Aliphatic carbon regulates soil water repellency in a chronosequence of grassland enclosure in the Loess Hilly Region

Considering the potential enhancement of soil water repellency (SWR) due to the increased accumulation of soil organic matter (SOM) under grassland enclosure, there may be an increased risk of soil erosion and degradation as it can reduce water infiltration and penetration into the soil. There remains a knowledge gap pertaining to the relationship between SWR and plant growth, soil physicochemical properties, SOM composition, and particle size in enclosed grassland. The main objective is to investigate the impact of different grassland enclosure years (14a, 23a, 32a, 40a, and 51a) on SWR in temperate grasslands of the Loess Hilly Region using the water drop penetration time (WDPT) method. Results showed that, at the early stage of enclosure (<32 years), in-situ grassland soils mainly showed slight water repellent and hydrophilic characteristics. In contrast, grassland soils at the late stage of enclosure (>32 years) exhibited a transition towards strong water repellency, accompanied by the emergence of severe hydrophobicity. The potential SWR also exhibited a significantly higher trend in the 40a and 51a grassland compared to the previous 32 years of enclosed grassland. Moreover, the SWR increased as the soil particle size decreased, and exhibited an upward trend with increasing years of grassland enclosure. Notably, in the 40a and 51a grasslands, SWR for sieve size of soils <0.05 mm was significantly higher than that observed in the initial 32a grasslands, reaching a strong water repellent level. These findings highlight that grassland enclosure significantly promoted the development of the SWR. Correlation analysis and random forest models showed that NO₃^--N, litter biomass, plant height, TN, CO, C–H, bulk density and plant richness were identified as the primary factors controlling SWR. The structural equation model (SEM) analyses further suggested that grassland enclosure indirectly affected SWR through aliphatic C–H groups, which was influenced by plant properties. Consequently, the consideration of SWR formation mechanism is imperative in order to mitigate the risk of soil erosion and degradation in enclosed grassland ecosystems.