Microbial Biomass and PLFA Profile Changes in Rhizosphere of Pakchoi (Brassica chinensis L.) as Affected by External Cadmium Loading

Abstract

The effects of root activity on microbial response to cadmium (Cd) loading in the rhizosphere are not well understood. A pot experiment in greenhouse was conducted to investigate the effects of low Cd loading and root activity on microbial biomass and community structure in the rhizosphere of pakchoi (Brassica chinensis L.) on silty clay loam and silt loamy soil. Cd was added into soil as Cd(NO3)2 to reach concentrations ranging from 0.00 to 7.00 mg kg−1. The microbial biomass carbon (MBC) and community structure were affected by Cd concentration, root activity, and soil type. Lower Cd loading rates (< 1.00 mg kg−1) stimulated the growth of pakchoi and microorganisms, but higher Cd concentrations inhibited the growth of microorganisms. The content of phospholipid fatty acids (PLFAs) was sensitive to increased Cd levels. MBC was linearly correlated with the total PLFAs. The content of general PLFAs in the fungi was positively correlated with the available Cd in the soil, whereas those in the bacteria and actinomycetes were negatively correlated with the available Cd in the soil. These results indicated that fungi were more resistant to Cd stress than bacteria or actinomycetes, and the latter was the most sensitive to Cd stress. Microbial biomass was more abundant in the rhizosphere than in the bulk soil. Root activity enhanced the growth of microorganisms and stabilized the microbial community structure in the rhizosphere. PLFA analysis was proven to be sensitive in detecting changes in the soil microbial community in response to Cd stress and root activity.