Regulation of nitrogen mineralization and nitrification in Southern Appalachian ecosystems: Separating the relative importance of biotic vs. abiotic controls

Abstract

Summary 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-oak/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.