Differential response of aggregate mechanical stability to antecedent water content for different types of soils

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.