Influence mechanisms of long-term fertilizations on the mineralization of organic matter in Ultisol

The influence mechanisms of long-term fertilizations with chemical fertilizer (NPK), a mixture of chemical fertilizer and straw (NPKS), animal manure (AM), and no fertilizer (Ck) on the mineralization of soil organic matter (SOM) in an Ultisol was studied in this study. The cumulative amount and rate of SOM mineralization were stimulated by the examined fertilizations, especially AM treatment. The order Ck <= NPK < NPKS < AM was found in the content of the soil total organic carbon, dissolved organic carbon, microbial biomass carbon, and organic carbon in each pool. The proportion of Fe/Al-associated organic carbon in the soil total organic carbon showed the opposite order. The fertilizations, especially the AM and NPKS treatments, increased the relative abundance of Actinobacteria, Proteobacteria and Ascomycota as well as the activity of beta-glucosidase and invertase. Structural equation modeling showed that the activity of the hydrolases, the content of labile organic carbon, and the composition and diversity of the microbes were positive factors in controlling SOM mineralization. Among the positive factors, the activity of the hydrolases was the most important, and the content of labile organic carbon was more important than the composition and diversity of the microbes. The information implied that the stimulation of fertilizations on the mineralization of organic matter in Ultisol progressed mainly through the increase in microbial activity.