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

TL;DR

Long-term apple farming in China's loess Plateau leads to heavy metal buildup, mainly from fertilizers, with moderate ecological risks that could threaten sustainable agriculture.

Contribution

The study uniquely compares soil depths and cultivation ages using PMF modeling to distinguish fertilizer and atmospheric sources of heavy metals in apple orchards.

Findings

Hg, Cu, and Pb concentrations in orchard soils increased significantly with cultivation age, while Zn, As, and Cr did not.

Fertilizers were the main source of Cu, Zn, As, Pb, and Cr, while Hg came mainly from atmospheric deposition.

Ecological risk was moderate overall, with Pb and Hg showing higher single-metal risks at different soil depths.

Abstract

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.

The online version contains supplementary material available at 10.1038/s41598-026-36342-3.

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