Splash erosion-induced soil aggregate turnover and associated organic carbon dynamics

The splash erosion-induced soil aggregate turnover is important to understand the mechanisms of soil degradation and soil organic carbon (SOC) cycling. However, the relationship between aggregate turnover and rainfall impact has not been explored directly because of methodological difficulties. The soil samples, developed from Quaternary red clay, were adopted from three land use types: natural forest (NF), garden/shrubs (AS), and abandoned meadow (AM). By using rare earth oxides to trace the transformation in soil aggregates of four size-fractions (>2 mm, large macroaggregate; 0.25–2 mm, small macroaggregate; 0.053–0.25 mm, microaggregate; <0.053 mm, silt and clay fraction), we investigated aggregate turnover based on splash experiments at four different rainfall kinetic energies (501.95, 326.00, 252.50 and 153.39 J·m−2·h−1). After splash erosion, the rare earth oxides concentrations and SOC content in different soil aggregate fractions were measured. Our results indicated that the soil aggregates mainly followed the direction of breakdown, with higher cumulative breakdown rates than formation rates. Specifically, with the rainfall kinetic energy increased to 501.95 J·m−2·h−1, the three soil samples (NF, AS, AM) exhibited the highest breakdown rates of 48.35%, 31.95% and 39.29% in large macroaggregates, respectively. Meanwhile, the formation pathway was mainly in the direction of forming small macroaggregates. Compared to soil aggregate from NF, AS and AM aggregates had higher proportion of silt and clay fraction transformed into larger aggregates. Moreover, the changes of SOC varied in aggregates during splash process. With the increase of rainfall kinetic energy, the organic matter content gradually decreased in large aggregates and microaggregates in AM and AS (P < 0.05), in contrast to a trend of first increasing and then decreasing (P < 0.05) in their small aggregates. In the clay particles, with the increase of rainfall kinetic energy, the SOC content gradually decreased in NF (P < 0.05) and fluctuated in AS, with both their organic matter content lower than before the splash test (P < 0.05). The loss of SOC in total soil was linearly related to the net breakdown rate of soil aggregates (P < 0.05, R2 =0.635). Therefore, we proposed a framework to highlight the quantitative relationship between aggregate turnover and SOC degradation under splash, which may enrich the mechanisms of soil erosion and soil degradation in the positions where erosion takes place.