Renaturation on the Loess Plateau: significant increase in soil organic carbon under different soil types over two decades

Following implementation of the Grain to Green Project (GTGP), various studies have attempted to analyze the impact of renaturation on soil organic carbon (SOC) content and regional C sequestration, but the effect of GTGP on SOC in various soil types is still vague. We analyzed the effects of land-use change on SOC according to soil properties and climate following the GTGP on the Loess Plateau from 1999 to 2020, based on the Harmonized World Soil Database (HWSD) and China's high-resolution National Soil Information Network. The results indicated that renaturation significantly increased SOC in the 0–30 cm soil layer on the Loess Plateau (Tg) by 122 Tg. This was due to the positive effects of afforestation, which contributed to an additional 260 Tg. However, grassland renaturation often decreased SOC (−57 Tg) primarily in the northwest with lower precipitation (<450 mm). The greatest SOC (Tg) increases occurred in Cambisols (+2.8) and Luvisols (+0.7) following cropland conversion to forest, whereas the greatest decreases occurred in Kastanozems (−3.7), and Anthrosols (−1.4) following conversion to grassland. Conversion of grassland to forest showed a strong increase in SOC density (kg m −2 ) increase (+1.34) compared to that of cropland conversion to forest (1.0) and grassland (0.1). The SOC in Kastanozems (−39 Tg) in natural grassland was the most vulnerable to degradation, whereas that in Cambisols in natural forest increased (+104 Tg). The SOC in Anthrosols (−33 Tg) in unchanged cropland degraded the most, however, that in Cambisols (+24 Tg) increased the most. In conclusion, to increase SOC via renaturation on the Loess Plateau, afforestation appears to be a more promising strategy, and climate and soil properties should be considered. Furthermore, this study establishes a foundation for future studies employing higher-resolution soil maps to conduct more detailed analyzes.