The effects of weeds and soil nutrient additions on the fate of 14C-labelled glucose and metsulfuron–methyl (metsulfuron) herbicide were determined in a growth-chamber soil–plant microcosm study. Nodding thistle (Carduus nutans) was used as a test weed species to which these compounds were applied. The experiment consisted of a full factorial design of N addition, P addition, planting with C. nutans, and addition of 14C-labelled compounds (glucose and herbicide metsulfuron). The experiment ran for 131 days following addition of the 14C-labelled compounds. There was a significant positive effect of C. nutans plants on soil microbial biomass-C and basal respiration. The addition of nutrients did not consistently affect any of the parameters measured in this study. The fate of 14C-labelled compounds was determined by quantifying the relative amounts of 14C absorbed by plants, incorporated into soil, taken up into microbial biomass and respired by soil organisms. Two days after application of the 14C compounds, 7–8% of the 14C was absorbed by the C. nutans (weed) canopy. By day 131, approximately 2–3% of the applied 14C was still detected in the tissues of dead thistle plants. The amounts and pattern of decomposition differed considerably between metsulfuron and glucose. Almost all (99%) of the applied 14C glucose was either respired by the microbes or incorporated into microbial biomass by day 131 while only 38–42% of the applied metsulfuron was respired or incorporated into microbes by this time. Greater amounts of metsulfuron pesticide were degraded in microcosms containing C. nutans plants than in those without plants. By day 131, 27.4% of 14C added from metsulfuron was respired in the planted treatment and only 22.8% from the non-planted treatment. This would suggest that the presence of decomposing weed litter (dying weed plants) has the potential to enhance the decomposition of added xenobiotics such as herbicides.
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