| Recently, many researches have been focused on the effects of earthworms on the phytoremediation of heavy metal contaminated soils which demonstrated that earthworms not only were able to increase heavy metal bioavailability in contaminated soils, but also enhance plant growth and heavy metal accumulations. These researches results indicated that earthworms had a potential role in modifying the efficiency of phytoremediation of heavy metal contaminated soils. However, the basic mechanisms of earthworms increase heavy metal bioavailability in contaminated soils, and enhance plant growth and heavy metal accumulations were still unclear. Therefore, we had designed a series of cultivated experiments to investigate the basic mechanisms of earthworms increase heavy metal bioavailability in contaminated soils, and enhance plant growth and heavy metal accumulation. These studies were mainly focused on:(1) Effects of earthworm (Metaphire guillemi) mucus on tomato(Lycopersicon esculentum, Hezuo 903) seedlings growth and cadmium (Cd) accumulations; (2) Influences of earthworm mucus and a solution of amino acids matching those in earthworm mucus on tomato seedlings physiological activities and Cd accumulations; (3) Effects of earthworm mucus and a solution of amino acids matching those of earthworm mucus on Cd subcellular distribution and chemical forms in tomato seedlings; (5) Influences of earthworm(Metaphire guillemi) on Cd species in spiked soils. The main results were shown as following:(1) A hydroponic study was conducted to investigate the effects of earthworm mucus (Metaphire guillemi) on tomato seedlings(Lycopersicon esculentum, Hezuo 903) growth and Cd accumulations. Results showed that, at 0 and 5 mg L-1 Cd concentrations, compared with control, earthworm mucus significantly increased dry weights of tomato seedlings roots and shoots by 12.4～33.2% and 13.8-44.5%, respectively. While, at 10 mg L-1 Cd concentration, earthworm mucus was no significant influence on tomato seedlings growth and even decreased dry weights of plant shoots by 4% at high earthworm mucus addition level treatment compared with control. Meanwhile, present study also showed, with Cd and earthworm mucus addition levels increasing, Cd concentrations and accumulations in tomato seedlings were significantly increased and the Cd concentrations and accumulations in tomato seedlings roots were much higher than those in corresponding shoots. Earthworm mucus contained abundant available nutrient materials, such as dissolved organic carbon (DOC), NH4+-N, NO3--N, P and K, which may be responsible for earthworm mucus increasing plants growth and heavy metals accumulation. This may be a mechanism of earthworm enhancing plant growth and heavy metal accumulations.(2) The results of a hydroponic study showed that, compared with CK, earthworm mucus significantly increased dry weights of tomato seedlings roots and stems; while a solution of amino acids matching those of earthworm mucus significantly increased dry weights of tomato seedlings roots only, but slightly decreased dry weights of plants stems and leaves. Meanwhile, both earthworm mucus and amino acids significantly increased the root average diameters and root volumes of tomato seedlings compared with CK, but the extents increased by earthworm mucus were much higher those than by amino acids. These results suggest that earthworm mucus could increase tomato seedlings growth through increasing chlorophyll contents of tomato seedlings leaves and enhancing microelements (including Fe, Mn, Zn and Cu) uptake and transport in tomato seedlings. While amino acids have slight influences on tomato seedlings growth, chlorophyll contents and microelements uptake and transport in tomato seedlings compared with earthworm mucus.(3) Earthworm mucus significantly increased chlorophyll contents and antioxidative enzymes activities, such as superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and significantly enhanced four essential elements, such as Fe, Mn, Zn and Cu, uptake and transport in tomato seedlings under Cd stress. These may be explaination for earthworm mucus enhancing tomato seedlings growth and Cd accumulations. However, although a solution of amino acids matching those of earthworm mucus significantly increased chlorophyll contents, antioxidative enzymes activities and microelements uptake and transport in plants, these effects of amino acids were much lower than those of earthworm mucus.(4) Under Cd stress, compared with control, earthworm mucus significantly increased the fresh weights of tomato seedlings roots, stems and leaves by 123.9%,16.2% and 32.0%, the Cd concentrations in tomato seedlings roots, stems and leaves by 22.5%,14.4% and 28.9%, and the Cd accumulations in tomato seedlings roots, stems and leaves by 173.2%, 14.5% and 75.4%, respectively. These might be due to earthworm mucus significantly increased soluble fraction Cd concentrations of Cd subcellular distribution and significantly increased inorganic and soluble forms Cd concentrations of Cd chemical forms in tomato seedlings. Meanwhile, a solution of amino acids matching those of earthworm muus had same functions as earthworm mucus that significantly enhanced tomato seedlings growth and Cd accumulation, and significantly increased soluble fraction Cd concentrations of Cd subcellular distribution and inorganic and soluble forms Cd concentrations of Cd chemical forms in tomato seedlings, but these effects of amino acids solution were much lower than those of earthworm mucus. These results indicated earthworm mucus increased tomato seedlings growth and Cd accumulation through changing Cd subcellular distribution and chemical forms in plants.(5) Earthworm (Metaphire guillemi) activities significantly increased cadmium bioavailability in spiked soils (10 Cd mg kg-1, Orthic aquisols). Especially, the Cd bioavailabilities in burrow walls and casts, which were earthworm frequently moving place and metabolizing production, respectively, were significantly higher than those in bulk soils with earthworm. Compared with control, earthworm activities significantly increased the Cd concentrations of exchangeable and bound to soil carbonates fractions in bulk soil with earthworms, and the Cd concentrations of exchangeable and bound to soil carbonates fractions in burrow walls and casts were significant higher than those in bulk soils with earthworms. Meanwhile, earthworm tissue and epidermis could accumulate and excrete Cd, respectively. The Cd concentrations in earthworm tissue and mucus were significantly increased with incubation time. There was a significant positive correlation between the Cd concentrations of earthworm tissue and epidermal excretion. These results indicated that earthworm increased Cd bioavailability in contaminated soils, which might be due to earthworms 1) significantly decreased the pH of soils, especially in burrow walls and casts in which the pH significantly lower than the pH of bulk soil with earthworms,2) produce organic materials and fully mixed them with Cd, and 3) excete Cd through their epidermis.In conclusion, these results indicated:1) Earthworm mucus significantly increased tomato seedlings growth and Cd accumulation, which might be a mechanism of earthworm activities increasing plants growth and Cd accumulation; 2) Earthworm mucus increase Cd accumulation by tomato seedlings might be due to earthworm mucus enhancing plants growth and metabolism activities; 3) A solution of amino acids matching those of earthworm mucus play a lower role than earthworm mucus on increasing plants growth and Cd accumulation; 4) Earthworm mucus increasing plants growth and heavy metals accumulation might be due to IAA-like substances and other unknown materials; 5) Earthworm activities could increase Cd bioavailability in contaminated soils through decreasing soil pH, producing organic matters and mixing Cd thoroughly with organic matters, and excreting Cd through epidermis.
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