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In the past years the observational evidence that s-process elements from Srto Pb are produced by stars ascending the socalled Asymptotic Giant Branch (orAGB) could not be explained by self-consistent models, forcing researchers toextensive parameterizations. The crucial point is to understand how protons canbe injected from the envelope into the He-rich layers, yielding the formationof 13C and then the activation of the 13C(a,n)16O reaction. Only recently,attempts to solve this problem started to consider quantitativelyphysically-based mixing mechanisms. Among them, MHD processes in the plasmawere suggested to yield mass transport through magnetic buoyancy. In thisframework, we compare results of nucleosynthesis models for Low Mass AGB Stars(M<=3Mo), developed from the MHD scenario, with the record of isotopicabundance ratios of s-elements in presolar SiC grains, which were shown tooffer precise constraints on the 13C reservoir. We find that n-captures drivenby magnetically-induced mixing can indeed account for the SiC data quite welland that this is due to the fact that our 13C distribution fulfils the aboveconstraints rather accurately. We suggest that similar tests should be nowperformed using different physical models for mixing. Such comparisons wouldindeed improve decisively our understanding of the formation of the neutronsource.

s-Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains