Effect of mixed sewage sludge and garden waste composts on potting soil amendment

To study the effects of sewage sludge and garden waste composts on soil amelioration and plant growth, pot experiments were carried out based on sewage sludge and garden waste in Beijing. The experiment consisted of four types of mixture of the two composts as modified substrate (volume ratios 1:0, 1:3, 1:1, and 0:1), at five concentration levels 0 (soil as control), 25%, 50%, 75%, and 100% compost mixture. The results showed that the pH value of the 1:1 mixture increased significantly compared to that of the control. Soil electrical conductivity increased at the beginning of the modified substrate addition, and then dropped to the normal range after four months. Meanwhile, additions also significantly increased the organic matter, total nitrogen, and total phosphorus. Soil available phosphorus was greatly increased, indicating enhanced soil phosphorus activity. Although the soil used in the pot experiment had relatively high amount of heavy metals, adding modified substrate could reduce their concentrations. After planting Festuca arundinacea, concentrations of heavy metals decreased significantly. For the four heavy metals studied, Pb had the strongest migration to plants, followed by Zn and Cr, while Cu was the weakest. Germinations of tall fescue were closely related to Cr, and the growth was related to the nitrogen content of the soil. In summary, tall fescue, as a common lawn grass, has a strong adsorption of heavy metals, and thus can be used to amend contaminated land. Although mixing sludge compost with garden waste does not reduce the total amount of heavy metals, it may affect the form of heavy metals, and promote their absorption and degradation by plants and microorganisms in soils.