Elevated Atmospheric CO₂ Combined With Cd-soil Contamination On Soil Biological Activity In The Rhizosphere Of Wheat Seedlings

The OTC(Open Top Chambers) system was used to test the effect of elevated atmospheric CO2 combined with Cd-soil contamination on soil biological activity in the rhizosphere of wheat seedlings, by determining the exudates of wheat seedlings, soil enzyme and microbial activities in rhizosphere soil. The results and conclusions were as follows: 1. Both Cd stress alone and the combined treatments improved polyphenol oxidase activity at low Cd concentration and inhibited it at high Cd concentration. Urease activity gradually reduced under either Cd stress alone or the combined treatments, however, the activity under combined treatments was higher than that under Cd stress alone. Under either Cd stress alone or the combined treatments, cellulose activity decreased gradually, and the activity under Cd stress alone(excepting the Cd concentration of 1.0 mg/kg dry soil Cd) was greater than that under the combined treatments; Asparagine activity was inhibited by either Cd stress alone or the combined treatments and the activity showed a decreased trend with increasing Cd concentration. The activities of invertase and amylase showed insignificant changes under either Cd stress alone or the combined treatments. Neutral phosphatase activity under Cd stress alone decreased at 1.0 and 10.0 mg/kg dry soil Cd and increased at 5.0 mg/kg dry soil Cd, however, it was improved by 1.0 and 10.0 mg/kg Cd dry soil and inhibited by 5.0 mg/kg Cd dry soil under the combined treatments. Neutral phosphatase activity under the combined treatments was greater than that under Cd stress alone. The results indicated that enzymes activities in rhizosphere soil were affected by Cd stress and elevated atmospheric CO2 to the extent, moreover, cellulose and polyphenol oxidase were the most sensitive to the Cd stress alone, cellulose and neutral phosphatase were the most sensitive to the combined treatments. 2. Cd stress alone improved the actinomycetes quantity, and the combined treatments improved actinomycetes activity at low Cd concentration and inhibited it at high Cd concentration; Under Cd stress alone, fungi quantity decreased significantly at 1.0 mg/kg dry soil Cd, the combined treatments improved fungi quantity at low Cd concentration and inhibited it at high Cd concentration; Cd stress alone and the combined treatments inhibited bacterial quantity, at 1.0 mg/kg dry soil Cd, the impact was the most significant; Cd stress alone and the combined treatments showed the antagonism to fungi and actinomycetes, showed the synergy to bacteria, under the combined treatments the sensitivity of soil microbial was followed by: bacteria > actinomyces > fungi. 3.Cd stress alone showed inhibition to the microbial biomass C and microbial quotient. the combined treatments showed inhibition to the microbial biomass C and microbial quotient. Cd stress alone and the combined treatments showed synergy to the microbial biomass C and microbial quotient in rhizosphere soil. 4.Compared with Cd stress alone, the effect of phenolic acids with fungi, actinomycetes and bacteria was significantly enhanced under the combined treatments, the correlation of sugar, amino acids and organic acid with fungi, bacteria and actinomyces was significantly reduced. The combined treatments enhanceed the correlation of rhizosphere soil amino acid with polyphenol oxidase, soil organic acid with phosphatase, cellulolytic enzyme with invertase, phenolic acids with soil urease, phosphatase, sugar with amylase, cellulolytic enzyme, polyphenol oxidase, soil invertase and asparagine enzymes, reduced the correlation of root secretion and other enzymes. Cd stress alone can improve spring wheat seedling root organic acids, amino acids, phenolic acids and sugar quantity to a certain extent, the combined treatments effectly inhibited root organic acids, amino acids and sugars.