Effects of cadmium on soil nitrification in the rhizosphere of Robinia pseudoacacia L. seedlings under elevated atmospheric CO2 scenarios
• Elevated CO2 increased root biomass of seedlings exposed to Cd at days 90 and 135.
• Elevated CO2 decreased Cd content and increased pH in rhizospheric soils.
• Elevated CO2 stimulated AMO and HAO activities under low Cd levels.
• Elevated CO2 stimulated NXR activity in Cd-contaminated rhizosphere soils.
• Elevated CO2 altered nitrifier abundance and community structure exposed to Cd.
The individual impacts of elevated CO2 and heavy metals on soil nitrification have been widely reported. However, studies on the combined effects of elevated CO2 and heavy metals on soil nitrification are still limited. Here, a 135-day growth chamber experiment was conducted to investigate the impacts of elevated CO2 and cadmium (Cd) levels on soil nitrification in the rhizosphere of Robinia pseudoacacia L. seedlings. Elevated CO2 combined with Cd pollution generally stimulated ammonia monooxygenase (AMO), hydroxylamine oxidase (HAO), and nitrite oxidoreductase (NXR) activities. Compared to the control, the abundance of ammonia-oxidizing bacteria (AOB) at day 135 and ammonia-oxidizing archaea (AOA) increased significantly (p < 0.05) and the abundance of AOB at days 45 and 90 and that of the nitrite-oxidizing bacteria (NOB) decreased under elevated CO2 + Cd. Elevated CO2 mostly led to a significant (p < 0.05) decrease in soil nitrification intensity in the rhizosphere of R. pseudoacacia exposed to Cd. The effects of Cd, CO2, and their interaction on HAO and NXR activities were significant (p < 0.01). Soil pH, the C/N ratio, water-soluble organic carbon, water-soluble organic nitrogen (WSON), and total carbon were the dominant factors (p < 0.05) affecting nitrifying enzyme activities and nitrification intensity in rhizosphere soils. Elevated CO2 clearly affected AOA, AOB, and NOB community structures and dominant genera by shaping C/N ratio, pH, and Cd and WSON contents in rhizosphere soils under Cd exposure. Overall, the responses of pH, C/N ratio, WSON, and Cd to elevated CO2 led to changes in rhizosphere soil nitrification under the combination of elevated CO2 and Cd pollution.
The full (paywalled) article appeared on the Science of the Total Environment website at https://www.sciencedirect.com/science/article/abs/pii/S0048969721000899]]>