Effects of exogenous carbon addition on the effectiveness of phosphorus under two soil phosphorus levels

Objectives  Addition of exogenous carbon is an alternative way to promote the release and supply potential of soil phosphorous. We studied the mechanism from soil microbial community structures and functions driven by the carbon addition.

Methods Indoor incubation method was used, glucose was used as the exogenous carbon source. The low and high P soils were collected from the maize and vegetable fields in Beibei District, Chongqing City. Each soil was divided into two groups, one group was added with glucose and the other was not, then incubated under shading condition at 60% of field water capacity. Soil samples were collected after 0, 2, 15, and 60 days of incubation, the BBP phosphorus fractionation method, Olsen method, and ignition method were employed to measure different forms of phosphorus contents, and high-throughput sequencing of 16S rRNA was used to analyze the microbial community structures.

Results The soil Olsen-P contents in glucose addition soil groups decreased after 2 and 15 days of incubation, then recovered to the original levels. Among the fractions by BBP method, CaCl₂-P, Enzyme-P, HCl-P, and Citrate-P content did not show obvious change at the end of incubation in low-P soils, regardless of glucose addition; microbial biomass phosphorus content and phosphatase activity increased significantly; the CaCl₂-P and Citrate-P contents in high-P soils were significantly decreased by glucose addition, while the Enzyme-P and HCl-P content did not show significant differences compared with the controls. Analysis of microbial community structure revealed that the bacterial community responded more strongly to glucose in low P soil than in high P soil. Glucose addition increased bacterial α-diversity in the low P soil, leading to clear separation of bacteria communities from the control group on the PC1 axis in the PCoA plot. However, glucose addition did not cause significant separation in high P soil. Results of the co-occurrence network analysis displayed a significant difference in the abundance of OTUs and a more pronounced modular pattern in the low P soil after glucose addition compared to the control group. The species Paraburkholderia, which had been shown in related studies to possess phosphorus solubilization function, was the most explanatory species for the differences between the treatment and control groups in terms of sample processing. In contrast, no microorganism with proven phosphorus solubilization function was observed in the high P soil after glucose addition. The plspm model results demonstrated that the addition of glucose induced changes in microbial community structure in both the high and low P soils, resulting in change differences of phosphorus fractions.

Conclusions The effect of exogenous carbon addition on phosphorus pool and bacteria community structure is more pronounced in low P soil than in high P soil. Glucose addition causes fluctuations of P pools, increases the species richness and compositional differences of the microbial communities, enriches the known P soluble bacteria Paraburkholderia in low P soil, resulting better stimulation effect on the bioavailability of soil P.