Effects of tree diversity on enzyme activity in litter of a subtropical forest ecosystem

Abstract

Aims: There is a growing concern regarding biodiversity loss and its effects on ecosystem functioning. Previous studies have primarily focused on the impact of biodiversity loss on plant productivity in forest ecosystems, but research regarding its effects on enzyme-mediated litter decomposition remains elusive. It has been well documented that the rate of litter decomposition is directly controlled by the activities of extracellular enzymes secreted by microbial decomposers, which is an important indicator of nutrient cycling in forest ecosystems. To explore the effect of biodiversity loss on extracellular enzyme activities in litter, and the underpinning mechanism that regulates litter decomposition, a field experiment was conducted in a subtropical forest. Methods: The experiment was conducted in a subtropical forest biodiversity-ecosystem functioning experimental area established in Xingangshan, Jiangxi Province. In this study area, 31 intensively studied plots which form a gradient of tree species diversity (i.e., 16 monocultures, eight 2-species mixtures, four 4-species mixtures, two 8-species mixtures and one 16-species mixture) were chosen. We collected fresh-fallen leaf litter in the plots of varying tree diversity levels and measured the extracellular enzyme activity, chemicals, and fungi in litter. We hypothesized that extracellular enzyme activities in litter, particularly those relevant to carbon (C), nitrogen (N), and phosphorus (P) cycling would significantly change with tree species diversity loss. We also expected to find that fungal decomposers could modulate the responses of enzyme activities to tree species diversity loss. Results: Our results showed that tree species diversity loss significantly affected the activities of extracellular enzymes, and extracellular enzyme activity showed a positive correlation with increasing tree species richness, except for single species. The activities of α-glucosidase (AG), β-glucosidase (BG), and cellubiosidase (CB), which are involved in C turnover, were the highest in the litter from the plot with the highest tree species diversity. Xylosidase (XS), N-acetyl-β-glucosaminidase (NAG), acid phosphatase (AP), and polyphenol oxidase (PPO), which are respectively involved in the turnover of N and P and the degradation of polyphenol, had higher rates of activity when the tree species diversity was lower. Moreover, we found that the extracellular enzyme activities showed a ‘single peak’ trend that correlated to changes of neighboring tree species diversity. This trend indicates that most of the extracellular enzyme activities relevant to the turnover of C, N and P were highest when the species richness of the six neighboring tree species was high as well. These findings indicate that both plot-level tree diversity and neighboring tree species can significantly alter extracellular enzyme activities in litter of target tree species. Fungal decomposers may play an important role in affecting the response of extracellular enzyme activities to tree species diversity loss. Conclusions: Our findings demonstrate that the loss of tree species diversity may indirectly affect extracellular enzyme activity by influencing the community structure and abundance of fungal decomposers. This study promotes our understanding of how tree diversity loss influences litter degradation and nutrient release through the modulation of extracellular enzyme activities in subtropical forests.