Effects of Long-Term Application of Cl-Containing Fertilizers on Chloride Content and Acidification in Brown Soil

Chloride is a necessary micronutrient for plant growth, and with long-term application of chlorine-containing fertilizer, significant amounts of Cl− are introduced into farmland ecosystems. Many recent studies have focused on chlorinated fertilizers’ effects on crop yield and quality, while few studies have examined their effects on soil properties. To determine the effects of the long-term application of chlorinated fertilizer on soil Cl− ions and soil acidification, we conducted a 35 year long-term study of chlorine-containing fertilizer in a peanut–corn rotation (since 1984). We chose three of eight experimental treatments: (1) no fertilization (CK), (2) urea + monoammonium phosphate + potassium chloride (lower Cl), and (3) urea + ammonium chlorophosphite + potassium chloride (higher Cl). We measured the Cl− concentrations, pH, cation exchange capacity (CEC), exchangeable H+ and Al3+, and exchangeable alkali ions (K+, Na+, Ca2+, and Mg2+) at different soil depths (0–20, 20–40, and 40–60 cm). Compared to CK, chlorine-containing fertilizer application significantly increased the content of Cl− in the soil profile. Compared to the control, the Cl− content of lower Cl treatment of 0–20, 20–40, and 40–60 cm soil layers increased 11.08, 9.01, 15.21 mg kg−1 respectively, and the higher Cl treatment increased 38.71, 34.71, 32.05 mg kg−1 respectively. Compared to CK, chlorine-containing fertilizer application significantly reduced the soil pH by 0.41, 0.17, and 0.25 and 1.25, 0.91, and 0.88, respectively, in the 0–20, 20–40, and 40–60 cm soil layers. The higher chlorine treatment significantly increased the exchangeable Al3+ content in the 0–20, 20–40, and 40–60 cm soil layers by 2.79, 1.64, and 0.94 mg kg−1, respectively, significantly increasing the risk of aluminum toxicity. Furthermore, the soil exchangeable Ca2+ and Mg2+ contents and soil base saturation were significantly reduced. Although the Cl− content in the high-chlorine-treated soil was far from endangering crop growth, it accelerates soil acidification and the loss of base ions and increases the risk of Al3+ toxicity, which will not only affect the topsoil, but also the subsoil. Therefore, the long-term application of high content chloride fertilizers should be avoided in agricultural production.