Different response of CO2 and N2O fluxes to N deposition with seasons in a temperate forest in northeastern China
Purpose: Elevated nitrogen (N) deposition had changed terrestrial carbon (C) and N cycling during the last several decades. Greenhouse gas (GHG) emission, serving as an important process of global C and N cyclings, was susceptible to elevated N deposition. However, the lack of GHG measurements in the spring freezing-thawing cycles (FTC) period and inconstant results of GHG emissions in growing season result in the uncertainties of GHG emission responding to elevated N deposition.
Materials and methods: Static chambers were used to monitor CO2 and N2O fluxes in growing season and spring FTC period in a simulated N deposition forest in northeastern China, which had been fertilized with different N amounts for 6 years.
Results and discussion: The temperate forest soil that subjected to N fertilizer was a net source of CO2 and N2O. The intensely fluctuant fluxes during the spring FTC period were due to stimulation of microbial activities induced by sharply changed soil temperature and moisture. The physical release of trapped gases beneath ice layer during the whole winter also contributed to the drastic increase of fluxes. No effect of N addition on the CO2 efflux was detected in the growing season or spring FTC period, indicating that N was not limited for microbial respiration. In contrast, N addition decreased the N2O flux by 72.8~85.7% in the growing season whereas increased by 11.2~17.8 times in the spring FTC period, which could be explained by the shift in structure of microbial community. Over the entire dataset, soil temperature and moisture could explain about 60 and 30% of seasonal variations of CO2 or N2O flux, respectively. By fitting the RS–T5–W5 model to separate season, poorer fit was achieved in the spring FTC period compared with growing season, suggesting a weakening effect of soil microclimates on CO2 efflux.
Conclusions: These results indicate that long-term elevated N deposition affected CO2 and N2O fluxes differently with seasons in a temperate forest soil, which will help to predict soil GHG flux under the conditions of global climate change.