Effects of cadmium on soil nitrification in the rhizosphere of Robinia pseudoacacia L. seedlings under elevated atmospheric CO₂ scenarios

Xia Jia^a, Yunfeng Gao^b, Xiaodi Li^a, Yonghua Zhao^b, Lu Wang^a, Chunyan Zhang^a

Key laboratory of Degraded and Unused Land Consolidation Engineering, the Ministry of Land and Resources, Key Laboratory of Subsurface Hydrology and Ecological Effects in Arid Region of Ministry of Education, Shaanxi Key Laboratory of Land Consolidation, School of Water and Environment, Chang'an University, Xi'an 710054, PR China.

Abstract

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.

Keywords: Ammonia-oxidizing bacteria, Ammonia-oxidizing archaea, Nitrite-oxidizing bacteria, Nitrifying enzyme activity, Nitrification intensity.