Soil Enzyme Stoichiometry and Its Influencing Factors in Different Mangrove Communities in Xinying Bay, Hainan

To investigate the differences in soil hydrolytic enzyme activity and enzyme stoichiometry among different mangrove communities, as well as the driving factors behind these variations, we will explore their implications for nutrient limitations of soil microbes and the availability of soil nutrients. This study will focus on the Rhizophora stylosa communities (RS), Aegiceras corniculatum communities (AC), and RS-AC mixed communities (MF) within the Hainan Xinying Bay mangrove conservation area, using adjacent bare flats (BF) as a control group. We will analyse soil enzyme activity and physicochemical properties in at soil depths of 0–20 cm and 20–40 cm across different mangrove communities to uncover the synergistic changes between these variables. The results indicate the following. (1) Except for acid phosphatase (ACP), the enzyme activities and their stoichiometric ratios in different mangrove soil communities differ significantly. In the layer of soil 0~20 cm, β-glucosidase (BG) activity is markedly diminished in the MF community relative to the other communities. Conversely, the activities of β-1,4-N-acetylglucosaminidase (NAG) and leucine aminopeptidase (LAP) are the most elevated in MF. In the 20~40 cm soil layer, the activity of cellulase (CBH) was found to be highest in the MF community, while the activities of BG and NAG in MF were significantly lower than those in other communities. Overall, the activity of the five enzymes decreased with increasing soil depth. (2) The ratios of ln (BG + CBH)/ln (NAG + LAP) and ln (BG + CBH)/ln (ACP) in different soil layers indicated that MF was significantly lower than other communities; in the 0~20 cm soil layer, the ln (NAG + LAP)/ln (ACP) ratio was highest in MF and lowest in RS, while no significant differences were observed between BF and AC. In the 20~40 cm soil layer, no significant differences in ln (NAG + LAP)/ln (ACP) ratios were found among the different communities. (3) The vector angles of the soil enzymatic stoichiometry in the three different mangrove communities and light beaches are all less than 45°. (4) The soil enzyme C:N:P ratio, after logarithmic transformation, measures at 1:1.36:1.28, deviating from the global average (1:1:1). This, in combination with the enzymatic stoichiometry, suggests that nitrogen and phosphorus both constrain the soil microorganisms in this study region, with nitrogen constraining them more strongly. Redundancy analysis indicates that the soil TK content is a primary driver regulating soil enzyme activity and its stoichiometric ratios. From the perspective of accelerating nutrient cycling and alleviating nutrient limitation, this study found that increasing exogenous inputs of nitrogen and potassium can alleviate nitrogen and phosphorus limitation in the mangrove ecosystem of Xinying Bay. These findings provide a basic theoretical basis for restoring and maintaining a healthy tropical mangrove ecosystem.