From Direct Application of Urban Organic Wastes to Nutrient Recycling - a Soil Science Perspective.

Plant nutrient cycling in society is only partially closed due to several reasons. Efficient recycling requires equitable redistribution on arable land, nutrients should be in plant-available form, and products applied must be ‘safe and clean’. Results from several long-term field experiments with urban organic wastes showed the following: Application of these amendments to soil at rates of 5–8.5 Mg dry matter ha⁻¹ yr⁻¹, which is higher than the regular supply in agriculture, increased soil organic carbon (SOC) stocks. The contribution of the organic amendments to SOC was in the order compost > manure > crop roots > sewage sludge. Domestic household waste compost was found to be highly recalcitrant; about 90% of C added was retained in SOC during a 13-year period. Efforts undertaken in Sweden to decrease metal loads through application of sewage sludge to soil have resulted in lower stress for soil organisms and increased turnover of soil organic matter. Yield effects of sewage sludge N were low to moderate corresponding to use efficiency of 13-40% calculated by difference to the control.  

We argue that application of urban organic wastes to soil is an efficient form of recycling for small municipalities but organic waste treatment from large cities requires other solutions. Urban organic wastes have generally high water content not allowing long-distance transportation. Therefore, nutrient extraction from urban wastes instead of direct organic waste recycling is a possible way forward. The trend to increase combustion of urban wastes makes ash to become a key waste type in the future. Two examples are discussed showing how inorganic fertilizers can be produced from contaminated ashes from cities - mono-ammonium phosphate fertilizer (MAP) from sewage sludge ash and water-soluble K fertilizer from fly-ash whereby metals are separated. The view on organic waste recycling needs to be diversified in order to close the urban-rural nutrient loop.