Potential Use of Tracer Methods, Especially Autoradiography, in the Study of the Relation of Herbicides to Soil Biology

Several tracer methods, designed originally for the study of general soil processes, are described. Their possible adaptation to investigations of the interaction of herbicides with soil biology (especially the soil microflora) is discussed. Rapid disposal of the herbicide-treated vegetation before replanting (minimal tillage) is essential. In comparing the efficiency of herbicidal destruction of vegetation with conventional ploughing, and also when evaluating the effect of various herbicides on the rate of decay of the treated crops, a method is required to measure the rate of the microbial decomposition of the plant residues. A technique based on autoradiography is described which makes possible the study of the progressive decay of herbage grasses and rye uniformly labelled with carbon-14, placed on the surface of soil and incubated for various periods of time. Photoelectric measurements of the density of images of autoradiographs prepared before the start of the experiment and at intervals during incubation show a statistically significant decrease in density with time of incubation. This progressive loss of carbon is an estimate of the rate of decomposition of the residues. The utilization of breakdown products of herbicides by micro-organisms has been demonstrated in pure culture using labelled herbicides but not directly in the soil. A method based on the stripping film technique demonstrates the incorporation of carbon-14 atoms into the cell material of fungi which decomposed 14C-labelled plant residues mixed with soil. This technique could be adapted to study the uptake of labelled atoms after the microbial decomposition of radioactive herbicides in the soil. Herbicides inhibit and under,certain conditions may also stimulate the growth of micro-organisms in the soil. The evolution of CO2 is frequently used as an index of microbial activity. It is, however, also a function of the carbon content of the soil. Soils not treated with herbicides will have a greater output of CO2 not only because microbial activity remains intact but also because of the additional supply of fresh organic carbon by the weed population. To obtain a more accurate evaluation of herbicide effects the soils should be supplemented with substrates of easily decomposed and labelled carbon substrates. The differences in the rate of production of labelled CO2 will be a measure of the changes that have taken place iñ the microbial activity. Autoradiography of soil profiles as well as of smears of individual soil crumbs showed the migration of soluble 14C labelled constituents from decaying plant residues. The latter method was used with some success in preliminary experiments with 14C labelled simazine. Since the detection of radioactivity proves the presence of the nuclide only and not necessarily that of the intact herbicide molecule, results, especially in studies on persistence of labelled herbicide, may under certain conditions present supporting rather than conclusive evidence. (author)