Linkage between aggregate stability of granitic soils and the permanent gully erosion in subtropical China

Abstract

As an integrated parameter for soil structure and function, aggregate stability is crucial to soil quality and erodibility, resulting in extensive studies on the formation and stability mechanisms of aggregates, especially for heavy-textured soils. Granitic soils with a wide range of particle composition are concentrated by large-scale permanent gullies in subtropical China, while limited information is available on the aggregate structure of granitic soils. To investigate the formation mechanism of these gullies, granitic soils were sampled systemically from different genetic horizons from temperate (no gully erosion) and subtropic (concentrated by gully erosion) areas, and the aggregate size distribution and stability of these soil were measured by Yoder method with sand correction. The results showed that macroaggregate (especially the 0.25 ~ 2 mm-sized fraction) occupied 56.92% of the aggregation. The disaggregation reduction (DR) ranging from 0.18 to 1.12 mm was shown to be more reasonable than the mean weight diameter in characterizing the aggregate stability of granitic soils. Additionally, macroaggregate and DR were unveiled to increase with the increase of weathering degree, implying that the humid climate with high rainfall conditions in the subtropic areas was responsible for the formation of the permanent gullies at a regional scale. The formation and stability of aggregates were generally determined by clay content (R2 = 30.6%), soil organic carbon (R2 = 20.1%), free aluminum oxides (R2 =11.1%), and free iron oxides (R2 =6.7%). Among these influential factors, clay fraction associated with aggregates in a size of > 2 mm was a promising variable for predicting DR (R2 =75%, p < 0.001). Collectively, the aggregate structure of granitic soils is directly determined by inorganic cementing matters controlled by the variation of phyllosilicate mineralogy under different rainfall conditions, while the coarse texture of granitic soils emphasizes the contribution of organic cementing agent (SOC). This study facilitates the understanding of aggregate structure and its stability mechanism of coarse-grained soils as well as erosion control in granitic soil areas.