Interactive influences of silvicultural management and soil chemistry upon soil microbial abundance and nitrogen mineralization

Abstract

The purpose of this study was to determine whether soil acidification, a widespread, chronic mode of disturbance and forest thinning, a site specific acute disturbance, produced interactive effects capable of producing changes in more general ecosystem properties and processes. Two forested sites in the Daniel Boone National Forest, KY which were similar in history, management, and parent material but which differed in the degree of soil acidity were selected for study. In each forest site we sampled two plots that were experimentally thinned in the 1960's and two adjacent unmanaged plots for soil chemical characteristics, microbial abundances, N mineralization and nitrification. There were significant differences between forest sites and significant effects of management for all soil chemical parameters, fungal biomass, N mineralization (site only) and nitrification. Soils from managed plots were generally higher in pH, nutrient availability and nitrification rates. There were significant interactions between site and management for NO 3, pH, Ca, Ca:Al ratio, and nitrification resulting from the greater magnitude of the management effect at the less acidified site. Additionally, there were interactive effects of site and management in organic C, NH 4 and fungal hyphal length because plots at the two sites showed dissimilar effects of management. Modeling using path analysis determined that N mineralization was most strongly predicted by combinations of organic C, Al, Ca:Al ratio, and fungal:bacterial ratio, while inorganic N availability and Ca:Al ratio were the main factors for nitrification. Fungal hyphal length was most strongly predicted by Al and pH; in contrast, path analysis failed to produce a model for bacterial abundance. These results suggest (1) that acute and chronic modes of disturbance have the potential to interact in a significant and ecologically meaningful manner and (2) that research to assess forest health should be based on key ecosystem processes, such as N mineralization and nitrification.