Evaluating the effect of soil internal forces on the stability of natural soil aggregates during vegetation restoration

Purpose Soil aggregate stability associates closely with many environmental and agricultural problems. Some studies using the model aggregates find that soil internal forces exert an important impact on soil aggregate stability. However, the effect of soil internal forces on the stability of natural soil aggregates during vegetation restoration has been little studied. Methods Five different succession stages (including farmland, grassland, shrubland, early forest, and climax forest) were chosen in the Ziwuling forest region (in the central part of the Loess Plateau, China), and the size distribution and stability of natural soil aggregates under different succession stages were investigated through dry sieving and wet sieving with ethanol and deionized water prewetting. Results The size distribution of aggregates determined by dry sieving showed decreased first and then increased with decreasing particle size, dominant sizes were 5–1- and < 0.15-mm fractions. The ethanol prewetting treatment showed a distribution similar to that determined by dry sieving. The size distribution of aggregates in deionized water prewetting treatment was mainly < 0.15-mm fractions. Moreover, the mean weight diameter values of farmland, grassland, shrubland, early forest, and climax forest soils determined in ethanol treatment (MWDe) were 4.62, 1.45, 1.31, 1.32, and 1.17 times than those in deionized water treatment (MWDw). In addition, in the fast wetting process, the preservation rate of soil aggregates was higher in 0.5–0.053-mm fractions than in > 0.5-mm fractions. The relative internal force index (RII) of grassland, shrubland, early forest, and climax forest soils decreased by 47%, 54%, 55%, and 64% compared with that of farmland soil, respectively. Conclusions These findings demonstrate that soil internal forces could significantly break down the aggregates and lead to decreasing water stability of aggregates. The vegetation restoration process decreased the repulsive soil internal forces, thereby decreasing the degree of disintegration of aggregates and consequently increased the water stability of aggregates.