Marked Spatial Variability in Acidity Characteristics of Purple Soil at Field Scale Induced by Citrus Plantation

Purple soil, predominantly found in the Sichuan Basin of China with a favorable climate, is renowned for its fertility, making it an ideal soil for citrus cultivation. To investigate the effect of citrus plantation on the acidification characteristics of purple soil, we selected one field where citrus trees coexist with varying ages of 3, 10, and 50 years. The soil is a neutral purple soil developed from Jurassic Shaximiao Formation mudstone. A total of 138 soil samples were collected at different depths (0–20, 20–40, and 40–60 cm) beneath the canopies of these citrus trees for physicochemical property analysis. Our results indicate that citrus cultivation caused significant spatial variability in the purple soil acidity within the same field. The pH values of these soils varied from 3.97 to 7.90. The degree of soil acidification under the citrus canopies adheres to the following order: 10-year-old > 50-year-old > 3-year-old citrus trees. Soil pH values were negatively correlated with the contents of N, P, and K available in the soil, particularly exhibiting a significantly negative correlation with these soil fertility indicators under the canopy of the 10-year-old citrus at p < 0.01, suggesting that the intensive fertilizer application typical in citrus plantations accelerated soil acidification. Additionally, soil acidification was associated with an increase in the exchangeable Al3⁺ (from 0 to 7.03 cmol kg−1) and a decrease in the exchangeable Ca2⁺ (from 25.07 to 6.48 cmol kg−1), exchangeable Mg2⁺ (from1.53 to 0.62 cmol kg−1), base saturation (from 100% to 53.4%), and effective cation exchange capacity (from 24.3 to 13.1 cmol kg−1).The acidification of the purple soil enhanced the extractability of metal elements, increasing the bioavailability of essential plant nutrients, such as Fe, Mn, Cu, Zn, and Ni, as well as enhancing the mobility of harmful heavy metals like Pb and Cd. In conclusion, unlike the widespread acidification observed in Oxisols or Ultisols at the field scale, citrus cultivation caused varying degrees of acidification within purple soil at this scale. This variability in soil acidification at the field scale of purple soil can lead to a series of soil degradation problems and should be given due attention in the management of citrus and similar high-economic-value fruit trees.