Rhizosphere Ecological Characteristics of Trifolium repens L. under Cadmium Stress: with Focus on the Spatial Distribution Pattern of Enzyme Activity

This study aims to illustrate the temporal and spatial patterns of rhizosphere microecological characteristics of plants highly adapted to heavy metals under different levels of heavy metal stress, to reveal the rhizosphere nutrient cycling and the mechanism of enhanced stress tolerance. The dynamic changes of rhizosphere microecology of Trifolium repens L. (white clover) were studied under different concentrations of Cd treatment. The spatial variation of soil enzyme activities was investigated in situ and microscopically by in situ zymography and the spatial distribution models of enzyme activities were developed. The results showed that the rhizosphere ecosystem remained relatively stable under 40 days of stress, with pH ranging from 7.13 to 7.26 and organic matter contents ranging from 14.83 to 18.09 g kg^− 1. In addition, pH and soil organic matter (SOM) were important ecological factors affecting Cd activation in the rhizosphere. Based on soil zymography analysis, both phosphatase and N-acetyl-glucosaminidase hotspots had root effects, and a maximum hotspot area of 21.51 and 10.19% at 10 mg kg^− 1 Cd treatment, respectively. Besides, the maximum activities of both enzymes were observed at 5 mg kg^− 1 Cd treatment and the rhizosphere extension distance up to 1.82 and 1.59 cm. This study reveals the potential rhizosphere regulatory mechanism of white clover. It was emphasized that the Cd bioavailability was increased in the rhizosphere, the activities of enzymes related to N and P cycling were stimulated under 5–10 mg kg^− 1 Cd stress, and thus soil N loss due to Cd could be compensated in the rhizosphere.