Diwu Fan

Hormesis is a phenomenon in which low doses of toxins stimulate organisms, while high doses have inhibitory effects. Soil heterogeneity nutrient spatial profoundly influences community structure and stress responses by altering the microenvironment around microorganisms. Studies on hormesis in soil ecosystems have made significant progress, but most of them have overlooked the impact of soil nutrient spatial heterogeneity on hormesis. To examine the effects of nutrient spatial heterogeneity on the hormesis of soil ecosystem under cadmium (Cd) stress, we constructed three patches with different spatial nutrient distribution but the same total nutrient content through different spatial discharge patterns. Different levels of Cd stress were applied to the patches, and after incubation, soil respiration rate, content of dissolved organic matter (DOM) and metagenomic characteristics were measured. The results indicated that higher nutrient spatial heterogeneity enhanced the tolerance of soil respiration to Cd stress (the maximum stimulating dose increased from 0.03 to 3.0 mg·kg⁻¹), and simultaneously improved the compensation capacity (Hor_zone increased from 0.04 to 21.59). The results also revealed that Cd stress had the least impact on soil microbial diversity of the high heterogeneity samples. The content of DOM initially displayed a hormesis-like phenomenon with increasing Cd stress, followed by a linear increase. Notably, the biphasic change trend became more pronounced as the degree of spatial heterogeneity increased (The maximum stimulation rate of DOM content increases from 10.8 to 22.9%). The hormetic response of soil respiration to nutrient spatial heterogeneity offers novel insights for the identification and mitigation of Cd pollution in ecosystems.

Hormesis in soil enzymes is well-established, yet the underlying mechanism remains elusive. In this novel study, we investigated the effects of low-dose Cd exposure (0, 0.03, 0.3, 3, and 30 mg·kg⁻¹) in farmland soil within a typical constructed wetland environment. We assessed the activities of four soil enzymes (urease (URE), denitrification enzyme (DEA), dehydrogenase (DHA), and alkaline phosphatase (ALP)) at varying exposure durations (0 h, 24 h, and 48 h), evaluating hormetic characteristics across these time intervals. Additionally, we determined kinetic parameters, specifically the Michaelis constant (K_m) and maximum reaction velocity (V_max), for these enzymes while examining potential alterations in microbial community structure. Our findings revealed hormesis in all four soil enzymes at 24 h of exposure, with varying stimulus width and maximum hormesis rates. Interestingly, heavy metals did not significantly influence the diversity of soil microbial communities, but they did inhibit the ability of soil microbial communities to secrete extracellular enzymes. This resulted in a reduction in the soil enzyme pool and a consequential shift in overall soil enzyme activities. The conclusion of this study is that low-dose Cd primarily reduced extracellular enzyme secretion by soil microorganisms, leading to a reduction in the size of the soil enzyme pool and thereby inducing hormesis in soil enzyme activities.