Neutral purple soil acidification and mineralogical property changes due to long-term urea application in southwest China

In recent decades, the soil acidification rates of farmland in China has increased rapidly due to increased nitrogen (N) fertilizer application. When soil acidification is severe, a mutual feedbacking mechanism between soil acidity and minerals may occur. However, the relationship between soil acidification and mineralogy under long-term urea application remains unclear. Therefore, a field experiment was established in 2010 with five N fertilizer rates (0 kg<middle dot>N<middle dot>ha(-1), N0; 90 kg<middle dot>N<middle dot>ha(-1), N90; 180 kg<middle dot>N<middle dot>ha(-1), N180; 270 kg<middle dot>N<middle dot>ha(-1), N270; 360 kg<middle dot>N<middle dot>ha(-1), N360), we collected surface soil (0-20 cm) from the long-term fertilization experiment which has lasted 12 years and determined soil acidification indexes and mineralogical properties. Compared with N0, the four N application treatments reduced soil pH by 0.3-1.0 units. The N180, N270, and N360 treatments reduced the cation exchange capacity (CEC), acid-base buffering capacity (pHBC), and base saturation (BS%), but increased the exchangeable acid (EA) content. The N270 and N360 treatments significantly increased the kaolinite, smectite, free iron oxide (Fe-d), and free aluminum oxide (Al-d) contents, but decreased the illite, amorphous iron oxide (Fe-o) and amorphous aluminum oxide (Al-o) contents (P < 0.05). Soil pH, CEC, BS%, and pHBC were significantly and positively correlated with illite, and significantly and negatively correlated with kaolinite (P < 0.05). And with the increase in urea application, average particle size width of clay particles decreased gradually. Therefore, long-term urea application led to soil acidification and changed soil mineralogy, promoted the evolution of 2:1 type mineral to 1:1 type mineral, thus reducing the amount of surface negative charge of soil colloid and their ability to adsorb base ions, ultimately leading to a decline of soil resistance to acidification.