Influence of moisture regime and tree species composition on nitrogen cycling dynamics in hardwood forests of Mammoth Cave National Park, Kentucky, USA

Understanding how natural factors interact across the landscape to influence nitrogen (N) cycling is an impor- tant focus in temperate forests because of the great inherent variability in these forests. Site-specific attributes, including local topography, soils, and vegetation, can exert important controls on N processes and retention. Seasonal monitoring of N cycling dynamics was carried out for 2 years in deciduous forest stands that differed in soil moisture status and geologic substrate, and thus, in tree species composition to determine the effects of tree species composition, mediated by moisture and soil chemistry, on N cycling. Geologic substrate influenced soil and soil leachate chemistry but did not appear to af- fect N cycling in the upper 10 cm. Moisture status was strongly correlated with tree species composition, which was sig- nificantly related to N cycling parameters. Sugar maple was associated with high net nitrification rates and soil solution NO3 concentrations, whereas in oak stands nitrification was low and soil solution NO3 was at or near detection limits. Tree species composition in the understory suggests that sugar maple may be increasing in mesic sites and that oak regen- eration in all sites is very limited, and a shift in species composition could result in changes to N retention and export.