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 ammonification 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.