Objective To explore the effect of long-term application of potassium fertilizer on the distribution of available potassium in organic and inorganic complexes by affecting different forms of Al and Fe oxides in soil, and to provide a theoretical basis for the healthy cultivation of the potassium pool in red soil.

Methods Three treatments of no fertilization (CK), combined application of chemical nitrogen and phosphorus fertilizer (NP), and combined application of chemical nitrogen, phosphorus and potassium fertilizer (NPK) were selected from a long-term experiment of red soil in dryland initiated in 1986. Large (>2 mm), small (0.25−2 mm) and micro-aggregate (<0.25 mm) soil samples were obtained by a dry sieving method. The organic and inorganic complexes (<2, 2−10, 10−50, and >50 μm) were further separated by the pipette method. The composition of the organic and inorganic complexes, the content and distribution of available potassium, and Al-Fe oxides at each particle size level were analyzed, and the relationships between different Al-Fe oxides and available potassium were explored.

Results (1) Compared with CK and NP treatments, the content of <2 μm granular complexes increased by 20.5% and 149.0%, respectively, while the content of 10−50 μm granular complexes increased by 18.6% and decreased by 31.0%, respectively. (2) In the range of 0−50 μm complex size, the content of 10−50 μm complexes in large, small, and micro-aggregates was the lowest. Soil available potassium mainly existed in <2 μm particle size complexes, accounting for 54.6−85.9%. Compared with CK and NP treatments, the content of <2 μm complexes in the NPK treatment increased by 44.6% and 23.0%, respectively. (3) Compared with the NP treatment, the content of free Al (Fe) oxides in large aggregate <2 μm particle size complexes under NPK treatment significantly increased by 467.0% (30%), and the content of amorphous Al (Fe) oxides significantly decreased by 59.3% (79.7%), respectively. Compared with the NP treatment, the content of organically complexed Al oxides in <2 μm particle size complexes under NPK treatment significantly increased by 9.8%, while the content of organically complexed Al oxides in the 10-50 μm complex significantly decreased by 33.2%. (4) Free Fe-Al oxides, amorphous Al oxides, and organically complexed Al oxides were positively correlated with available K in <2 μm particle size complexes. Amorphous and organically complexed aluminum oxides explained 11.2% and 11.0% of the distribution of available potassium, respectively.

Conclusion Long-term application of K fertilizer significantly increased the proportion of available K content and free Fe and Al oxide content in large aggregate <2 μm granular complexes, while decreasing the content of amorphous Al oxides in red soil dryland. Additionally, amorphous and organically complexed aluminum oxides can positively regulate the available potassium in <2 μm complexes, helping to enhance the soil potassium pool.