Document: FRA-3-59-15

Effects of harvesting on soil nitrogen dynamics in a nitrogen-saturated hardwood forest.

GILLIAM, F.S.* ¹ and M.B.ADAMS ²

Marshall University, Huntington, WV 25755 USA ¹
USDA Forest Service, Parsons, WV USA ²

Abstract:
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 nitrate 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 in the first post-treatment year; addition of N in combination with harvesting did not increase nitrification significantly over harvesting alone. However, addition of cations in combination with harvesting and N resulted in nitrification rates significantly higher than the other harvest treatments. Effects of harvesting appeared transient, generally disappearing by the third post-treatment year. These results illustrate the complexities and challenges facing management of hardwood forests in the context of pollutant conditions.

Keywords: nitrogen saturation, hardwood forests

This abstract is being presented at: 3:45 PM in session:
Oral Session #28: Effects of N Deposition in Ecosystems.