Rare microbial taxa rather than phoD gene abundance determine hotspots of alkaline phosphomonoesterase activity in the karst rhizosphere soil

The spatial distribution of alkaline phosphomonoesterase (ALP) activity in the rhizosphere and bulk soil of three plants (Zea mays, Medicago sativa, and Cyperus rotundus) grown in a karst soil (pH 8.27) was visualized using in situ soil zymography. According to the zymogram images, we identified and precisely collected soil samples from hotspots and non-hotspots of ALP activity, and then analyzed the phoD genes that encoded alkaline phosphomonoesterase and assessed the microbial community. The results showed that (1) the phoD abundance in the plant types varied and was highest in the alfalfa and lowest in the maize; (2) Proteobacteria dominated the phoD-harboring microbes in the rhizosphere and Actinobacteria dominated the phoD-harboring microbes in the bulk soil, and (3) the ALP activity was positively correlated with the relative abundance of the diazotrophic Azotobacter, but negatively correlated with the phoD gene abundance, microbial community richness, and diversity. By coupling zymography and microbial molecular approaches, we identified hotspots of enzymatic and microbial activity in rhizosphere soil and evaluated the relative contributions from potential active microorganisms. We found that the function of specific phoD-harboring microorganisms in these hotspots differed depending on the plant in the soil, which had implications for phosphorus (P) management in P-limited karst soils. The results also suggested that free-living N-2-fixing bacteria (Azotobacter) might promote ALP activity, thereby emphasizing vital linkages between, and coupling of, the soil nutrient cycles.

Automated summary

The spatial distribution of alkaline phosphomonoesterase (ALP) activity and related microbial communities in the rhizosphere and bulk soil of different plants in karst soil varied, with positive correlation between ALP activity and abundance of Azotobacter but negative correlation with phoD gene abundance, microbial community richness, and diversity. These findings highlight the importance of specific phoD-harboring microorganisms in nutrient cycling, particularly in phosphorus-limited karst soils, with free-living N-2-fixing bacteria potentially promoting ALP activity.