Soluble Organic and Inorganic Nutrient Fluxes in Clearcut and Mature Deciduous Forests

nitrification due to a high C/N ratio in litter (Vitousek et al., 1979), temporary sorption on ion exchange sites The mechanisms by which forest ecosystems retain or lose soluble (Vitousek et al., 1979) and in the case of P, fixation or inorganic nutrients after disturbance are well known, but substantial amounts of soluble organic nutrients may also be released from cut sorption on soil (Wood et al., 1984; Walbridge et al., vegetation. Our objective was to compare the leaching of dissolved 1991). The increase in water flux from the root zone organic and inorganic nutrients in cut and mature forest stands and to due to cutting and the concomitant reduction in evapodevelop hypotheses about factors controlling the retention of soluble transpiration also plays an important role in controlling organic nutrients after disturbance. Solution chemistry was measured the leaching of nutrients (Likens and Bormann, 1995). for 2 yr after clearcutting a small area in the surrounding undisturbed The factors which control the leaching of organic nutrideciduous forest on a reference watershed at the Coweeta Hydrologic ents after clearcutting or other disturbances, however, Laboratory in the Appalachian Mountains. Concentrations of dishave not been extensively investigated. solved organic C (DOC) and N (DON) in slash leachate were 2.6 to Dissolved organic N is the major form of N in stream3.2 times the concentrations in throughfall from undisturbed forest. water draining from many mature forest watersheds Concentrations in forest floor, A horizon, and B horizon solutions from cut plots were 1.2 to 3.8 times those from undisturbed forest. (Sollins and McCorison, 1981; Hedin et al., 1995). In a Dissolved organic P (DOP) concentrations in cut plots were 3.1 and study of 31 watersheds with unpolluted old-growth for3.6 times those of uncut plots in solutions from slash and forest floor, ests in Chile representing an area without anthropogenic respectively, but did not differ in mineral soil. Fluxes of DOC, DON, atmospheric N deposition, DON comprised about 95% and DOP in all strata were greater in cut plots than uncut plots. of total N in streamwater (Hedin et al., 1995). Relatively Fluxes of DON were greater than those of ammonium plus nitrate high concentrations of DON drain from the forest floor, N in all strata of both cut and uncut plots. We hypothesize that the welland this DON also generally comprises most of the total recognized retention mechanisms for inorganic nutrients combine with N draining from the forest floor of intact forests (Qualls equilibrium adsorption of dissolved organic matter to efficiently buffer et al., 1991; Johnson and Lindberg, 1992, Appendix; against leaching of both soluble inorganic and organic nutrients Northup et al., 1995; Currie et al., 1996). The importance after clearcutting. of DON in solution transport in intact forests and the sudden inputs of potentially soluble nutrients in logging slash suggest that transport of soluble organic nutrients M of the emphasis on the cycling and leaching may be important in the retention or loss of nutrients of nutrients after disturbance of forests has been after clearcutting. For example, Bormann and Likens focused on inorganic nutrients. Historically, this empha(1979) estimated that a large amount of N lost from the sis is partly a product of the cases of severe leaching of forest floor after clearcutting could not be accounted nitrate from some clearcut forests, such as the Hubbard for in either streamwater, inorganic N, or in vegetation Brook Experimental Forest (Bormann and Likens, regrowth and they hypothesized that it may have been 1979), and of concern for the eutrophication of downtranslocated to deeper soil horizons and stored. stream waters. Soluble organic nutrients are also reOur objectives in this study were to (i) compare fluxes leased as vegetation grows, dies, and decomposes. For of the dissolved organic nutrients DOC, DON, and DOP example, 27% of the C in freshly fallen autumn leaf in a clearcut area and an adjacent, mature reference litter in a deciduous forest was soluble in water (Qualls area, (ii) determine whether concentrations of dissolved et al., 1991). Even such insoluble material as lignin in organic nutrients or inorganic nutrients were greater in wood can generate substantial quantities of soluble orclearcut than in reference areas, and (iii) identify the ganic matter as it is degraded by the white rot fungus strata where the greatest net leaching and deposition Phanaerochaete chrysosporium (Reid et al., 1982). occur. The mechanisms by which inorganic nutrients are reBy examining concentrations and fluxes we can evalutained or lost after clear cutting are generally well ate the relative importance of dissolved organic nutriknown and illustrated in many studies. These include ents. Identifying the strata where most of the dissolved loss of root uptake (Bormann and Likens, 1979), or the organic matter (DOM) originates or is removed from rapid recovery of root uptake by stump sprouts (Boring solution will help us develop hypotheses about the proet al., 1988), recovery of root uptake by seedling growth cesses controlling the retention or loss of dissolved or(Marks, 1974), delayed mineralization and subsequent ganics after disturbance. By comparing concentrations with fluxes in soil, we will evaluate the effects of inR.G. Qualls and S.W. Tyler, Dep. of Environmental and Resource creased leaching due to reduction in evapotranspiration. Sci., Univ. of Nevada, M.S. 370, Reno, NV 89557; B.L. Haines, Botany Dep., Univ. of Georgia, Athens, GA, 30602; W.T. Swank, USDA Forest Service, Coweeta Hydrologic Lab., Otto, NC, 29763. Received Abbreviations: DOC, dissolved organic carbon; DOM; dissolved or4 May 1999. *Corresponding author ([email protected]). ganic matter; DON, dissolved organic nitrogen; DOP, dissolved organic phosphorus; ET, evapotranspiration; WS, Watershed [label]. Published in Soil Sci. Soc. Am. J. 64:1068–1077 (2000).