Fate of C-14 from glucose and the herbicide metsulfuron-methyl in a plant-soil microcosm system

The effects of weeds and soil nutrient additions on the fate of C-14-labell ed glucose and metsulfuron-methyl (metsulfuron) herbicide were determined i n a growth-chamber soil-plant microcosm study. Nodding thistle (Carduus nut ans) 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 addit ion, planting with C. nutans, and addition of C-14-labelled compounds (gluc ose and herbicide metsulfuron). The experiment ran for 131 days following a ddition of the C-14-labelled compounds. There was a significant positive ef fect of C. nutans plants on soil microbial biomass-C and basal respiration. The addition of nutrients did not consistently affect any of the parameter s measured in this study. The fate of C-14-labelled compounds was determine d by quantifying the relative amounts of C-14 absorbed by plants, incorpora ted into soil, taken up into microbial biomass and respired by soil organis ms. Two days after application of the C-14 compounds, 7-8% of the C-14 was absorbed by the C. nutans (weed) canopy. By day 131, approximately 2-3% of the applied C-14 was still detected in the tissues of dead thistle plants. The amounts and pattern of decomposition differed considerably between mets ulfuron and glucose. Almost all (99%) of the applied C-14 glucose was eithe r respired by the microbes or incorporated into microbial biomass by day 13 1 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 pl ants. By day 131, 27.4% of C-14 added from metsulfuron was respired in the planted treatment and only 22.8% from the non-planted treatment. This: woul d suggest that the presence of decomposing weed litter (dying weed plants) has the potential to enhance the decomposition of added xenobiotics such as herbicides. (C) 2001 Elsevier Science Ltd. All rights reserved.