Let (M, g) be a closed, connected and orientable Riemannian manifold with nonnegative Ricci curvature. Consider a Lagrangian L(x, v): TM → ℝ defined by \(L(x,\, v) := {1 \over 2}gx(v, \, v)-\omega(v) + c\) , where c ∈ ℝ and ω is a closed 1-form. From the perspective of differential geometry, we estimate the Laplacian of the weak Kolmogorov-Arnold-Moser (KAM) solution u to the associated Hamilton-Jacobi equation H(x, du) = c[L] in the barrier sense. This analysis enables us to prove that each weak KAM solution u is a constant if and only if ω is a harmonic 1-form. Furthermore, we explore several applications to the Mather quotient and the Mañé Lagrangian.