Importance of soil interparticle forces and organic matter for aggregate stability in a temperate soil and a subtropical soil

Soil organic matter (SOM) and interparticle forces, including the electrostatic repulsive force (ERF), surface hydration repulsive force (SHRF) and van der Waals attractive force (vDWAF), play crucial roles in aggregate stability. However, few studies investigated their coupled effects on aggregate stability in variably charged soils, in which soil particle interactions are more complex than in permanently charged soils due to the coexistence of positive and negative charges and the variable surface charge characteristics. Therefore, this study aims to: 1) investigate the combined effects of soil solution pH and the overlapping of electric double layers (EDLs) between positively charged Fe/Al (hydro)oxides and negatively charged SOM on the ERF and aggregate stability before (control) and after SOM removal and after straw incubation for 240 days; and 2) evaluate the importance of soil interparticle forces and SOM for aggregate stability in a permanently charged temperate soil (Vertisol) and a variably charged subtropical soil (Ultisol). Soil aggregate stability was determined by measuring the release of small particles (w(< d)%) after aggregate breakdown at different KCl concentrations (10(-1) to 10(-5) mol L-1). Soil solution pH was adjusted by varying KCl concentration. In both soils, the w(< d)% increased exponentially with the net force of the soil interparticle forces. However, in contrast to the Vertisol, the w(<d)% of the differently treated Ultisol showed almost no change before the KCl concentration decreased to 10(-2) mol L-1 and continuous changes after the KCl concentration decreased to 10(-2) MOl L-1. In addition, the differences in the w (< d)% between the SOM removal treatment and the control and straw incubation treatments were much larger in the Ultisol than in the Vertisol. In the Ultisol, the ERF did exhibit little and continuous increase over the whole range of tested pH. When considering the impact of the overlapping of oppositely charged EDLs between Fe/Al (hydro)oxides and SOM, a much stronger reduction in ERF was observed in the control and straw incubation treatments and at lower KCl concentrations. Consequently, in both the Vertisol and Ultisol, aggregate stability is essentially controlled by the soil interparticle forces, whereas the effects of soil solution pH and the overlapping of oppositely charged EDLs between SOM and Fe/Al (hydro)oxides on the ERF must be considered in variably charged soils (i.e. Ultisol). Moreover, SOM can play a more important role in aggregate stability in subtropical soil than in temperate soil.