Trace Metal Content Following the Tillage of An Old Grassland Soil Fertilized with Pig Manure.

In Quebec, most grasslands receive regular applications of manure that can result in the accumulation of trace metals in the surface layers of soil and could be transferred to streams and water bodies is an environmental concern during land use change. The objective of this study was to evaluate the effect of occasional tillage of an old grassland fertilized with liquid pig manure for 25 years on the soluble and total contents of trace metals in soil and in soil solution. Two long-term forage fields were studied and divided into 32 subplots (4 treatments x 4 replicates x 2 long-term plots). One of the two fields had received 100 m³ ha^-1 of liquid swine manure annually since 1978 and the other was and an unmanured grassland. The four treatments were randomly designated to subplots and were: (i) left as control (grass undisturbed) (Ctrl); (ii) chemically treated with herbicide (chemical fallow, CF), then (iii) ploughed by full inversion tillage (FIT) in the autumn (autumn-FIT), or (iv) ploughed in the spring (spring-FIT). Soil samples were taken regularly to monitor the various trace metals contents (Cd, Co, Cr, Cu, Ni, Pb and Zn) in the soil solid phase. In addition, lysimeters were installed in all subplots at depths of 15, 30 and 45 cm to monitor the concentration of trace metals in the soil solution. Overall, our results indicate that long-term application of liquid swine manure has contributed to increase the soluble and the total content of several trace metals in soil and also in the soil solution. However, tillage also appears to have had little effect on the soluble and total contents of trace metals.