Changes in the Physical, Chemical, and Bacterial Community Characteristics of Soil in Response to Short-Term Combined Organic-Inorganic Fertilizers in a Dry Direct-Seeded Paddy Field

Dry direct-seeded rice cultivation is a simple and labor-saving planting method wherein the combined application of organic and inorganic fertilizers can improve yield. However, the effects of combined fertilizers on soil properties and bacteria in dry direct-seeded rice paddy soil are unclear. Here, four treatments, conventional fertilization (NPK), seaweed bio-organic fertilizer + NPK, Jishiwang bio-organic fertilizer + NPK, and attapulgite organic fertilizer + NPK applied for three consecutive years were tested to explore their effects on soil physical, chemical, and bacterial community characteristics in a dry direct-seeded rice paddy field. The combined fertilizers increased alkaline hydrolysis-nitrogen and available potassium while decreasing the bulk density and pH; in addition, a marked increase in the relative abundance of soil macroaggregates (>5 mm) and clay particles and a decrease in that of sand was observed. Urease and neutral phosphatase activities were the highest with the Jishiwang organic fertilizer + NPK, whereas invertase and catalase activities were the highest with attapulgite organic fertilizer + NPK. All combined fertilizers considerably increased the bacterial richness index (ACE) and Chao index; Jishiwang bio-organic fertilizer + NPK also increased the Simpson index, whereas the seaweed bio-organic fertilizer + NPK reduced it. Proteobacteria and Acidobacteria accounted for 54.25-70.49% of the total bacterial relative abundance. The relative abundance of Verrucomicrobia, Chloroflexi, Firmicutes, and Nitrospirae increased with the combined fertilizers. The correlation network analysis showed predominant antagonistic relationships. A redundancy analysis demonstrated that total nitrogen, soil organic matter, urease, and invertase were the main environmental factors affecting bacterial composition. Combined fertilizers may improve soil physical and chemical properties, fertility, and bacterial richness.