Elevated CO₂ increases glomalin-related soil protein (GRSP) in the rhizosphere of Robinia pseudoacacia L. seedlings in Pb- and Cd-contaminated soils

Glomalin-related soil protein (GRSP), which contains glycoproteins produced by arbuscular mycorrhizal fungi (AMF), as well as non-mycorrhizal-related heat-stable proteins, lipids, and humic materials, is generally categorized into two fractions: easily extractable GRSP (EE-GRSP) and total GRSP (T-GRSP). GRSP plays an important role in soil carbon (C) sequestration and can stabilize heavy metals such as lead (Pb), cadmium (Cd), and manganese (Mn). Soil contamination by heavy metals is occurring in conjunction with rising atmospheric CO₂ in natural ecosystems due to human activities. However, the response of GRSP to elevated CO₂ combined with heavy metal contamination has not been widely reported. Here, we investigated the response of GRSP to elevated CO₂ in the rhizosphere of Robinia pseudoacacia L. seedlings in Pb- and Cd-contaminated soils. Elevated CO₂ (700 μmol mol⁻¹) significantly increased T- and EE- GRSP concentrations in soils contaminated with Cd, Pb or Cd + Pb. GRSP contributed more carbon to the rhizosphere soil organic carbon pool under elevated CO₂ + heavy metals than under ambient CO₂. The amount of Cd and Pb bound to GRSP was significantly higher under elevated (compared to ambient) CO₂; and elevated CO₂ increased the ratio of GRSP-bound Cd and Pb to total Cd and Pb. However, available Cd and Pb in rhizosphere soil under increased elevated CO₂ compared to ambient CO₂. The combination of both metals and elevated CO₂ led to a significant increase in available Pb in rhizosphere soil compared to the Pb treatment alone. In conclusion, increased GRSP produced under elevated CO₂ could contribute to sequestration of soil pollutants by adsorption of Cd and Pb. - Highlights: • Elevated CO₂ increased GRSP contents in the rhizosphere under Cd and Pb stress. • Elevated CO₂ increased the amount of GRSP-bound Cd and Pb. • Elevated CO₂ improved the ratio of GRSP-bound Cd and Pb to total Cd and Pb. • Increased GRSP under elevated CO₂ contributed to metal sequestration in soil. - Increased GRSP produced under elevated CO₂ could contribute to sequestration of soil pollutants via adsorption of Cd and Pb.