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Nutrient Cycling in Forests of Walker Branch Watershed, Tennessee: Roles of Uptake and Leaching in Causing Soil Changes

D. E. Todd^*

Environ. Sci. Div., Oak Ridge National Lab., Oak Ridge, TN 37831-6038.

^* Corresponding author.

ABSTRACT

Previous studies showed that subsoil exchangeable Ca²⁺ and Mg²⁺ decreased over the period 1971 to 1982 in several plots on Walker Branch Watershed. It was hypothesized that wood accumulation (i.e., uptake and sequestering in both living and dead tree biomass) was the dominant cause of the exchangeable Ca²⁺ decreases, whereas atmospheric deposition-induced leaching was the dominant cause of the exchangeable Mg²⁺ decreases. As hypothesized, wood accumulation far exceeded leaching of Ca²⁺ in those plots where soil exchangeable Ca²⁺ decreased. The hypothesis regarding causes of exchangeable Mg²⁺ decrease was neither accepted nor rejected from the data on hand. Leaching exceeded wood accumulation of Mg²⁺ in three of the four plots studied (pine, Pinus; yellow-poplar, Liriodendron tulipifera; and chestnut oak Quercus prinus). Cation leaching in these three plots was balanced by SO^2–₄, and input-output balances indicated that soil solution SO^2–₄ originated primarily from atmospheric deposition. Thus, the trends toward decreased subsoil exchangeable Mg²⁺ in those particular plots (while not all statistically significant at the 95% level), are attributable primarily to atmospheric S deposition. However, leaching was minimal in the fourth plot (oak hickory, Quercus-Carya spp.) due to very low soil solution SO^2–₄ concentrations, yet subsoil Mg²⁺ decreases were significant at the 95% level. In the latter case, wood uptake was the dominant mechanism of Mg²⁺ removal from the soil. The importance of slope position in determining nutrient cycling patterns and the long-term changes in nutrient status is also discussed.

Current address of D.W. Johnson: Biological Sci. Center, Desert Res. Inst., P.O. Box 60220, Reno, NV 89506 and Dept of Range, Wildlife, and Forestry, College of Agriculture, University of Nevada, Reno, NV 89512.

Research sponsored by the Office of Health and Environ. Res., U.S. Dep. of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc., Oak Ridge National Lab., Oak Ridge, TN 37831-6038. Publication no. 3417, Environ. Sci. Div., ORNL.

Received for publication September 16, 1988.

This article has been cited by other articles:

				D. W. Johnson, D. E. Todd Jr., C. F. Trettin, and J. S. Sedinger Soil Carbon and Nitrogen Changes in Forests of Walker Branch Watershed, 1972 to 2004 Soil Sci. Soc. Am. J., August 27, 2007; 71(5): 1639 - 1646. [Abstract] [Full Text] [PDF]