Soil Carbon Storage Characteristics of Alfalfa ( Medicago sativa ) Artificial Grasslands in the Semi-Arid Hilly Gully Region of the Loess Plateau, China

Seven artificial alfalfa grasslands (aged 3, 5, 6, 7, 8, 11, 12 a) and an abandoned land plot were selected to examine the changes in soil organic carbon content, active organic carbon content, soil organic carbon storage and active organic carbon storage in the 0–200 cm soil layer and their correlation with soil nutrients, soil water content, soil bulk density, alfalfa cover, density, height and aboveground biomass. The results show that the soil organic carbon content was between 2.02 and 4.54 g kg–1 within depths of 0–200 cm, which was higher than that in the abandoned land. The soil organic carbon storage in the 0–40 cm layers accounted for, on average, 29.50 ± 7.13% of the total storage in the 0–200 cm soil layers in the 7-year-old alfalfa grasslands and the abandoned land, and the 0–40 cm layer was the main layer of soil organic carbon storage. The soil organic carbon storage in 12 a grassland was the highest, and the abandoned land had the smallest reserve, with values of 10.22 and 5.06 kg m–2, respectively. In addition, carbon storage was, on average, 1.84 kg m–2 higher in 7-year alfalfa artificial grassland than in abandoned farmland. The proportion of soil AOC storage to SOC storage ranged from 30.24 to 58.11%; the proportion in 5-year grassland was the lowest, while it was the highest in the abandoned land. SOC storage had significant positive correlations with TN and soil active organic carbon storage, and it had negative correlations with soil bulk density at the 0.01 level (p < 0.01).