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Watersheds within the Catskill Mountains, New York, receive among the highest rates of nitrogen (N) deposition in the northeastern United States and are beginning to show signs of N saturation. Despite similar amounts of N deposition across watersheds within the Catskill Mountains, rates of soil N cycling and N retention vary significantly among stands of different tree species. We examined the potential use of delta (15)N of plants and soils as an indicator of relative forest soil N cycling rates. We analyzed the delta (15)N of foliage, litterfall, bole wood, surface litter layer, fine roots and organic soil from single-species stands of American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), and sugar maple (Acer saccharum). Fine root and organic soil delta (15)N values were highest within sugar maple stands, which correlated significantly with higher rates of net mineralization and nitrification. Results from this study suggest that fine root and organic soil delta (15)N can be used as an indicator of relative rates of soil N cycling. Although not statistically significant, delta (15)N was highest within foliage, wood and litterfall of beech stands, a tree species associated with intermediate levels of soil N cycling rates and forest N retention. Our results show that belowground delta (15)N values are a better indicator of relative rates of soil N cycling than are aboveground delta (15)N values.

Forest defoliation by insects can lead to severe disruptions of the nitrogen (N) cycle resulting in elevated NO 3 - levels in stream water. To trace the movement of insect-mobilized N in a forest soil, 15 N-labeled gypsy moth frass or 15 N-labeled oak leaf litter was added to trenched plots in an oak forest over 29 months. Nitrogen movement from the frass or litter was measured in the available, mineralizable, microbial and total soil pools. Uptake of 15 N by oak seedlings and inorganic N leaching losses were also measured. No significant differences were found between the frass or leaf treatments for total N in any of the pools. Significant differences were found among the treatments in the distribution of the 15 N tracer. Forty percent of the 15 N added as frass became incorporated in the soils, with less than 1% found in oak seedlings. Almost 80% of 15 N added as leaves remained in the undecomposed leaf material after 2 years. Less than 0.001% of the added 15 N was leached in both treatments. Our data indicate that N in frass is mobilized more quickly than N in leaf litter. However, this frass N may be largely unavailable to plants and microorganisms as little of it was found in the extractable, microbial, or readily mineralizable pools.

The gypsy moth (Lymantria dispar L.), a major defoliator of hardwood forests in the eastern U.S., has a highly alkaline midgut pH. We hypothesized that the high pH would cause high rates of ammonia (NH 3 ) volatilization as larvae consumed foliage, leading to potentially large losses of N from the ecosystem to the atmosphere during gypsy moth outbreaks. We measured NH 3 emission during the consumption of oak foliage by larvae in the laboratory. Surprisingly, we found very low amounts of NH 3 release of about 0.1% of the N consumed in foliage. We speculate that digestive mechanisms may limit NH 3 production in the midgut, and that the acidic environment of the hindgut traps most of the small amount of NH 3 that is produced, effectively preventing a potentially very large N loss from both larvae and ecosystem. The estimated rate of NH 3 emission from a defoliated forest is small compared to other inputs and outputs of N from the ecosystem, but could potentially enhance the neutralization of atmospheric acidity during the defoliation period.

Defoliation of forests by insects is often assumed to produce a pulse of available nitrogen (N) from the decomposition of frass pellets. In this study we measured rates of carbon (C) and N mineralization from gypsy moth frass incubated with and without soil, and for soil alone. Incubations were at constant temperature and soil moisture conditions and lasted for 120 days. We found that gypsy moth frass contains much labile C as well as extractable N, and that the stimulation of microbial growth by the labile C results in immobilization of essentially all of the extractable N in the frass. The response of the microbes is fast, beginning within 1 day and lasting at least 90 days. This immobilization response represents an efficient mechanism for conserving N within a forest ecosystem after a defoliation event.