Hormetic effects of Cd on alkaline phosphatase in soils across particle-size fractions in a typical coastal wetland.

Hormetic responses in soil ecosystem are increasingly reported recently. Soil enzymes are involved in almost all biochemical reactions, but insufficient investigations were conducted to define its hormetic responses. The objective of this study is to investigate the hormetic responses across soil particle-size fractions with cadmium (Cd) as a stressor and alkaline phosphatase (ALP) as a potential endpoint. Soils were treated by adding CdCl2·2.5H2O solution with 0, 0.003, 0.03, 0.3, 3.0 and 30.0mg·kg-1 of Cd, respectively. A low-power ultrasonic method was used to separate the bulk soil into 0.1-2, 2-63, 63-200 and 200-2000μm fractions. In 2-63μm, ALP activity at doses of 0.3-3.0mg·kg-1 of Cd was significantly higher than that of CK (0.0mg·kg-1 of Cd), showing a typical U-shaped dose-response with the amplitude of 72.3-118.6%. Similarly, ALP activity at 0.003-0.3mg·kg-1 of Cd was 36.4-66.1% higher than that of CK in 63-200μm. However, no similar phenomenon was observed in 0.1-2 and 200-2000μm fractions. This suggested that low doses of Cd induced the hormetic responses of soil ALP, particularly in 2-63 and 63-200μm. In addition, analysis of the microbial community structure and diversity indicates that, at genus level, the relative abundance (RA) of Gillisia at 0.03-0.3mg·kg-1 of Cd was significantly higher than that of CK with the amplitude of 3.7-37.5% in 2-63μm. The similar responses were observed that the RA of Pontibacter at 0.003-0.03mg·kg-1 of Cd was 4.0-85.4% higher than that of CK in 63-200μm. This showed that Gillisia and Pontibacter possibly contribute to the hormetic responses of soil ALP when low contents of Cd presented in soils. This study will provide a good insight into the hormetic phenomenon at soil ecosystem scales.