Effects of Long-Term Fertilization on Soil Physical and Chemical Properties of “Dangshansuli” Pear Orchard

Inefficient fertilization practices frequently take place in orchards in Dangshan County, leading to substantial changes in soil properties and pear tree growth. To comprehensively evaluate the long-term impact and identify limiting factors, this study assessed the effects of 30-year fertilization across different soil layers in “Dangshansuli” pear orchards. In May 2020, 30 soil samples were collected from a long-term fertilized plot and an unfertilized sandy control. The analyses focused on the physicochemical properties, mineral elements, heavy metals, chemical compound diversity, and allelopathic effects. The results showed that long-term fertilization significantly reduced soil pH (e.g., from 8.1 to 7.3 in the topsoil) and increased the content of soil organic matter by about 3.7-fold in the 0–20 cm layer. The contents of available potassium, exchangeable calcium, and magnesium in fertilized soil were optimal for pear growth, whereas available iron was deficient. Although fertilization led to the accumulation of heavy metals (Cu, Hg, Ni, Cr, As, Mn), their concentrations remained within national safety limits. The number of chemical compounds detected in fertilized soil was over 40% higher than in the control. Allelopathy tests indicated that 0.18 mmol·L−1 of octadecane strongly inhibited the root growth of “Shanli” (Pyrus ussuriensis Maxim.) tissue-cultured seedlings by more than 50%. These findings provide a scientific basis for optimizing fertilization strategies in “Dangshansuli” pear orchards.