Effects of fenpropimorph on bacteria and fungi during decomposition of barley roots

The study examined the effects of the fungicide fenpropimorph (in the formulation Corbel) on primary decomposer organisms in soil. Bacterial and fungal succession was followed on dead young barley roots buried in fungicide-treated or untreated soil. Fenpropimorph was added to the soil in a concentration realistically achieved in field topsoil when using the recommended dose. Over a 56-day period we measured the length of active fungal hyphae, the abundance of total culturable bacteria, the abundance of two culturable subgroups relevant to the soil environment (hyphae-forming actinomycetes and Pseudomonas), and the diversity of bacterial DNA. The soil fenpropimorph content followed first order disappearance concomitant with the first order formation of fenpropimorphic acid. Fenpropimorph inhibited the growth of active fungi during the first 10 days, when fungal activity peaked in both the treated and non-treated root samples. The number of total culturable bacteria was significantly lowered by fenpropimorph at day 17 and stimulated at day 56, indicating a possible indirect effect of the fungicide on the culturable bacteria as a whole. Nevertheless, culturable Pseudomonas and actinomycetes were not affected. The succession of the two bacterial subgroups differed considerably, so, whereas Pseudomonas peaked in the early decomposition stages, actinomycetes were most abundant after 2 months, and on the very first sampling day. Thus, the present findings suggest a possible effect of fenpropimorph on soil fertility. The diversity of total bacterial DNA measured by denaturing gradient gel electrophoresis (DGGE) was unaffected by fenpropimorph treatment, there being a clear and highly reproducible succession in bacterial diversity during decomposition of the barley roots. The diversity in the hotspot on the last sampling occasion at two months was similar to the diversity of the bulk soil, thus indicating a return to the level prior to addition of the roots.