Effect of Broccoli Residues on Enzyme Activity of Cotton Rhizosphere Soil and Relationships Between Enzyme Activity and Carbon Metabolism Characteristics

The objective of this study is to research the effect of broccoli residues (BR) on enzyme activity of cotton rhizosphere soil and relationships between enzyme activity and carbon source utilization capacity, and to provide a theoretical basis for revealing the ecological mechanism of BR to prevent Verticillium wilt disease and promote the growth of cotton.

In this study, soil treated with BR and soil treated without BR (CK) were set, and the activities of peroxidase, neutral phosphatase, arylsulfatase, chitinase, urease, β-glucosidase and N-acetyl-β-D-glucosaminidase in the rhizosphere soil of different treatments were determined, respectively. Shannon-Wiener index, Simpson index, Pielou index, McIntosh index, and richness index of different treatments were studied by Biolog-ECO technology. Principal component analysis (PCA) was used to compare the characteristics of microbial metabolic activity in rhizosphere soil under different treatments, and redundancy analysis (RDA) was used to analyze the correlations between soil enzyme activity and the utilization capacity of different types of carbon sources.

Compared with the CK, the plant height, branch and boll numbers treated with BR significantly increased by 12.73%, 16.95%, and 10.36%, respectively. The disease index of cotton Verticillium wilt treated with BR decreased by 64.19%. BR treatment significantly increased pH, nitrate nitrogen (NO₃^--N) and organic matter (OM) contents in rhizosphere soil. Meanwhile, there was no significant difference in the functional diversity index of rhizosphere microorganisms between treatments. However, BR changed the metabolic activities of rhizosphere microorganisms. BR treatment significantly increased the activity of enzymes related to cycles of different nutrient elements. In the aspect of carbon cycle, the activity of peroxidase and β-glucosidase was 2.70 and 1.95 times that of the control, respectively. In the nitrogen cycle, the activity of urease, chitinase and N-acetyl-β-D-glucosidase was 1.42, 1.59 and 1.52 times that of the control, respectively. In the phosphorus cycle, the activity of neutral phosphatase was 1.33 times that of the control. In the sulfur cycle, the activity of arylsulfatase was 1.22 times that of the control. RDA showed that soil enzyme activities under the treatment of BR were positively correlated with the utilization capacity of L-phenylalanine, L-threonine, glycogen, 2-hydroxybenzoic acid, itaconic acid, and D-malic acid, respectively.

BR changed the carbon metabolic activity of rhizosphere soil microorganisms, significantly increased the activity of soil nutrient cycling-related enzymes, pH, nitrate nitrogen, and organic matter contents in rhizosphere soil. Meanwhile, different degrees of correlations were found between soil enzyme activity and carbon metabolism characteristics.