Impact of applied phenolic acids on the microbes, enzymes and available nutrients in paddy soils

Phenolic acids are important compounds in plant allelopathy, hence the effects of 5-applied phenolic acids (p-hydroxybenzoic acid, ferulic acid, salicylic acid, vanillic acid, and cinnamic acid), were determined on the microbial community, enzyme activities and available nutrients in paddy soil. The application of phenolic acids significantly enhanced the soil microbial biomass carbon (MBC), soil microbial respiration rate (MBR), soil microbial populations, improved the soil enzyme activities and proved beneficial to the available nutrients (nitrogen, phosphorus and potassium) in soil. Three days after application of phenolic acids in soil they increased the soil MBC by 23.4% to 110.1% and MBR by 37.6% to 86.1% over the control, but these declined later on. The population of bacteria was increased by 43.3% to 255.0%, 11.1% to 44.4%, for fungi, and 87.8% to 226.8% for actinomycetes over the control. The activity of urease was 56.6% to 163.3%, protease activity was 75.0% to 162.5%, and sucrase activity was 109.3% to 220.9% higher than control. The available nitrogen, phosphorus and potassium in the soils were significantly increased by 99.0% to 168.5%, 134.6% to 241.7% and 11.5% to 12.9%, respectively. The promoting effects of applied phenolic acids followed the order: p-hydroxybenzoic acid > ferulic acid > vanillic acid > salicylic acid > cinnamic acid. Furthermore, all the bacteria, available phosphorus and potassium were significantly positively correlated with the five applied phenolic acids. While the actinomycetes, urease and nitrogen had significantly positive correlation with p-hydroxybenzoic acid; the fungi, actinomycetes and protease had significant correlation with ferulic acid, the urease, protease, sucrase and available nitrogen were significantly positively correlated with salicylic acid; the actinomycetes, urease, protease and nitrogen had significantly positive correlation with vanillic acid. The protease and sucrase had significantly positive correlation with cinnamic acid. It suggested that phenolic acids are useful carbon resources to establish soil microbial community, enhance the activities of soil enzyme, and accelerate the nutrients cycling in soil.