Abstract:

【Objective】 Hormesis is characterized as low doses stimulation and high doses inhibition with a typical biphasic dose-response curves under stressors. The phenomenon has been widely reported in different species and individuals (plant, animal, bacteria, protozoa, virus, and so on) within various endpoint types (such as growth, metabolic, reproduction, survival, immune response, and so on). Hormesis is considered to be evolutionarily based with broad potential implications for toxicology, risk assessment, and numerous other areas. In the field of environment science and technology, the Hormesis concept possibly upset the basis for environmental regulations. The government and the public are beginning to rethink the answer to this question “How clean is clean?”. However, in the previous studies almost all experimental models involved in the realm of Hormesis are organisms, hardly linking the findings to the community- or ecosystem-levels of organization. The fundamental problem “How clean is clean” did not find the expected solution from Hormesis yet. Recently, soil hormesis, stress-induced hormetic responses occurred in soils in absence of plants, is increasingly attracting researchers' interest. The soil itself is assumed to be an experimental model, focusing on revealing the population expression of potential hormetic responses of all active subjects in the soil. Soil enzyme, one of the crucial soil components, involves in catalyzing reactions necessary for organic matter decomposition, nutrient cycling and energy transfer in soil ecosystem, which was considered a typical biological indicator for assessing soil quality and reflect the microbiological activity in soils. Soil enzyme was found to be the best experimental models known for revealing soil hormesis. Land use change has profound effects on soil properties, including soil enzymes. Assuming conversion of land use types to be a special disturbance to soil ecosystems, we attempt to link the compensation potentials of soil ecosystems for the disturbance to soil enzymes’ Hormesis. 【Method】Based on the integrality of mutual feedback net among soil enzymes, soil alkaline phosphatase (ALP), a key enzyme that hydrolyzes organic phosphate to inorganic form thereby increasing soil available phosphorus supply, was selected as a representative to investigate the potential soil hormetic responses with Cd as an inducer under five land use types emerged chronologically, including mudflat (Mud), Phragmites australis (Phr), Salix babylonica (Sal), Populus alba L. (Pop) and farmland (wheat-rice rotation, WRR) in the confluence area of the Hung-tse Lake and Huaihe River, China. 【Result】 Under the natural cover of Mud and Phr, the maximum stimulation rates (y_max) of ALP induced by Cd were 8.81% and 5.84% with dose range (D₁-D₂) of 0.39-3.02 mg/kg and 0.22-3.77 mg/kg, and dose interval (Q_i) of 7.74 and 17.14, respectively. The potential for coping Cd stress (R) was 11.34 and 7.85, respectively. In contrast, parameters for WRR were 5.22 % (y_max), 0.09-1.03 mg/kg (D₁-D₂), 11.44 (Q_i), and 3.02 (R), respectively. This showed that conversion of Phr into WRR resulted in a significant decrease in y_max, Q_i, and R, implying that conversion from natural land use to farmland may lead to a decrease in the compensation potential of the soil to Cd stress. In addition, redundancy analysis showed Q_i was mainly affected by the contents of soil DOC and 0.1-2.0 μm particles, and the main factors affecting R parameters were pH and $SO_{4}^{2-}$ content. 【Conclusion】 Man-made reclamation of wetlands would substantially reduce the capacity of the soil to cope with stress. Soil enzymes’ Hormesis may provide new insight for identifying and eliminating the damage factors induced by land-use change.

Key words: Hormesis, Cd, alkaline phosphatase, confluence area of Hungtse Lake and Huaihe River (wetland), land use type