Long-term apple orchard cultivation drives selective accumulation and moderate ecological risk of heavy metals in loess Plateau, China.

The long-term intensive cultivation in apple orchards has led to certain heavy metals accumulation yet the relative contributions of fertilizer inputs versus atmospheric deposition remain poorly resolved. Therefore, it is crucial to assess the effect of long-term cultivation on heavy metal pollution in soils of apple orchards, for safe and sustainable fruit production. Unlike the most studies that analyzed only surface soils, this study compares two soil depths with multiple cultivation-age classes and deep-profile background values, supported by positive matrix factorization (PMF) modeling, to distinguish fertilizer-derived and atmospheric sources. A total number of 128 soil samples were collected from two depths (0–20 and 20–40 cm) in apple orchards of varying ages up to 30 years. Overall, the soil pH was alkaline with high soil organic matter, total nitrogen, total phosphorus, total potassium and heavy metal contents. The concentrations of Hg, Cu, and Pb in orchard soils significantly increased with cultivation age at rates of 0.00165 mg kg−1, 0.244 mg kg−1, and 0.208 mg kg−1 per year (equivalent to 4.34%, 0.88%, and 1.11% per year relative to background levels), respectively, while Zn, As, and Cr showed no significant accumulation over time. The cumulative pollution load index of heavy metals was at moderate level in both depths (1.10 and 1.05 respectively). The single heavy metals pollution load index was variable Pb pollution load was high at 0–20 cm depth and Hg at 20–40 cm depth. The cumulative ecological risk index was at considerable level at both depths. However, the single ecological risk index of Hg was only at moderate level. Hg ecological risk is dominated by atmospheric deposition, while Cu, Pb, Zn, As, and Cr are fertilizer-driven. Ecological risk is based on total concentrations; bioavailability was not measured. The PMF model identified inorganic and organic fertilizers as the major contributing factors in Cu, Zn, As, Pb and Cr accumulation, whereas Hg accumulation was mainly due to atmospheric deposition. The study suggests regulating the use of fertilizers inputs and implementing control and remediation practices for sustainable fruit production in apple orchards.