An indicator to evaluate the proportion of exogenous organic matter (EOM) remaining in soils over the long-term after application has been developed. A database was constructed with analytical data corresponding to 83 EOMs, including sludges, composts, animal wastes, mulches, plant materials and fertilizers. The data included results of proximal analysis (soluble, SOL, hemicellulose-, HEM, cellulose-, CEL, and lignin-like, LIC, fractions, in g kg-1 total organic matter) and of carbon (C) mineralization during long-term incubations under laboratory conditions (in g kg-1 exogenous organic C, EOC). The potential residual organic C after EOM application to soil was assessed from the extrapolation of the incubation results. Then, partial least square regression was used to relate EOM characteristics to the proportion of potentially residual organic C previously determined from the incubations. The biochemical fractions of EOM were not predictive enough to develop the indicator. The proportion of organic C mineralized during 3 days of incubation (C3d) was cumulated and appeared to be the most predictive variable of residual organic C. The proposed indicator of residual organic carbon in soils (expressed as g EOC kg-1) was IROC = 445 + 0.5 SOL - 0.2 CEL + 0.7 LIC - 2.3 C3d. The indicator was calculated for the main types of EOM applied to soils. When compared with the few field data of residual C measured in long-term field experiments, the values provided by the indicator seemed to be over-estimated (i.e. EOC degradation could be faster under field conditions than during laboratory incubations)

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To cite this version:EricKeywords soil, PAH, sewage sludge, maize, crop, pollutant IntroductionPolycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds widely occurring in various media such as soils, sediments, waters, aerosols, food and living organisms (Lichtfouse et al. 2005). PAHs are of environmental concern because some PAHs such as benzo [a]pyrene have shown mutagenic and carcinogenic properties. Light aromatic hydrocarbons occurring in contaminated soils and tars inhibit the growth of plants (Henner et al. 1999). Some PAHs such as phenanthrene and benzo[a]pyrene can easily be transferred through the food chain to end in blood (Laurent et al. 2001). Nonetheless, in many instances, it is difficult to assess the precise origin of PAHs, e.g. natural versus anthropogenic as discussed by Lichtfouse et al (1997). It is even more difficult to make a clear relationship between PAH contact to living organisms and the later occurrence of an illness such as cancer.On earth, PAHs are produced either rapidly during combustion processes, e.g. vegetation fires and vehicle exhausts, or slowly during the maturation of organic matter in sediments. They usually occur in minute amounts in most natural media, with exceptions such as petroleum pools, organic-rich sediments and soils after vegetation fires. On the other hand, PAHs are usually concentrated in locations of high human activity, e.g. near major highways, in major cities and in some industrial soils (Henner et al. 1997, Bryselbout et al. 2000.PAHs are concentrated in sewage sludges. Urban sewage sludges are very complex media inherited from the decay of animal, vegetal and rain wastes. Huge quantities of sewage sludges are produced, amounting yearly to about 850,000 tons dry weight in France and 7.7 million tons in Europe in 1998. There are two main options for the treatment of sewage sludges. Firstly, incineration is fast but costly and leads to a rapid input of the greenhouse gas carbon dioxyde in the atmosphere. Secondly, recycling in crop soils is convenient because sewage sludges are excellent fertilisers as they are rich in NP K elements (Sommers, 1977). Moreover, this process should be more sustainable that incineration regarding carbon dioxyde emissions, as a part of sludge carbon should stay in the soil for some time. Nonetheless sewage sludges contain pollutants that could be further transferred to the food chain and ground waters (Wilson et al. 1996).The knowledge on the fate of PAHs in soils amended with sewage sludges in scarce, notably in the long term (Wilson et al. 1997). Here we report PAH levels in experimental maize crop soils during sludges addition (1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992) and after sludges addition (1993)(1994)(1995)(1996)(1997)(1998)(1999) Revised version Experimental SamplesThe INRA experimental crop field "Ambarès" in Bordeaux, France, is splitted into 25 plots including plots amended with sewage-sludges and non-amended plots. The 0-20...

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