Geographic Variations of Aggregate Mechanical Stability Controlled by Inorganic Cementing Agents Across the East Asian Monsoon Region

Soil aggregate mechanical properties are of vital importance for plant growth, tillage and soil erosion, and strongly conditioned by the amount and type of cementing agents that differ with soils. As most research focus at the site scale, how aggregate mechanical stability vary across different types of soils at regional scales and underlying mechanisms remain poorly understood. Herein, seven typical zonal soils in heavy textures with an increased status of soil development were collected under two land uses (arable and forest) and at three pedogenic horizons along the mid-temperate to south-subtropical climatic gradient in the East Asian monsoon region. Aggregate tensile strength (Y), specific rupture energy (Esp) and friability (FI) were measured on air-dried aggregates in a compression test, as well as soil cementing agents related to particle size distribution, organic matter and its components, metal oxides, phyllosilicates and exchangeable cations. Y and Esp were most affected by soil type (F = 37.7 and 21.4, p < 0.001), and less affected by land use and soil horizon (F < 8.2). Y and Esp were overall remarkably larger for temperate moderately (257 ∼ 700 kPa and 40 ∼ 261 J kg−1) than for subtropical highly developed soils (171 ∼ 329 kPa and 22 ∼ 52 J kg−1); FI (0.35 ∼ 1.30) showed no distinct geographic trend. Among soil properties, exchangeable polyvalent cations, crystalline Fe and Al oxides, and vermiculite had much larger and significant explanatory power (R2 = 0.33 ∼ 0.54, p < 0.01) than traditional soil properties (organic matter, soil texture and bulk density) (R2 < 0.25, p > 0.01), suggesting the predominant roles of the inorganic cementing agents in aggregate mechanical stability across soil types. Aggregate mechanical stability was increased by exchangeable polyvalent cations and vermiculite (r = 0.71 ∼ 0.81, p < 0.001), and weakened by crystalline Fe and Al oxides (r = −0.77 ∼ −0.74, p < 0.001). Additionally, mean annual precipitation and temperature were negatively correlated with aggregate mechanical stability (r = −0.80 ∼ −0.76, p < 0.001). This study demonstrates the remarkable geographic variations of aggregate mechanical stability under the influence of climate and highlights the importance of clay mineralogy (mainly swelling clay and crystalline Fe and Al oxides) and exchangeable cations at the regional scale.