It has been speculated that quantum-gravity might induce a foamy space-time structure at small scales, randomly perturbing the propagation phases of free-streaming particles (such as kaons, neutrons, or neutrinos). Particle interferometry might then reveal nonstandard decoherence effects, in addition to standard ones (due to, e.g., finite source size and detector resolution.) In this work we discuss the phenomenology of such nonstandard effects in the propagation of electron neutrinos in the Sun and in the long-baseline reactor experiment KamLAND, which jointly provide us with the best available probes of decoherence at neutrino energies $E\sim \mathrm{\text{few}}$ MeV. In the solar neutrino case, by means of a perturbative approach, decoherence is shown to modify the standard (adiabatic) propagation in matter through a calculable damping factor. By assuming a power-law dependence of decoherence effects in the energy domain ($E^n$ with $n=0$, $\pm 1$, $\pm 2$), theoretical predictions for two-family neutrino mixing are compared with the data and discussed. We find that neither solar nor KamLAND data show evidence in favor of nonstandard decoherence effects, whose characteristic parameter ${\gamma }_0$ can thus be significantly constrained. In the “Lorentz-invariant” case $n=-1$, we obtain the upper limit ${\gamma }_0<0.78\times {10}^{-26}\mathrm{GeV}$ at 95% C.L. In the specific case $n=-2$, the constraints can also be interpreted as bounds on possible matter density fluctuations in the Sun, which we improve by a factor of $\sim 2$ with respect to previous analyses.

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