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