Medicago sativa improves soil carbon sequestration following revegetation of degraded arable land in a semi-arid environment on the Loess Plateau, China

Revegetation of degraded arable land with legume pasture has been proposed as an effective approach to improve vegetation conditions and control soil erosion on the Loess Plateau, China. This paper investigated the effects of revegetation on soil carbon pool and its fractions. Three treatments (natural revegetation, Medicago sativa L. introduction and Melilotus suaveolens L. introduction) were applied over 11 years from 2003 to 2013 on the semi-arid Loess Plateau; both aboveground biomass and soil organic carbon (SOC) stocks as well as its fractions (i.e., the light-fraction organic carbon (LFOC) and heavy-fraction organic carbon (HFOC)) were measured. We found that plant aboveground biomass was greater under Melilotus treatment than under Medicago treatment and natural revegetation in the first two years. Afterward, the aboveground biomass was greatest under Medicago treatment. Medicago treatment yielded higher SOC and HFOC stocks than the other two treatments during the experimental period. After revegetation for 10 years, the SOC stocks at the 0-1.5 m soil layer under Medicago treatment were 52.8 and 44.8 Mg ha−1 higher than those under natural revegetation and Melilotus treatments, respectively. The mean rates of SOC and HFOC sequestration in the top 20 cm of the soil layer under Medicago treatment were 0.27 and 0.23 Mg ha−1 year−1 in year 11, which were greater than the other two treatments (0.22, 0.15 and −0.07, −0.03 under the two treatments, respectively). Revegetation through introduction of Medicago to degraded arable land can potentially promote SOC accumulation by enhancing the transformation from LFOC to HFOC on the semi-arid Loess Plateau.