Activities of CO2 Emission, N2 Fixation, and Denitrification during the Decay of Norway Spruce Coarse Woody Debris in Southern Taiga

The activities of CO 2 emission, N 2 fixation, and denitrification, as well as the physiological state of the community of microbial decomposers are assessed at different stages of decay of coarse woody debris (CWD) in the incubation experiments with the Norway spruce ( Picea abies L.) and the humus horizon of podzolic soil (Retisol). The CWD of five decomposition stages and soil are sampled at the experimental plots of the Central Forest State Reserve (Tver oblast, Russia). The maximum CO 2 emissions are associated with CWD decay stages III and IV. In addition, characteristic of these stages are the maximum values of the important indices of CWD and soil microbial activity, such as the substrate-induced respiration (SIR, 50 µg C–CO 2 /(g h), share of easily decomposable С in organic matter ( A 1 , 66%), and metabolic quotient q CO 2 (0.78). Unlike the CO 2 emission, the maximum activity of N 2 fixation is recorded earlier, during decay stage II. The N 2 fixation and denitrification activities indicate a gradual and intricately regulated transition process from the properties of CWD bacterial and fungal communities to those in soil during stages II, III, and IV. A dramatic decrease (more than threefold) is observed only for the C : N ratio in CWD at decay stage IV as compared with stage III. Although the CO 2 emission at stage V sharply decreases, the CWD organic matter is less stable as compared with that of the Retisol.