Clover nodule bacteria as bioindicators of soils contaminated with heavy metals

The article presents the results of a study of vital activity of nodule bacteria of Trifolium pratense, Trifolium hibridum, Trifolium alexandrinum and Trifolium incarnatum as environmentally friendly bioindicators in soil toxicity assessment. The study focused on the number of nodules and their stain with leghemoglobin. The materials were taken from the plots artificially contaminated with heavy metals (lead (Pb), arsenic (As), mercury (Hg), fluorine (F), zinc (Zn) and cobalt (Co)). The results of the experiments showed that environmental distress occurs when the nitrogen-fixing nodules are stained pink or red in the range of 8-15%, which indicates that the lead content in the soil exceeds the threshold limit value (TLV) by 1.2-1.5 times. A significant decrease in nodule activity and weak or no stain were observed with an increase in mercury concentration by 1.1 times. It was found that the mass of rhizobium or the supply of organic nitrogen obtained by plants from fixation from the air is directly dependent on the grass density. At high concentrations of heavy metals in the soil, the nodule bacteria lose the amount of leghemoglobin, therefore the stain of bacteria is disrupted, so this indicator can be used as a bioindicator in soil toxicity assessment.