Influence of Liquid Swine Manure Application On the Trace Metal Contents of an Old Grassland Soil.

In Quebec, most managed grasslands receive regular applications of animal manure which could lead to the accumulation of trace metals in the soil surface layers. These trace metals could be transferred to streams and water bodies during land use change and impact the environment. The objective of this study was to evaluate the effect of ploughing and long-term liquid swine manure (LSM) applications on the accumulation of trace metals in a grassland soil. Two adjacent grassland sites, one amended with 100 m³ ha^-1 of LSM annually since 1978 and an unamended grassland were either (i) left with vegetation intact (Ctrl) or killed by glyphosate in the autumn. Glyphosate-treated plots were either (ii) left as an undisturbed chemical fallow (CF), (iii) ploughed by full inversion tillage in the autumn (A-FIT), or (iv) in the spring (S-FIT). Soil samples were taken regularly to monitor metal contents (Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) in the soil solid phase. In addition, lysimeters were installed at depths of 15, 30 and 45 cm to monitor concentration of trace metals in the soil solution. For most metals, the highest Mehlich-3 concentration in the soil surface (0-5 cm) were found in the no-till treatments (Ctrl and CF), whereas at lower depths (10-20 and 20-30 cm) the highest metals concentrations were found in the tillage treatments (A-FIT and S-FIT). Overall, our results indicate that long-term application of LSM increased the content of several trace metals in the soil solid and liquid phases. Tillage modified the distribution pattern of trace metals in the soil profile, but its effect on the soil solution was not as clear.