  We study the decoupling effects in one-loop corrected N=1 supersymmetric
theory with gauge neutral chiral superfields, by calculating the one-loop
corrected effective Lagrangian that involves light and heavy fields with the
mass scale M, and subsequently eliminating heavy fields by their equations of
motion. In addition to new non-renormalizable couplings, we determine the terms
that grow as log(M) and renormalize the fields and couplings in the effective
field theory, in accordance with the decoupling theorem. However, in a theory
derived from superstring theory, these terms can significantly modify low
energy predictions for the effective couplings of light fields. For example, in
a class of heterotic superstring vacua with an anomalous U(1) the vacuum
restabilization introduces such decoupling effects which in turn correct the
low energy predictions for certain couplings by 10-50%.
