Regulation of nitrogen mineralization and nitrification in Southern Appalachian ecosystems: separating the relative importance of biotic vs. abiotic controls
Long-term measurements of soil nitrogen (N) transformations along an environmental gradient within the Coweeta Hydrologic Laboratory basin in western North Carolina showed a strong seasonal pattern and suggested that vegetation community type through its influence on soil properties-was an important regulating factor. Our objective was to determine the relative effects of biotic vs. abiotic factors on soil N transformations. During the 1999 and 2000 growing seasons we transplanted soil cores from each of the five gradient plots to all other gradient plots for their 28-day in situ incubation. N mineralization and nitrification rates in soils from the northern hardwood (NH) site were significantly increased when soils were transplanted to warmer sites. N mineralization rates also increased in transplanted soil from the dry mixed-oak/pine site to a wetter site. Multiple regression analysis of N mineralization from all five sites found that biotic (total soil N and C:N ratios) and climatic factors (moisture and temperature) regulate N mineralization. Regression analyses of individual sites showed that N mineralization rates responded to variation in temperature and moisture at only the high elevation northern hardwood site and moisture alone on the dry warm mixed-oak1 pine site. N mineralization was unrelated to temperature or moisture at any of the other sites. Results indicate that soil properties plus climatic conditions affect soil N transformations along the environmental gradient at Coweeta. Environmental controls were significant only at the extreme sites; i.e., at the wettest and warmest sites and soils with highest and lowest C and N contents. The high degree of temperature sensitivity for the northern hardwood soils indicates potentially large responses to climatic change at these sites.