Uniform first-order definitions in finitely generated fields

We prove that there is a first-order sentence in the language of rings that is true for all finitely generated fields of characteristic 0 and false for all fields of characteristic >0. We also prove that for each n in N, there is a first-order formula psi_n(x_1,…,x_n) that when interpreted in a finitely generated field K is true for elements x_1,…,x_n in K if and only if the elements are algebraically dependent over the prime field in K.