Winter climate change affects growing-season soil microbial biomass and activity in northern hardwood forests.

Understanding the responses of terrestrial ecosystems to global change remains a major challenge of ecological research. We exploited a natural elevation gradient in a northern hardwood forest to determine how reductions in snow accumulation, expected with climate change, directly affect dynamics of soil winter frost, and indirectly soil microbial biomass and activity during the growing season. Soils from lower elevation plots, which accumulated less snow and experienced more soil temperature variability during the winter (and likely more freeze/thaw events), had less extractable inorganic nitrogen (N), lower rates of microbial N production via potential net N mineralization and nitrification, and higher potential microbial respiration during the growing season. Potential nitrate production rates during the growing season were particularly sensitive to changes in winter snow pack accumulation and winter soil temperature variability, especially in spring. Effects of elevation and winter conditions on N transformation rates differed from those on potential microbial respiration, suggesting that N-related processes might respond differently to winter climate change in northern hardwood forests than C-related processes.

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.

Caterpillar guts and ammonia volatilization: retention of nitrogen by gypsy moth larvae consuming oak foliage.

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 (NH3) volatilization as larvae consumed foliage, leading to potentially large losses of N from the ecosystem to the atmosphere during gypsy moth outbreaks. We measured NH3 emission during the consumption of oak foliage by larvae in the laboratory. Surprisingly, we found very low amounts of NH3 release of about 0.1% of the N consumed in foliage. We speculate that digestive mechanisms may limit NH3 production in the midgut, and that the acidic environment of the hindgut traps most of the small amount of NH3 that is produced, effectively preventing a potentially very large N loss from both larvae and ecosystem. The estimated rate of NH3 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.