The fate of nitrogen in gypsy moth frass deposited to an oak forest floor

Forest defoliation by insects can lead to severe disruptions of the nitrogen (N) cycle resulting in elevated NO₃^- levels in stream water. To trace the movement of insect-mobilized N in a forest soil, ¹⁵N-labeled gypsy moth frass or ¹⁵N-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 ¹⁵N 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 ¹⁵N tracer. Forty percent of the ¹⁵N added as frass became incorporated in the soils, with less than 1% found in oak seedlings. Almost 80% of ¹⁵N added as leaves remained in the undecomposed leaf material after 2 years. Less than 0.001% of the added ¹⁵N 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.