Effect of natural vegetation restoration age on slope soil anti-scourability in gully region of Loess Plateau

Some water and soil conservation projects were continuously conducted on the Loess Plateau since 1970s. Especially, in the gully region, some targeted measures were implemented in different geomorphic positions (gully heads, hill slopes and gully channel) to control soil and water loss. To investigate the effect of vegetation restoration age on soil anti-scourability, an in-situ runoff scouring experiment was carried out in grasslands of five natural vegetation restoration ages with the slope farmland as CK. This study explored the variations in soil anti-scourability (ANS) with vegetation restoration years and its relationships with soil properties and root characteristics. In this study, soil properties of soil bulk density (SBD), saturated hydraulic conductivity (SHC), soil organic matter content (OMC), soil disintegration rate (SDR), soil particle size distribution and water-stable aggregate (WSA) and root mass densities (RD) of different root diameters (<0.5, 0.5-1.0, 1.0-2.0, >2.0 mm) were sampled and measured in each site. Each soil property and root index was repeated three times for each sampling site. The scouring experimental plot size was set as 2.0 m×0.25 m to test soil anti-scourability, which included flow transition section (0.5 m length) and soil test section (1.5 m length). The flow discharge was determined as 10 L/min. The results indicated that 1) Vegetation restoration could significantly improve soil properties. The SBD generally decreased after 3 years restoration, and the SDR decreased 28.85%~88.22% compared with the CK (slope farmland). The SBD and SDR exponentially decreased with the increase of restoration time (P<0.01). The SHC, OMC, WSA and MWD were 0.85-4.25 times, 1.12-5.61 times, 13.36%-91.82% and 10.18%-145.16% greater than those of the CK, respectively, and the four indicators linearly increased with the restoration time increasing (P<0.01). Moreover, the RD of different root diameters (<0.5, 0.5-1.0, 1.0-2.0, >2.0 mm) also exponentially increased with the increase of restoration time (P<0.01). 2) Vegetation restoration could create more stable soil structure by improving the effect of root physical consolidation and the biochemical effect of root exudate matters, and thus improves the soil resistance to flow scouring. Soil anti-scourability after 3 years restoration was greater than that of the CK, but had no significant difference with the CK. While the ANS significantly increased by 1.98-9.82 times when the restoration time was higher than 3 years. Regression analysis showed that the ANS increased linearly with the increase of restoration time (R2=0.98, P<0.01). 3) Partial correlation analysis showed that the ANS had a negative related with SBD and SDR (P<0.05), but positive related with SHC, OMC, WSA and MWD (P<0.01). The degree of correlation between ANS and OMC and MWD was higher than other soil property parameters. The Hill curve function could simulate well the relationships between ANS and RD of different root diameters (R2=0.913-0.978, P<0.01). The root diameter level of <0.5 mm was the optimal root diameter in improving ANS. 4) The enhancement of soil anti-scourability was closely related with the improvement of soil properties and the increase of root mass density. The SBD, MWD and RD of <0.5 mm were the key factors influencing ANS. The results could improve scientific reference for the assessment of soil and water conservation benefits and the optimization of vegetation measures.