Regional variation and its influencing factors for the aggregate mechanical stability of typical cultivated soils

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.