Sludge-derived organic carbon in an agricultural soil estimated by 13C abundance measurements.
Résumé
The objective of this study is to develop a method to follow the dynamics of sludge-derived organic carbon, which will allow us to understand the behaviour of trace metals in the sludge-treated soils. We studied, in a sandy agricultural soil of southwest France, cultivated with maize and amended with sewage-sludge over 20 years, the dynamics of different sources of organic matter and compared this with a control, which had never received any treatment. For the first time, a method is proposed that will distinguish and quantify sludge-derived organic carbon, maize-derived organic carbon, and native organic carbon. This method is based on the mean differences in d13C abundances between native (-2.65%), maize (-1.25%) and sludge (-2.54%) organic carbon. Three hypotheses on the dynamics of soil organic matter sources are proposed: (i) isotopic differences observed between control and sludge-treated soils are due only to the incorporation of sludge C, whereas in the others, the control was used to model the incorporation of (ii) maize C or (iii) native C in the sludge-treated soils. The comparison of the stocks of each source (native C, maize C and sludge C) found in the bulk soil with the sum of corresponding stocks found in particle-size fractions allowed us to reject the two first hypotheses and to validate the last one. Repeated applications of sewage-sludge induced accumulation of sludge-derived organic carbon in the topsoil, and simultaneously contributed to the preservation of maize-derived organic carbon. When sludge applications ceased, the rapid decrease in soil organic matter stocks was mostly caused by the degradation of the sludge-derived organic carbon sources. At the same time, the maize-derived organic carbon shifted from the coarsest fraction (200–2000 μm) to the finest fraction (0–50 μm). Therefore, this study has shown that repeated applications of sewage-sludge induced changes in soil organic matter dynamics over time.