Soil potassium regulation by changes in potassium balance and iron and aluminum oxides in paddy soils subjected to long-term fertilization regimes

Abstract

Southern China’s paddy soils are poor in potassium (K) and rich in iron (Fe) and aluminum (Al) oxides, both of which are affected by fertilizer application. However, the response of soil K budget to long-term K fertilization and Fe and Al oxides remains unclear, especially in the subsurface horizons in different soil types. Here, four long-term fertilization treatments (no fertilizer, CK; inorganic nitrogen and phosphorus fertilizers, NP; NPK; and the combined NPK and manure, NPKM) were selected to determine the effects of K input and different forms of Fe and Al oxides on soil K status at two soil layers (0–20, surface; and 20–40 cm, subsurface) in red (Ferralsols) and purple (Cambisols) paddy soils across China. Overall, treatments where K fertilizer application was withheld had lower surface soil exchangeable K (EK), non-exchangeable K (NEK), and total K contents than treatments applied with K fertilizer. In contrast, the treatment including K with manure fertilizer increased EK and NEK contents. Regardless of fertilization regimes, the contents of EK and NEK in both soil depths of purple soil were significantly higher than those in their corresponding depths of red soil. Moreover, there were significantly lower EK and NEK contents in the subsurface layer than those in the surface layer of red soil, while no significant differences were observed in purple soil. A positive correlation was obtained between K balances and soil EK contents (P < 0.05) and the slopes of linear regressions in red soil was higher than that in purple soil. A three-way ANOVA showed that the lone and interactive effects of experimental site, fertilization regime, and soil depth significantly influenced the contents of Fe and Al oxides in both soils. The application of NPKM can inhibit the decrease of free Fe and Al oxides in the surface layer of soil and increase the amorphous and chelated Fe and Al oxides, especially in red soil. Redundancy analysis showed that the amorphous Fe and Al oxides were the most important factors for regulating surface soil EK content. The free and chelated oxides were the most important factors for regulating NEK contents in red and purple soils, respectively, particularly in subsurface soil. Our results imply that the combined application of inorganic fertilizer and manure is a viable strategy for improving soil K availability by increasing K balance and regulating the contents and forms of Fe and Al oxides in different depths of paddy soil.