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.

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SUMMARY(1) Early forest regeneration in southern Appalachian hardwood forests is dominated by the woody nitrogen-fixing legume, black locust (Robinia pseudo-acacia). Although it grows most prevalently on clear-felled areas, abandoned pastures, or disturbed roadsides, it may have historically been an important colonizer of burned sites. Although it commonly reproduces from seed germination, sprouting from stumps and roots is its most prevalent means of regeneration. Early sprout growth is rapid, attaining heights up to 8 m in 3 years.(2) Except for stands on high-nutrient sites, growth decreases after 10-20 years. In less vigorous stands, stem mortality may be high due to attacks by the locust stem borer (Megacyllene robiniae). The high mortality of black locust is an early successional mechanism that releases cpdominant species such as Liriodendron tulipifera, and creates canopy gaps favourable for growth of longer-lived individuals.(3) Total biomass accretion in 4, 17 and 38-year-old black locust stands growing on fertile, mesic sites was 33, 174 and 399 t ha~', respectively, in comparison to 198 t ha~' for an older, uneven-aged mixed oak forest with a history of disturbance.(4) Biomass accumulation was the predominant fate of fixed N in all three stands, with an addition to total soil N apparent only in the 38-year-old stand.(5) Symbiotic N fixation by black locust apparently increased the concentration of NO 3 in the soil. The source of elevated soil NO 3 is hypothesized to be high fluxes of N from leaf and root litter mineralization and nitrification, and perhaps from canopy insect frass.(6) Total stand N increased at a net average annual rate of 48, 75 and 33 kg ha~' year" 1 , respectively, for ages 4, 17 and 38. Nodule biomass was 8, 106 and 4 kg ha" 1 in the 4, 17 and 38-year-old stands, respectively.(7) These patterns of N accretion are similar to those reported for other woody nitrogen-fixing species on secondary successional sites. They indicate that peak N fixation occurred from early to intermediate stages of forest succession, and declined with later successional development.

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The components of hardwood forest regeneration on a southern Appalachian watershed were assessed during the 1st yr following clear—cutting. First—year net primary production (NPP) on the clear—cut was 1955 kg/ha, representing 22% of the NPP of a nearby undisturbed hardwood forest. First—year nutrient pools in NPP for N, P, K, Mg, and Ca were estimated at 29—44% of those in the NPP of the control. The greatest NPP and nutrient pools were represented in descending order by hardwood sprouts, herbs, vines, and seedlings. Woody successional species (Robinia pseudo—acacia, Liriodendron tulipifera, and Vitis aestivalis var. argentifolia) and herbs (Aster spp., Solidago spp., and Erechtites hieracifolia) were important in revegetation due to competitive advantages in growth rates, growth forms, and propagative capacities. The woody successional species had higher tissue concentrations of N and P than most other woody species. Herbs as a group had significantly higher foliar concentrations of K than woody species. Woody successional and herbaceous species collectively had higher biomass and elemental pools than other woody species. Following forest disturbance, these fast—growing species conserve substantial pools of nutrients in their biomass and initiate a rapid recovery of forest elemental cycling processes.