Application of an antibiotic (streptomycin) and a fungicide (benomyl) in order to separate root respiration and microbial respiration of exudates was tested using two plant species (ryegrass and spring wheat) growing on a Haplic Luvisol. Both xenobiotics were added to the soil with growing plants either separately or in combination. Plants were 14C-pulse labelled and both labelled and total CO2 emission from soil were measured. After seven days plants were harvested, dried and total carbon and C content in shoots and roots were determined. Growing plants increased the total CO2 emission from soil by about 2.5 times in comparison to unplanted soil. The temporal pattern of 14CO2 evolution in control treatments was similar as reported in the literature with maximum emission rates (0.55 % and 0.15 % of assimilated C per hour for wheat and ryegrass, respectively) during the first day after labelling. It was affected by both xenobiotics used but in different ways. With ryegrass streptomycin decreased CO2 emission rates during the first day after labelling, while it left the second phase of tracer emission unchanged; with spring wheat the 14CO2 evolution rates were reduced on the second day. Applied xenobiotics did not change the 14C content in the shoots, but smaller tracer amounts were found in the roots and in soil-derived CO2. This was more pronounced for ryegrass than for spring wheat plants.
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