Revegetation decreases soil carbon loss through preventing and controlling rill erosion in steep loess gully-slopes under extreme rainfall

Abstract

Soil erosion-induced carbon loss exacerbates land degradation and carbon pool depletion. Rill erosion, a dominant form of slope erosion, has an unclear contribution mechanism to carbon transport under extreme rainfall, particularly the regulatory effect of revegetation on pathways of carbon loss during rill development, which urgently requires quantification. This study established six experimental plots (20 m×2.5 m) on a 35° gully-slope in the Loess Plateau of China, with three grass coverages: 0% (bare land, BL), 30–40% (low-coverage, LC) and 70–80% (high-coverage, HC). Six simulated extreme rainfall events were conducted, each at a constant intensity of 60 mm·h−1 and with a cumulative rainfall of 180 mm per event. HC plots completely inhibited rill erosion, while LC plots only mitigated it. As coverage increased, rill morphology transitioned from narrow-deep to wide-shallow and eventually disappeared. Cumulative rainfall and vegetation restoration jointly reversed the runoff-sediment relationship during rill erosion development. Sediment-bound organic carbon (SBOC) loss was the dominant form of total organic carbon (TOC) loss, exceeding 91% in all plots. Rill erosion contributed more than 67% to soil organic carbon (SOC) loss in both BL and LC plots. Rill erosion influenced the runoff-TOC relationship more strongly than the sediment-TOC relationship. Sediment, rather than runoff, was the primary carrier of organic carbon loss; the inhibitory effect of vegetation on organic carbon loss was mainly indirect (-0.552), with a weaker direct effect (-0.111). Revegetation effectively reduces organic carbon loss by inhibiting erosion, particularly rill erosion. These findings provide a crucial scientific basis for achieving the dual objectives of synergistic erosion control and soil carbon pool protection through vegetation restoration.