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

摘要

Global climate change in the real world always exhibits simultaneous changesin multiple factors. Prediction of ecosystem responses to multi-factorglobal changes in a future world strongly relies on our understanding oftheir interactions. However, it is still unclear how nitrogen (N) depositionand elevated atmospheric carbon dioxide concentration [CO₂] wouldinteractively influence forest floor soil respiration in subtropical China.We assessed the main and interactive effects of elevated [CO₂] and Naddition on soil respiration by growing tree seedlings in ten large open-topchambers under CO₂ (ambient CO₂ and 700 μmol mol−1) andnitrogen (ambient and 100 kg N ha−1 yr−1) treatments. Soilrespiration, 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 withhigher values observed in the wet season (April–September) and lower valuesin the dry season (October–March) in all treatments. Significant exponentialrelationships between soil respiration rates and soil temperatures, as wellas significant linear relationships between soil respiration rates and soilmoistures (below 15%) were found. Both CO₂ and N treatmentssignificantly affected soil respiration, and there was significantinteraction between elevated [CO₂] and N addition (p<0.001, p=0.003,and p=0.006, respectively). We also observed that the stimulatory effect ofindividual elevated [CO₂] (about 29% increased) was maintainedthroughout the experimental period. The positive effect of N addition wasfound only in 2006 (8.17% increased), and then had been weakened overtime. Their combined effect on soil respiration (about 50% increased) wasgreater than the impact of either one alone. Mean value of annual soilrespiration was 5.32 ± 0.08, 4.54 ± 0.10, 3.56 ± 0.03 and 3.53 ± 0.03 kg CO₂ m−2 yr−1in the chambers exposed to elevated[CO₂] and high N deposition (CN), elevated [CO₂] and ambientN deposition (CC), ambient [CO₂] and high N deposition (NN), andambient [CO₂] and ambient N deposition (CK as a control), respectively.Greater above-ground biomass and root biomass was obtained in the CN, CC andNN treatments, and higher soil organic matter was observed only in the CNtreatment. In conclusion, the combined effect of elevated [CO₂] and Naddition on soil respiration was apparent interaction. They should beevaluated in combination in subtropical forest ecosystems in China where theatmospheric CO₂ and N deposition have been increasing simultaneouslyand remarkably.