binkley 1986, Tritton and others 1987, Martin 1995, Gilliam and Adams 1995, Johnson 1995). It also has been learned, however, that there is great inter-site variability in these responses, precluding broad generalizations across forest ecosystems. Despite their great ecological and economic importance, forests of the central Appalachian region have not been studied extensively with respect to nutrient responses to forest harvesting. We know of no studies that have examined N saturation in the context of commercial forest harvesting. Accordingly, the purpose of this study was to examine the effects of harvesting on N mineralization and nitrification in a Nsaturated forest. Harvesting was carried out alone and in combination with additional treatments to simulate different pollutant and mitigation scenarios. In addition, because mineralization and nitrification rates are often found to be directly related to soil temperature, the effect of harvesting on the temperature of surface soils was examined in a subset of sample plots. Finally, we will review current knowledge on N dynamics at the Fernow Experimental Forest as an example of a central Appalachian forest under conditions of N saturation.