Responses of soil respiration to elevated carbon dioxide and nitrogen addition in young subtropical forest ecosystems in China

Abstract. Global climate change in the real world always exhibits simultaneous changes in multiple factors. Prediction of ecosystem responses to multi-factor global changes in a future world strongly relies on our understanding of their interactions. However, it is still unclear how nitrogen (N) deposition and elevated atmospheric carbon dioxide concentration [CO₂] would interactively influence forest floor soil respiration in subtropical China. We assessed the main and interactive effects of elevated [CO₂] and N addition on soil respiration by growing tree seedlings in ten large open-top chambers under CO₂ (ambient CO₂ and 700 μmol mol⁻¹) and nitrogen (ambient and 100 kg N ha⁻¹ yr⁻¹) treatments. Soil respiration, soil temperature and soil moisture were measured for 30 months, as well as above-ground biomass, root biomass and soil organic matter (SOM). Results showed that soil respiration displayed strong seasonal patterns with higher values observed in the wet season (April–September) and lower values in the dry season (October–March) in all treatments. Significant exponential relationships between soil respiration rates and soil temperatures, as well as significant linear relationships between soil respiration rates and soil moistures (below 15%) were found. Both CO₂ and N treatments significantly affected soil respiration, and there was significant interaction between elevated [CO₂] and N addition (p<0.001, p=0.003, and p=0.006, respectively). We also observed that the stimulatory effect of individual elevated [CO₂] (about 29% increased) was maintained throughout the experimental period. The positive effect of N addition was found only in 2006 (8.17% increased), and then had been weakened over time. Their combined effect on soil respiration (about 50% increased) was greater than the impact of either one alone. Mean value of annual soil respiration was 5.32 ± 0.08, 4.54 ± 0.10, 3.56 ± 0.03 and 3.53 ± 0.03 kg CO₂ m⁻² yr⁻¹ in the chambers exposed to elevated [CO₂] and high N deposition (CN), elevated [CO₂] and ambient N deposition (CC), ambient [CO₂] and high N deposition (NN), and ambient [CO₂] and ambient N deposition (CK as a control), respectively. Greater above-ground biomass and root biomass was obtained in the CN, CC and NN treatments, and higher soil organic matter was observed only in the CN treatment. In conclusion, the combined effect of elevated [CO₂] and N addition on soil respiration was apparent interaction. They should be evaluated in combination in subtropical forest ecosystems in China where the atmospheric CO₂ and N deposition have been increasing simultaneously and remarkably.