Effects of elevated atmospheric CO and nitrogen deposition on leaf litter and soil carbon degrading enzyme activities in a Cd-contaminated environment: A mesocosm study.

Rising atmospheric CO concentration and nitrogen (N) deposition are changing terrestrial carbon (C) cycle; however, little has been known about such impacts in a heavy-metal-contaminated environment. This study conducted an open-top chamber experiment to explore the impacts of rising atmospheric CO concentration and N deposition on the leaf litter and soil C cycle in cadmium (Cd)-contaminated environment. The experiment include five treatments: control, Cd (30 g ha yr) addition, Cd addition under elevated CO (700 ppm CO), Cd and N(100 kg ha yr) additions, and Cd and N additions under elevated CO, with three replicates per treatment. Leaf litter and soil C cycle were indexed by microbial biomass C concentration and the activities of four key C-degrading enzyme (β-glucosidase (BG), cellobiohydrolase (CBH), polyphenol oxidase (PPO), and peroxidase (POD)) in litter and soil. Results showed that, after one year treatment, Cd addition negatively affected the activities of all four C-degrading enzyme in litter and soil; while elevated CO and N addition essentially alleviated these negative effects. Elevated CO and N addition increased C-degrading enzyme activities more of the non-legume (i.e., Cinnamomum camphora) litter than those of the legume (i.e., Acacia auriculiformis) litter. Elevated CO, N addition, and Cd addition all affected C-degrading enzyme activities via their effects on the microbial biomass C concentration and C and N availability of the litter and soil samples. We suggest that rising atmospheric CO concentration and N deposition can offset the detrimental effect of Cd on the litter and soil C-degrading enzyme activities in forest ecosystems.