Agricultural Use of Different Residual Waste Composts - Current Situation and Experiences in France
Quality standards have been recently established in France for sewage sludge composts and other urban composts (greenwaste, biowaste and municipal solid waste composts). These standards allow to guarantee the innocuousness of the composts. Other standards have been defined in order to predict the compost effects on soil properties: the index of biological stability (ISB), based on biochemical fractionation, evaluates the proportion of compost organic matter contributing to increase organic C in soils. Kinetics of organic C and N mineralisation in soil-compost mixtures inform on the short term effects of composts, especially on N availability after compost addition. A longterm field experiment located in Feucherolles (Yvelines, France) has been initiated in 1998 to compare the agronomic value and the environmental impacts of three composts and farm yard manure (FYM): biowaste compost (BIO), municipal solid waste compost (MSW) and compost made from green wastes co-composted with sewage sludge (GWS). Three applications have been realised in October 1998, September 2000 and 2002, respectively. The applied amounts were calculated in order to bring 4 t organic carbon per hectare. The field was cultivated with a wheat-maize rotation. Some examples of results of soil and crop quality evolution are given: soil organic C evolution, crop yields and compost N availability, soil structure stability, trace element concentrations in grains. The results are related to the characteristics of composts. The MSW composts were still highly biodegradable and contributed to the stimulation of microbial activity improving soil structure stability. The largest crop yields were measured with the MSW composts that presented the largest availability of N for plants (10 to 15% of the applied N during 3 years after the application). On the contrary, the largest increases of soil organic C were observed with the BIO and GWS composts that had the largest ISB indicators and were characterised by a more stabilised organic matter. These 2 other composts improved soil structure stability through their humified organic matter. No increase in trace element concentrations was observed with none of the composts although the trace element concentrations in the MSW composts did not fit the quality standards. Trace elements concentrations should be improved in all type of composts and their cumulative effects must be watched out.