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

In Quebec, municipal sewage sludge application on agricultural lands is relatively recent and there are many concerns related to potential contamination and loss of plant productivity. This study aimed at monitoring the impacts of repeated, long-term application of sewage sludge on agricultural lands under operating field conditions. Eight paired study sites (treated and control) in three regions of the Eastern Townships (Que., Canada) were selected. Vegetation surveys were carried out twice a season for 2 years and soil samples were collected. Diversity index (D), yield production and percentage 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 control and treated plots for all sites were not significant. The diversity indices significantly varied between sites due to original soil composition differences between locations. In some cases, significant differences in diversity existed between paired treated and control fields. However, weed percentage did not necessarily vary in those same fields suggesting that change in diversity did not lead to changes in number of weeds. Soil chemical composition showed that significant variation existed between sites but not between paired fields. Correlation performed between diversity, production, weed frequency and soil parameters showed that variation in D was negatively correlated 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 sewage sludge produced little direct effect on the ecological parameters, analyses suggest that some soil chemical factors may affect plant diversity and percentage of weeds. These results indicate that plant community ecology may be affected by changes in environmental conditions but in a complex manner. Monitoring programs should be developed to evaluate the long-term impacts of repetitive sludge application on agricultural lands. © 2000 Elsevier Science B.V. All rights reserved.