Gross rates of ammonification and nitrification at a nitrogen-saturated spruce (Picea abies (L.) Karst.) stand in southern Germany

We investigated the magnitudes of temporal and spatial variabilities of gross ammonitication and nitrification, in an N-saturated temperate forest ecosystem. Forest soil gross ammonification, gross nitrification and heterotrophic soil respiration were measured in the forest floor and uppermost mineral layer over a period of 3 years. Total annual gross fluxes for the organic layer and uppermost mineral horizon (0-4 cm) were in the range of 800-980 kg N ha(-1) year(-1) for gross ammonification and 480-590 kg N ha(-1) year(-1) for gross nitrification. Annual heterotrophic soil respiration was 8000-8900 kg C ha(-1) year(-1). Highest soil C and N turnover rates occurred in summer, and a consistent pattern was observed throughout the observation period, with highest values for plots located at a clear-cut area and lowest values for plots located at an unmanaged, approximately 100-year-old, spruce control site. Soil moisture, soil temperature and substrate availability accounted for most of the observed variability of C and N turnover rates. Because gross rates of inorganic N production were more than an order of magnitude larger than ecosystem N losses along hydrological and gaseous pathways, our study underlines the importance of internal microbial N turnover processes for ecosystem N cycling and retention.