Nutrient limitation status in a subtropical mangrove ecosystem revealed by analysis of enzymatic stoichiometry and microbial abundance for sediment carbon cycling

Mangrove ecosystem plays an important role in global cycling of nutrients. According to nutrient availability, microorganisms can optimize the allocation of C-, N- and P-degrading enzymes. Thus, in this study, enzymatic stoichiometry involved in C, N and P acquisition, and the microbial abundance were analysed in coastal mangrove sediments in two different seasons. Activities of β-1,4-glucosidase (GLU), N-acetyl-β-glucosaminidase (NAG), acid phosphotase (ACP) and phenol oxidase (PHO) were determined, and then applied to calculate the enzymatic C:N (GLU:NAG), C:P (GLU:ACP) and N:P (NAG:ACP) acquisition, respectively. Besides, GLU:PHO was applied as relative recalcitrance of sediment organic matter (SOM). The results showed the enzymatic stoichiometry and microbial abundance had strongly seasonal variations. Compared to other ecosystems, GLU:ACP and NAG:ACP ratios were largely lower, suggesting that sediments in this ecosystem might be microbial P-limitation. Moreover, the higher GLU:PHO ratio implied that SOM in this ecosystem might be more decomposable. Furthermore, the imbalance of nutrients for microbial abundance was revealed by the strong correlations relating sediment N:P to GLU:NAG, GLU:ACP, and GLU:PHO. On the other hand, microbial P limitation in this ecosystem may enhance C storage and sequestration because the higher C contents in sediments of March than November occurred with lower GLU:ACP and NAG:ACP ratios as well as bacterial abundance.