Effects of long-term afforestation and natural grassland recovery on soil properties and quality in Loess Plateau (China)

Long-term afforestation has important implications on soil properties and quality in semi-arid areas. A large-scale afforestation project has been carried out in the Loess Plateau in the last 20 years. This work aims to study the afforestation (Robinia pseudoacacia, Caragana korshinskii and natural grassland recover 10, 20, 30, and 40 years after) impacts on soil properties and quality. The results showed that coverage and root biomass (RB) was the highest 30 years after the restoration in Robinia pseudoacacia and Caragana korshinskii treatments, while the highest 40 years post-restoration in natural grasslands. Sand content and BD showed the highest values 10 years post afforestation in all study areas. Clay, Silt, mean weight diameter (MWD), and geometric mean diameter (GMD) in Robinia pseudoacacia, Caragana korshinskii had the highest values 30 years after the afforestation, while in natural grasslands, this was observed 40 years after. In Robinia pseudoacacia, Caragana korshinskii treatments, soil moisture content (SMC) reached the highest levels 30 years post afforestation at 20–40 and 40–60 cm. Regarding natural grasslands, SMC had the highest values 40 years post-afforestation. Sand content and BD increased with soil depth, while the opposite was identified in RB, clay, silt, MWD, GMD and SMC. In Robinia pseudoacacia and Caragana korshinskii treatments, soil organic matter, total nitrogen, available nitrogen, total phosphorous, and available phosphorus had the highest levels 40 years post-restoration at 0–20 cm, while at 20–40 and 40–60 cm, the highest concentrations were identified 30 years after. In all the treatments, the soil quality index (SQI) was the highest 40 years post-restoration. The values of SQI were always higher in natural grasslands than in Robinia pseudoacacia and Caragana korshinskii treatments. Overall, natural recovery (natural grasslands) is more efficient than afforestation (Robinia pseudoacacia and Caragana korshinskii treatments) in soil quality.