Potassium organomineral fertilizer alters the microbiome of a sandy loam tropical soil

Soil fertility depends on a series of physical, chemical, and biological factors that interact to create an environment favorable to plant growth. In conditions of low fertility, mineral and organic fertilizers are commonly used in agricultural systems. However, studies on the impact of these fertilizers on the soil microbiota are limited, particularly those focusing on fertilizers containing potassium, a crucial plant macronutrient. In this study, we evaluated how potassium organomineral fertilizer (OMF) influences soil bacterial and fungal communities compared to potassium chloride (KCl), a conventional agricultural source in the humid tropics. Both the conventional particle size and the ground form used to produce OMF were examined. Samples of a sandy loam Typic Hapludox were incubated with the fertilizers, and the bacterial and fungal communities were assessed through sequencing of the 16S rRNA and ITS regions, respectively. OMF reduced the richness and diversity of bacterial and fungal communities, and this effect was attributed to the nutrient composition of OMF, rich in calcium and magnesium, which were absent in other treatments. Different fertilizers selected specific bacterial and fungal phyla, demonstrating their ability to influence community structure. Notably, OMF favored Proteobacteria and Bacteroidota, while KCl increased the abundance of Actinobacteriota and Firmicutes. The granulometry of KCl also influenced the soil microbial community, with smaller granules having greater soil contact, thus affecting chemical conditions and microbial composition. The OMF treatment enriched several bacterial genera, including Microvirga, Phenylobacterium, and Azospirillum, while increasing only the fungal genus Ascobolus. While OMF application reduced microbial richness and diversity, it favored specific microbial groups beneficial for agriculture, such as those involved in organic compound degradation and nitrogen cycling. These compositional changes may have significant implications for nutrient cycling and soil organic matter decomposition, highlighting the need for further studies to understand the underlying mechanisms and broader ecological impacts.