Disentangling the effects of edaphic and vegetational properties on soil aggregate stability in riparian zones along a gradient of flooding stress

Riparian zone is an ecotone between aquatic and terrestrial ecosystems, where soil aggregate stability is fundamental in supporting ecosystem functions and associated services. Yet, little is known about the resilience of riparian soil aggregate stability along a gradient of flooding stress. Here, we studied a riparian ecosystem across hillslopes in an early successional stage (a grass-dominated successional stage) along a gradient of flooding intensity (FI) of a hydrological regime triggered by operation of the Three Gorges Dam (TGD), China, which is one of the biggest dams in the world. Size distribution and stability of soil aggregates were tested by the wet-sieving method. Path analysis and structural equation modelling (SEM) were applied to determine the contributions of abiotic and biotic factors (including environmental variables, soil physicochemical properties, vegetational properties) to soil aggregate stability. Results showed that soil aggregate stability decreased with FI (a ratio of flooded days to whole days) which was a major determinant of soil aggregation. Also, increasing soil water content (SWC) greatly contributed to the decrease of aggregate stability, while contents of soil organic matter and total nitrogen correlated with an increase of aggregate stability. In terms of vegetational properties, aboveground biomass, plant species richness, and plant functional types partially explained soil aggregation. Significantly, distribution of the riparian dominant pioneer species, Cynodon dactylon, was affected by FI and SWC, and a higher density of the species led to a lower amount of small macro-aggregates (0.25–2 mm). Our study reveals that the soil stabilizing process is mainly governed by FI and edaphic factors, although vegetational properties have potentials to influence the soil aggregate stability. Our study highlights the vital processes for ecological restoration of riparian ecosystem in an early successional stage under a drastic change in hydrological regime.