Determining the effects of organic manure substitution on soil pH in Chinese vegetable fields: a meta-analysis

Purpose Substituting chemical fertilizers with organic manure may alleviate soil acidification in vegetable fields, but the optimum organic manure substitution ratio (SR, the ratio of organic nitrogen (N) rate to total N application rate) may vary due to different climatic conditions (mean annual precipitation (MAP), mean air temperature, and climate zone), initial soil properties (soil texture, initial soil pH, and soil organic carbon content), and management practices (vegetable type, cultivation type, planting season, growth cycle, planting years, and N application rate). Methods Based on a meta-analysis of 348 pairwise observations collected from 74 studies in Chinese vegetable fields, we quantified the responses of topsoil (0–20 cm) pH and vegetable yields to various organic manure (commercial and farmyard manure) substitution ratios (low, SR ≤ 35%; medium, 35% < SR ≤ 70%; and high, SR > 70%) with chemical fertilizer alone as the control. Results The soil pH increased overall with the increase of SRs. Compared with the control, low, medium, and high SRs increased the soil pH by 2.6%, 5.6%, and 9.0% and the vegetable yield by 11.0%, 12.6%, and 3.2%, respectively. The soil pH in open (33.7%) and greenhouse (66.3%) vegetable fields under medium SR increased by 6.3% and 5.0%, respectively. The initial soil pH, MAP, and climate zone are the key factors affecting soil pH in the vegetable field, and their total contribution ratio on soil pH change accounted for 32.1% of all variables. When the initial soil pH ≤ 6, the soil pH increased with the increase of SRs. Conversely, when the initial soil pH > 8, all SRs decreased the soil pH. The soil pH also tended to increase with increasing MAP and subtropical climate zone. The contribution rate of the N application rate on soil pH change was 7.5%. The growth cycle, planting season, and vegetable type had the least impact on soil pH change. Conclusion In general, manure substitution can significantly improve the soil pH and vegetable yield. The optimal SR ranges from 35 to 70%. However, the interactions (among factors) and trade-offs during the process remain complicated, which requires further validation based on local conditions.