Rhizosphere effect decreases soil pH in arid but not humid areas across global agricultural ecosystems

Abstract

The pH of the rhizosphere strongly regulates nutrient transformations, microbial activity, and plant hormone metabolism in agroecosystems, thereby shaping crop growth, stress tolerance, and yield. Yet a global synthesis quantifying the magnitude, direction, and drivers of rhizosphere effects on soil pH has been lacking. To address this gap, we conducted a meta-analysis incorporating 906 individual cases of the rhizosphere effect on soil pH extracted from 102 peer-reviewed studies, along with associated biotic and abiotic variables across global agricultural ecosystems. Overall, the rhizosphere effect significantly decreased in soil pH by 1.04 % ± 0.20 %, but the effect varied with climate, management, crop identity, and soil properties. Specifically, the rhizosphere effect reduced soil pH by 1.34 % ± 0.10 % in arid areas (aridity index <0.65), while showing no significant change (ranging from −0.26 % to 0.08 %) in humid areas (aridity index >0.65). This arid humid contrast likely reflects differences in initial alkalinity/buffering and the balance of plant–microbial processes. Beyond climatic, the pH lowering effect of the rhizosphere was more pronounced in soils with an initial pH of 6.5–7.5 and soil organic carbon content of 10–20 g kg⁻¹. Among crops, soybean showed the largest decreases (arid: 3.00 % ± 0.43 %; humid: 1.73 % ± 0.34 %). Moreover, in humid areas, soil nutrient availability and microbial communities/functions were more sensitive to rhizosphere-induced pH shifts than in arid regions. Collectively, these findings highlight the potential for agricultural practices, particularly soybean cultivation in arid regions, to contribute substantially to global scale acidification, especially in soils with near neutral pH.