Aggregate mechanical stability greatly influences plant root growth, energy efficiency and ease of tillage, wind and water erosion. A mechanistic understanding of soil aggregate mechanical stability is of great significance for evaluating and improving soil quality. To date, how aggregate mechanical properties vary over different types of soils remains poorly understood. To this end, four cultivated soils including Black soil, Cinnamon soil, Red soil, and Latosol were separately selected in typical agricultural producing areas from the north to south in eastern China. The objectives of this study was to investigate the difference of aggregates among differet typical arable soils in China and its influential factors. The soil samples were collected from the top 20 cm. Tensile strength, specific rupture energy, friability index of different aggregate sizes (1-2, >3-5, >5-8, >8-10 mm) were calculated and measured by a compression test to analyze their relationships with soil physicochemical properties and climate factors. The results were shown as follows: Tensile strength and specific rupture energy decreased gradually from north to south and decreased with increasing aggregate size. The tensile strength of 1-2 mm aggregates (a mean value of 661.5 kPa) was significantly higher than that of ≥3 mm aggregates (a mean value of 240.3 kPa); there was no significant difference between the tensile strength of >5-8 mm and >8-10 mm aggregates (P>0.05). The specific variation of specific rupture energy differed with soil types. The friability index increased first and then decreased from south to north, with the highest and lowest values in red soil (0.87) and black soil (0.47), respectively. Compared to the Black and Cinnamon soils in temperate zones, the variations of friability index among aggregate sizes were more remarkable for Red soil and Latosol in subtropical and tropical zones, where friability index was larger for small aggregates (1.10 and 0.76) than for large aggregates (0.65 and 0.58). Tensile strength and specific rupture energy were correlated positively with vermiculite (r=0.73 and 0.70, P<0.01), while correlated negatively with mean annual precipitation (r=−0.72 and −0.72, P<0.01). In addition, The tensile strength also showed significant negative correlations with free aluminum oxides, 1.4 nm intergrade mineral, and mean annual temperature (r=−0.67-−0.66, P<0.01). The friability index was correlated positively with mean annual precipitation (r=0.66, P<0.01), while correlated negatively with vermiculite, carbon-nitrogen ratio, and amorphous manganese oxides (r=−0.75-−0.66, P<0.01), indicating that climate factors controlled the spatial differentiation of aggregate mechanical stability through the type of clay minerals and the content of metal oxides at the regional scale. Stepwise regression analysis showed that aggregate size, vermiculite or mean annual precipitation, and amorphous manganese oxides were good indicators to predict aggregate mechanical stability (R²_adj≥0.47, P<0.01). The obtained results provide the valuable information for the prediction and improvement of agricultural soil quality across different climate zones.

Aggregate mechanical stability represents the aggregates' capability to endure mechanical forces that tend to break them apart and is an essential index for appraising soil quality. Aggregate mechanical stability is influenced by both intrinsic soil properties and external factors. Most previous studies focused on a single soil type and the conclusions of different studies were different. To date, there is still a lack of systematic research on the response characteristics of aggregate mechanical stability to antecedent water content in different soil types. In this study, we collected four typical cultivated soils from the major agricultural regions in eastern China (Black soil, Cinnamon soil, Red soil and Latosol). Compression test was conducted to determine the tensile strength, specific rupture energy and friability index of soil aggregates at different antecedent water contents (4%, 6%, 8%, 10%, 12%, 15% and 20%); the relationship between the tensile strength, specific rupture energy and friability index of these aggregates and the physicochemical properties of the soils were analyzed to identify the influencing factors. The results showed that tensile strength and specific rupture energy were mainly affected by the antecedent water content (F=99.6 and 43.9), followed by soil type (F=28.3 and 21.9). Both indices overall decreased with antecedent water content in different manners, which could be well characterized by an exponential function relationship (R²=0.70-0.98, P<0.05). The value of parameter β of the exponential function reflects the sensitivity of the aggregate mechanical stability to the antecedent water content. For Black soil and Cinnamon soil, The tensile strength of aggregates and the specific rupture energy declined swiftly as the soil water content rose, while for Red soil and Latosol, they varied gently. Among soil types, Cinnamon soil had the greatest aggregate mechanical stability; except for Latosol, the friability index of Black, Cinnamon and Red soil aggregates were significantly higher at >10% than <10% antecedent water contents. Regression analysis showed that parameter α was negatively correlated with amorphous manganese oxide and 1.4 nm transition minerals (R²=0.95-0.99, P<0.05), and positively with exchanged calcium ions (R²=0.87, P<0.05); parameter β was positively correlated with amorphous manganese oxide and 1.4 nm transition minerals (R²=0.88-0.99, P<0.05), and negatively with exchangeable calcium ions and vermiculite contents (R²=0.94-0.98, P<0.05); friability index had a significant positive correlation with the content of free iron and aluminum oxides (R²=0.91-0.94, P<0.05), and a negative one with silt content(R²=0.92, P<0.05). For different types of soils, amorphous manganese oxide, free state iron and aluminum oxides can weaken the sensitivity of aggregate mechanical stability to antecedent water content, which was opposite to silt, exchangeable calcium ions, and vermiculite. In conclusion, as controlled by inorganic cementing agents, the sensitivity of aggregate mechanical stability to soil water content decreased with the increase of soil development. The obtained results will be of great significance to reveal the characteristics and mechanisms of aggregate mechanical stability against environmental changes and facilitate the scientific managements of soil quality in different agricultural regions.