Effects of repeated sewage sludge application on plant community diversityand structure under agricultural field conditions on Podzolic soils in eastern Quebec

In Quebec, municipal sewage sludge application on agricultural lands is rel atively recent and there are many concerns related to potential contaminati on and loss of plant productivity. This study aimed at monitoring the impac ts of repeated, long-term application of sewage sludge on agricultural land s under operating field conditions. Eight paired study sites (treated and c ontrol) in three regions of the Eastern Townships (Que., Canada) were selec ted. Vegetation surveys were carried out twice a season for 2 years and soi l samples were collected. Diversity index (D), yield production and percent age of weeds were measured in each field. The average number of species per site and treatment varied from 10 to 36 but the differences between contro l and treated plots for all sites were not significant. The diversity indic es significantly varied between sites due to original soil composition diff erences between locations. In some cases, significant differences in divers ity existed between paired treated and control fields. However, weed percen tage did not necessarily vary in those same fields suggesting that change i n diversity did not lead to changes in number of weeds. Soil chemical compo sition showed that significant variation existed between sites but not betw een paired fields. Correlation performed between diversity, production, wee d frequency and soil parameters showed that variation in D was negatively c orrelated with Mg and positively with pH. At the same time, the percentage of weeds was negatively associated with K, Ca and pH (P<0.05). Although sew age sludge produced little direct effect on the ecological parameters, anal yses suggest that some soil chemical factors may affect plant diversity and percentage of weeds. These results indicate that plant community ecology m ay be affected by changes in environmental conditions but in a complex mann er. Monitoring programs should be developed to evaluate the long-term impac ts of repetitive sludge application on agricultural lands. (C) 2000 Elsevie r Science B.V. All rights reserved.