Effect of chronic nitrogen fertilization on soil CO2 flux in a temperate forest in North China: a 5-year nitrogen addition experiment

Abstract

Mounting evidence has indicated that there was dramatic increase in atmospheric nitrogen (N) deposition. The objectives of this study were to characterize how soil carbon dioxide (CO2) flux responds to different forms and levels of N addition in 5 years. We hope to provide further understanding and detailed information of the impact of N addition on CO2 flux in temperate forests in North China. A 5-year field experiment was conducted at the Xi Mountain Research Station of Beijing Forestry University, northern China, from 2011 to 2016. Multiple levels and forms of N addition experiment included control with no N added, NH4NO3, NaNO3, and (NH4)2SO4 at two levels (low N (L) 50 kg N ha−1 year−1 and high N (H) 150 kg N ha−1 year−1). Additional N was administered equally once per month during the growing season (March to October), and CO2 flux was measured three times every month. Soil temperature, water-filled pore space, and NO3 −-N and NH4 +-N concentrations were measured monthly to determine the relationships between CO2 flux and soil physicochemical variables. Cumulative CO2 flux increased by more than 50% under all high-level N addition and by 27% under L-NH4NO3 addition, while other N additions had no significant effect. H-NaNO3 and H-NH4NO3 exerted stronger effects on cumulative CO2 flux in initial years, especially the second year when maximum increases were 99 and 129%, respectively. Increasing inorganic N concentration could change soil from N-limited to N-rich, and then N-saturated, and so the promotion increased and then decreased. The effect of high-level N addition was stronger than that of low-level, and exhibited a general order: NH4NO3 > (NH4)2SO4. Considering the amount and decrease in NH4 +-N/NO3 −-N in local actual N deposition, there might be an increase in soil CO2 flux in our study area in the future. The performance of N addition on cumulative CO2 flux depended on N forms and levels. If the experimental period had been less than 3 years, the effect of N addition on temperate forest soil would be overestimated. Our findings highlighted the importance of experimental time and multiple forms and levels of N addition with regard to the response of soil CO2 flux to N deposition.