Earthworms, one of the most important macroinvertebrates in terrestrial ecosystems of temperate zones, exert important influ- ences on soil functions. A laboratory microcosm study was conducted to evaluate the influence of the earthworm Eisenia fetida on wheat straw decomposition and nutrient cycling in an agricultural soil in a reclaimed salinity area of the North China Plain. Each microcosm was simulated by thoroughly mixing wheat straw into the soil and incubated for 120 d with earthworms added at 3 different densities as treatments： control with no earthworms, regular density （RD） with two earthworms, and increased density （ID） with six earthworms. The results showed that there was no depletion of carbon and nitrogen pools in the presence of the earthworms. Basal soil respiration rates and metabolic quotient increased with the increase in earthworm density during the initial and middle part of the incubation period. In contrast, concentrations of microbial biomass carbon and microbial biomass quotient decreased in the presence of earthworms. Earthworm activity stimulated the transfer of microbial biomass carbon to dissolved organic carbon and could lead to a smaller, but more metabolically active microbial biomass. Concentrations of inorganic nitrogen and NO^-N increased significantly with the increase in earthworm density at the end of the incubation （P ~ 0.05）, resulting in a large pool of inorganic nitrogen available for plant uptake. Cumulative net nitrogen mineralization rates were three times higher in the ID treatment than the RD treatment.
PANG Jun-ZhuQIAO Yu-HuiSUN Zhen-JunZHANG Shuo-XinLI Yun-LeZHANG Rui-Qing
Earthworms （Eiseniafetida） were used to study the impact of low-dose cadmium in treated artificial soil （0, 0.6, 3, 6, 15, 30 mg/kg） and contaminated natural soil （1.46 mg/kg）. The changes of earthworms＇ physiological related gene expressions of metallothionein （MT）, armetocin, calreticulin and antimicrobial peptides were detected using real-time PCR after a 70-day incubation period. The results showed that low doses of cadmium could up regulate earthworms＇ MT and down regulate annetocin gene expression and show a significant positive and negative correlation respectively. The expression of two other genes, calreticulin and anti-microbial peptides, was induced at low doses of cadmium （highest gene expression at 0.6 mg/kg for calreticulin and 6 mg/kg for anti-microbial peptides） and inhibited at high doses. No significant correlation was found for these two genes. This study shows that MT and annetocin genes expression found in earthworms in contaminated soil have the potential to be developed as biomarkers of soil cadmium pollution.
