Alfalfa monocultures promote soil organic carbon accumulation to a greater extent than perennial grass monocultures or grass-alfalfa mixtures

Abstract

Perennial forages provide a range of environmental benefits in managing water, nutrients and carbon. This study examined the changes in soil organic carbon stocks and the underlying mechanisms in mixtures and monocultures of alfalfa-Medicago sativa and perennial grass-Leymus chinensis in established grasslands in a semi-arid region. In constructed grasslands three systems (L. chinensis monocultures, L. chinensis-M. sativa mixture and M. sativa monoculture) were assessed for biological N2 fixation (BNF), biomass accumulation and nutrient store in aboveground shoots and roots, soil total organic carbon (SOC) and labile organic carbon stocks, as well as soil nitrogen (N) and phosphorus (P) availability over four years. BNF decreased by 59% from mixtures to M. sativa monocultures due to lower legume N fixation efficiency. However, the M. sativa monocultures had greater aboveground biomass and a significantly increased root biomass compared with other treatments, through increased soil N and P acquisition. M. sativa cultivation, especially as a monoculture increased SOC stocks at the 0–150 cm soil depth compared to L. chinensis monocultures but were dependent on root biomass carbon inputs. In comparison to L. chinensis monocultures and mixtures, M. sativa monocultures significantly increased soil labile carbon [permanganate oxidizable extract] at 0–150 cm soil depth, resulting from increased root biomass carbon input. We conclude that M. sativa monocultures, rather than mixtures can promote greater SOC, and in particular the soil labile organic carbon sequestration compared to L. chinensis monoculture in the short term.