High concentrations of dissolved organic matter (DOM) were leached into rainwater passing through the canopy and forest floor of an oak (Quercus spp.)-hickory (Carya spp.) forest in the southern Appalachian Mountains. More than 95% of this dissolved organic C (DOC) and N (DON) was removed as water percolated through the soil profile and left the ecosystem in stream water. Our objective was to examine the importance of decomposition in the removal of DOC and DON.

Dissolved organic matter (DOM) is a major vehicle for the translocation and loss of N and P from forest ecosystems. The chemical properties of DOM and its interactions with soil surfaces are crucial in determining the mobility of these organic nutrients. We fractionated DOM from throughfall; all soil horizons (Ultisols and Inceptisols), and stream water from an Appalachian mountain forest ecosystem into hydrophobic or hydrophilic acids, neutrals, and bases. \Ve analyzed each fraction for dissolved organic C (DOC), N (DON), and P (DOP). Most of the DOC was in the acid fractions, with the humic fractions (hydrophobic acids and phenols) comprising 35 to 57% of the DOC in all samples except summer throughfall. Concentrations of all fractions declined with depth in the soil. As a percent of total DOC, the humics declined with depth, whereas the hydrophilic neutrals increased. Bases, which we expected to contain cationic amino groups, were < 2.5% of the DOC. Instead, most DON was in the humic. hydrophilic acid, and hydrophilic neutral fractions. Most DOP occurred in the hydrophilic acid, humic, and hydrophilic neutral fractions. The functional groups in which N and P occur had little influence on the behavior of most of the DOM as a whole since: (i) cationic DOM was such a minor component, and (ii) P was simply too rare to influence the anionic behavior of many molecules. Nevertheless, for those molecules in which P did occur, P may have influenced their behavior since a large percentage of the DOP was in the hydrophilic acid (i.e., anionic) fraction. The carboxylic and phenolic functional groups, or in some cases the neutrality, of the DOM molecules appeared to be much more important than N-containing groups in influencing the behavior of the N carried passively by the DOM. D ISSOLVED ORGANIC MATTER plays an important role in interrestrial and stream ecosystems because it: (i) is a major mode of export of N and P in many ecosystems that are not experiencing severe erosion (Sollins and McCorison, 1981); (ii) plays a major role in determining the balance of soil N and P over the time of soil development; (iii) affects soil structure, e.g., resulting in a deeper redistribution of soil organic matter and the coating of clay particles with organic matter, (iv) is the principal vehicle for movement of Al and Fe in soil; and (v) provides a potential source of carbon for microbial growth (Meyer et al., 1987). Characterization of DOM in soil and stream water is very difficult because it consists of a myriad of compounds, none of which is present in large proportions. Four general approaches have been used to characterize DOM: (i) cataloging individual compounds; (ii) analyzing for broad biochemical classes of compounds such as proteins, free amino acids, monosaccharides, pplysaccharides, lipids, pplyphenols, and tannins; (iii) dividing into molecular-size classes; and (iv) compre

We evaluated the importance of dissolved organic matter as a vehicle for the movement of N and P from the canopy and the forest floor into the mineral soil of a deciduous forest. We also examined the origin and nature of dissolved organic matter from the forest floor to see whether it was simply soluble plant material or highly humidified matter. The average annual output from the forest floor in the form of dissolved organic matter was 18, 28, and 14% of the input in solid litterfall for C, N, and P, respectively. In throughfall, about half of the dissolved N and P was organic. But, in solution percolating from the forest floor, 94% of the N and 64% of the P was organic. Leaching from the forest floor was not a source of inorganic N and P for the mineral soil. Instead, the forest floor was a sink for the removal of these inorganic nutrients delivered in throughfall. Microbial immobilization was the most likely explanation for much of the inorganic nutrient removal. In contrast, the forest floor was an abundant contributor of N and P to the mineral soil in the form of dissolved, and possibly particulate, organic matter. Much of the dissolved organic matter entering the A horizon originated from the upper (Oa and Oe horizon) forest floor, but it was modified in several respects compared to the original soluble material. The solution percolating from the forest floor over most of the year was much richer in nitrogen, contained a much larger proportion of hydrophilic acids, and contained a much smaller proportion of carbohydrate—rich hydrophilic neutrals, than did the original water—extractable material in autumn litter. However, the fresh autumn litter did contain a similar proportion of soluble hydrophobic acids that resembled dissolved humic substances in several respects. Most of the flux of nitrogen from the forest floor to the A horizon was carried by humic substances and highly colored hydrophilic acids.

Forests are often subject to multiple, compounded disturbances, representing both natural and human-induced processes. Predicting forest dynamics requires that we consider how these disturbances interact to affect species demography. Here we present results of an individual-based, spatially explicit forest simulator that we developed to analyze the compounded effects of hurricane disturbance and land use legacies on the dynamics of a subtropical forest. We used data from the 16-ha Luquillo Forest Dynamics Plot in Puerto Rico, together with a reconstruction of historical wind damage, to parameterize the simulator. We used the model to ask two questions. (1) What are the implications of variation in hurricane frequency and severity for the long-term dynamics of forest composition, diversity, and structure? Both storm severity and frequency had striking effects on forest dynamics, composition, and structure. The periodicity of disturbance also played an important role, with periods of high hurricane activity fostering the establishment of species that may become rare in the absence of severe storms and quiescent periods allowing these species to reach reproductive size. Species responses to hurricane disturbance could not be predicted from their life history attributes. However, species perceived to be primary forest species exhibited low temporal variation in abundance through the simulations. (2) How do hurricanes and legacies from human land use interact to determine community structure and composition? Our results suggest that, over time, regardless of the storm regime, land use legacies will become less apparent but will lead to a forest community that contains a mixture of secondary and primary forest species formerly dominant in areas of different land use. In the long term, hurricane disturbance generated two communities with slightly greater similarity than those not subject to storms. Thus, the inclusion of hurricane disturbance does not alter the prediction that land use legacies in this tropical forest will diminish over time. Our simulations also highlight the contingent effects of human legacies on subsequent community dynamics, including the response to hurricane disturbance, therefore supporting the notion that compounded disturbances can interact in ways that cannot be predicted by the study of single disturbances. The widespread importance of land use as a large-scale disturbance makes it imperative that it be addressed as a fundamental ecological process.

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