Evaluation of the Effectiveness of Soil Improvement Measures in Apple Orchards on the Loess Plateau: A Meta-Analysis

[Objective] Soil acidification, structure and fertility degradation and poor water-holding capacities are common problems in the apple orchard on the Loess Plateau, effective soil improvement techniques are crucial for enhancing soil quality, and promoting the healthy and sustainable development of the orchards. [Methods] Based on 75 publications and 1 302 field trial datasets on soil improvement in the apple orchards of the Loess Plateau, meta-analysis and principal component analysis (PCA) were used to quantitatively evaluate the effects of different improvement measures, including mulching, fertilization, and soil conditioners, on soil quality. [Results] The results indicated that grass mulching, plastic film mulching, straw mulching, and gravel mulching effectively reduced soil bulk density by 6.63%, 5.96%, 6.04%, and 6.40%, respectively. Additionally, grass mulching and straw mulching also significantly increased soil nutrient contents. Chemical fertilizer effectively increased available nitrogen (by 111.17%), available phosphorus (by 112.29%), and available potassium (by 43.95%), respectively. Organic fertilizer not only increased soil available nutrients, but also significantly increased soil organic matter (by 57.19%), total nitrogen (by 57.64%), and nitrate-nitrogen (by 94.51%). Combined application of organic and inorganic fertilizer significantly reduced soil bulk density (5.58%) and increased nutrient contents. Water-retaining agents effectively reduced soil bulk density (by 4.88%), increased total nitrogen (by 40.90%) and nitrate-nitrogen (by 143.51%). Biochar significantly increased soil organic matter (by 39.35%), nitrate-nitrogen (by 91.23%), and available phosphorus (by 89.70%). Soil quality assessment results indicated that soil quality index (SQI) ranked as 7.72 (organic fertilizer) > 7.68 (straw mulching) > 7.20 (organic-inorganic fertilizer) > 6.05 (water-retaining agent) > 5.99 (grass mulching) > 5.94 (biochar) > 5.79 (chemical fertilizer) > 3.53 (plastic film mulching) > 2.63 (gravel mulching). [Conclusion] Considering the practical conditions of soil quality and apple orchards yields, it is suggested that straw mulching and combined application of organic and inorganic fertilizer should be prioritized as measures to improve orchard soil quality, followed by organic fertilizers and grass mulching.