EFFECTS OF CO_2 ENRICHMENT, HIGH NITROGEN DEPOSITION AND HIGH PRECIPITATION ON A MODEL FOREST ECOSYSTEM IN SOUTHERN CHINA
CO_2浓度倍增、高氮沉降和高降雨对南亚热带人工模拟森林生态系统土壤呼吸的影响

Aims Responses of soil respiration to global change play an important role in global carbon cycling, but the effects of increasing atmospheric carbon dioxide concentration (CO_2]), nitrogen (N) deposition and precipitation on soil respiration in subtropical China are unclear. Our objective was to test the effects of increased CO_2], N deposition and precipitation on soil respiration and to determine how they influence soil respiration in subtropical China. Methods A model forest ecosystem was constructed of six tree species native to South China. The species were exposed to four experimental treatments in open-top chambers beginning March 2005. Three chambers were used for elevated CO_2] (CC), two for high N treatment (NN) and the control (CO) and one for elevated precipitation (HR). The CC treatment was achieved by supplying additional CO_2 from a tank until the chambers had a concentration of (700 ± 20) μmol CO_2·mol~(-1). The NN treatment was achieved by spraying seedlings once a week for a total amount of NH_4NO_3 of 100 kg N·hm~(-2)·a~(-1). The HR treatment was achieved by weekly irrigation with 100 L water. Important findings For two years, soil respiration displayed strong seasonal patterns with higher values observed in the wet season (April to September) and lower values in the dry season (October to March) in the control chambers (CO) and the CC and NN treatments (p<0.001). There was no seasonal difference in the HR treatment (p>0.05). The CC enrichment affected soil respiration significantly (p<0.05), and there were no significant differences in annual CO_2 effluxes between CO and the other two treatments. The annual CO_2 effluxes reached 4 241.7, 3 400.8, 3 432.0 and 3 308.4 g CO_2·m~(-2)·a~(-1) in the CC, NN, HR and CO treatments, respectively. Soil respiration showed diverse responses between dry and wet seasons under different treatments. Higher soil respiration in the CC and NN treatments occurred in the wet season (p<0.05). In the dry season, soil respiration increased in the HR treatment (p<0.05) and decreased in the NN treatment (p<0.05). We found significant exponential relationships between soil respiration rates and soil temperature and significant linear relationships between soil respiration rates and soil moisture (below 15%).