The changes of Tea Bag Index parameters depending on the radionuclide contamination level of soils in northern Ukraine

Huge soil areas in the northern Ukraine were contaminated with artificial radioactive isotopes after the accident at the Chornobyl nuclear power plant in 1986. Meanwhile, the levels of radionuclide pollution in these areas vary widely, which creates unique opportunities to conduct studies on the impact of the ionizing radiation (IR) on the soil health including functioning of soil microorganisms in field conditions.

We aimed to study the cellulose-destroying activity of the soil microflora at elevated radionuclides contamination. Two experimental sites were chosen – 1) near the exclusion zone (site 1), but outside its borders, and 2) in the exclusion zone (site 2). Both sites were characterized by a significant gradient of radionuclide contamination and the absorbed dose rate (ADR) of IR was formed due to activity of two isotopes ¹³⁷Cs and ⁹⁰Sr. We selected three points with ADR of 0.2, 1.0  and 1.6 μGy/hour in the upper soil layer at the site 1 and three points with ADR 3.7, 22.2 and 61.6 μGy/hour in the upper soil at the site 2. The physico-chemical soil properties and climate condition did not differ between all points of one site.

The rate of OM decomposition by soil microorganisms at all experimental points was determined by using the standardized Tea Bag Index (TBI) method (Keuskamp et al., 2013). We used two types of tea bags TM Lipton^©: green tea and rooibos as a standardized plant material to determine decomposition rate (k) and stabilization factor (S), percentage of decomposed fast disintegrated litter compounds - green tea (g) and more recalcitrant litter compounds – rooibos (r), respectively.

Two burials of plant material (=tea bags), each time for 90 days, were made: 1) from April to June 2021; and 2) from July to September 2021.

At the site 1, S and g values did not differ among points with different ADR levels. At the same time, k and r values statistically reliably increased with an increase of IR ADR.

At the site 2, S and g values significantly changed as the IR ADR increased. S values decreased, and g values, on the contrary, increased. k and r values increased statistically significantly with IR ADR increase as at site 1.

These results indicate the stimulating effect of relatively small IR doses (site 1) on the functioning of cellulose-destroying microorganisms, which capable to use more recalcitrant litter compounds (i.e., rooibos). At the same time, high IR doses (site 2) accelerate decomposition processes of both litter compounds. We conclude, that small doses of ionizing radiation will not affect soil quality in terms of the functioning of cellulose-destroying microorganisms. In general, our results could help to better understand how IR affects the processes of soil organic matter (OM) transformation.

 

We acknowledge the National Research Foundation of Ukraine for the financial support of this research (Project number 2020.01/0489).