Planting age of peach affects soil metal accumulation and distribution in soil profile

Changes in soil available metal, particularly, distribution changes in the soil profile relative to long-term peach cultivation, have not been studied thoroughly. Soil samples at depths of up to 100 cm in the soil profile were taken from peach orchards that were cultivated for 7, 15, and 50 years. We analyzed available metals (Zn, Fe, Mn, Al, and Cu), soil pH, total nitrogen (TN), nitrate nitrogen (NO₃^--N), and ammonium nitrogen (NH₄⁺-N) in different soil layers (0-10 cm, 10-20 cm, 20-40 cm, 40-60 cm, 60-80 cm, and 80-100 cm). The results showed that available metals were enriched in the topsoil (0-20 cm) after 50 years of peach cultivation, with the highest contents of available Fe (1.0 mg kg^-1), Al (188.2 mg kg^-1), and Cu (0.7 mg kg^-1) in the 10-20 cm layer and Zn (11.7 mg kg^-1) in the 0-10 cm layer. The soil pH in the 0-40 cm layer decreased with increasing periods of peach cultivation, with the lowest pH (4.2) in the 10-20 cm layer after 50 years of peach cultivation. Soil pH was negatively correlated with available metals (R = - 0.579, P < 0.05 for Zn, R = - 0.727, P < 0.01 for Fe, R = - 0.792, P < 0.01 for Mn, R = - 0.690, P < 0.01 for Al, and R = - 0.783, P < 0.01 for Cu). The highest contents of NO₃^--N (212.9 mg kg^-1) and NH₄⁺-N (10.2 mg kg^-1) were observed in the 50-year-old 0-10 cm layer, and soil pH was correlated negatively with the contents of NO₃^--N and NH₄⁺-N. Overall, our results indicated that the continuous input of nitrogen fertilizers may play an important role in soil acidification, and soil acidification may result in high accumulation of available metals in soil after long-term peach cultivation.