No abstract

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.

Incorporation of 35S‐sulfate into phosphate‐extractable S, hydriodic acid‐reducible S (HI‐S), and total S was measured in three horizons of Spodosols from the Huntington Forest, New York and Hubbard Brook Experimental Forest, New Hampshire. After 56 d 35S incorporated into nonwater‐extractable S constituents was 92, 65, 92, 63, 72, and 91% of the total 35S‐activity for the Huntington Forest Oa, Bh, Bsl, and Hubbard Brook Oa, Bh, and Bsl horizons, respectively. Immobilization of 35S‐sulfate into carbon‐bonded S (total S — HI‐S) was the major incorporation pathway in the Oa horizons. Adsorption of 35S‐sulfate (phosphate‐extractable) was most evident in the Bh and Bsl horizons. Incorporation of 35S‐sulfate into ester sulfate (HI‐S — inorganic S) occurred in all horizons. The influence of immobilization‐mineralization and adsorption‐desorption on S dynamics of these forest soils was evaluated.

Forests in northeastern North America are influenced by varying climatic and biotic factors; however, there is concern that rapid changes in these factors may lead to important changes in ecosystem processes such as decomposition. Climate change (especially warming) is predicted to increase rates of decomposition in northern latitudes. Warming in winter may result in complex effects including decreased levels of snow cover and an increased incidence of soil freezing that will effect decomposition. Along with these changes in climate, moose densities have also been increasing in this region, likely affecting nutrient dynamics. We measured decomposition and N release from 15 N-labeled sugar maple leaf litter and moose feces over 20 months in reference and snow removal treatment (to induce soil freezing) plots in two separate experiments at the Hubbard Brook Experimental Forest in New Hampshire, USA. Snow removal/soil freezing decreased decomposition of maple litter, but stimulated N transfer to soil and microbial biomass. Feces decomposed more rapidly than maple litter, and feces N moved into the mineral soil more than N derived from litter, likely due to the lower C : N ratio of feces. Feces decomposition was not affected by the snow removal treatment. Total microbial biomass (measured as microbial N and C) was not significantly affected by the treatments in either the litter or feces plots. These results suggest that increases in soil freezing and/or large herbivore populations, increase the transfer rate of N from plant detritus or digested plants into the mineral soil. Such changes suggest that altering the spatial and temporal patterns of soil freezing and moose density have important implications for ecosystem N cycling.