  We study the symmetry and strength of the superconducting pairing in a
two-orbital t-J1-J2-K model for iron pnictides using the salve boson mean-field
theory. We show that the nearest-neighbor biquadratic interaction -K (S_i *
S_j)^2 inflences the superconducting pairing phase diagram by promoting the
d_{x2-y2} B_{1g} and the s_{x^2+y^2} A_{1g} channels. The resulting phase
diagram consists of several competing pairing channels, including an isotropic
s_{\pm} A_{1g} channel, an anisotropic d_{x2-y2} B_{1g} channel, and two s+id
pairing channels. We have investigated the evolution of superconducting states
with electron doping, and find that with the biquadratic interaction various
pairing channels may dominate at different doping concentrations. We show that
this is crucial in understanding the doping evolution of superconducting gap
anisotropy observed in some angle resolved photoemission spectrum measurements.
