Let G be a finite abelian group. This paper is concerned with nonnegative functions on G that are sparse with respect to the Fourier basis. We establish combinatorial conditions on subsets S and T of Fourier basis elements under which nonnegative functions with Fourier support S are sums of squares of functions with Fourier support T . Our combinatorial condition involves constructing a chordal cover of a graph related to G and S (the Cayley graph Cay( G, S)) with maximal cliques related to T . Our result relies on two main ingredients: the decomposition of sparse positive semidefinite matrices with a chordal sparsity pattern, as well as a simple but key observation exploiting the structure of the Fourier basis elements of G (the characters of G).We apply our general result to two examples. First, in the case where G = Z n 2 , by constructing a particular chordal cover of the half-cube graph, we prove that any nonnegative quadratic form in n binary variables is a sum of squares of functions of degree at most n/2 , establishing a conjecture of Laurent. Second, we consider nonnegative functions of degree d on ZN (when d divides N ). By constructing a particular chordal cover of the dth power of the N -cycle, we prove that any such function is a sum of squares of functions with at most 3d log(N/d) nonzero Fourier coefficients. Dually this shows that a certain cyclic polytope in R 2d with N vertices can be expressed as a projection of a section of the cone of positive semidefinite matrices of size 3d log(N/d). Putting N = d 2 gives a family of polytopes in R 2d with linear programming extension complexity Ω(d 2 ) and semidefinite programming extension complexity O(d log(d)). To the best of our knowledge, this is the first explicit family of polytopes (P d ) in increasing dimensions where xcPSD(P d ) = o(xcLP(P d )) (where xcPSD and xcLP are respectively the SDP and LP extension complexity).The authors are with the