Can stable isotopes quantify soil carbon build-up from organic fertilizers?

International audienceCan stable isotopes quantify soil carbon build-up from organic fertilizers? Application of organic fertilizers (OF) can supply carbon (C) to the soil in crop fields. OF-derived C (OF-C) is often estimated using the differential method that can be biased due to indirect effects of OF on soil C. This study tested three methods to quantify OF-C: (i) the widespread differential method, (ii) the synchronic isotopic method comparing plots with and without OF and (iii) the asynchronic isotopic method mimicking a trial without a control plot. These methods were implemented on an Arenosol and an Andosol supplied during 13 years with slurry or compost. The results highlighted the relevance of using the isotopic synchronic method, which focuses on the direct effect of OFs on the soil organic matter (without bias of vegetation change) and considers control soil's evolution. Considering limit of resolution in the Arenosol, the method was suitable after 4 years of fertilisation for an initial δ 13 C difference of 7.5 ‰ between OF and soil, and after 9 years for a difference of 3.5 ‰. A sensitivity analysis showed that particular attention must be paid to OF-δ 13 C variability to guarantee the method validity. The method proved to be suitable to study the factors controlling the OF-C fate in tropical soils