On subspaces of non-commutative L_p-spaces

We study some structural aspects of the subspaces of the non-commutative (Haagerup) L_p-spaces associated with a general (non necessarily semi-finite) von Neumann algebra A. If a subspace X of L_p(A) contains uniformly the spaces \ell_p^n, n>= 1, it contains an almost isometric, almost 1-complemented copy of \ell_p. If X contains uniformly the finite dimensional Schatten classes S_p^n, it contains their \ell_p-direct sum too. We obtain a version of the classical Kadec-Pel czynski dichotomy theorem for L_p-spaces, p>= 2. We also give operator space versions of these results. The proofs are based on previous structural results on the ultrapowers of L_p(A), together with a careful analysis of the elements of an ultrapower [L_p(A)]_U which are disjoint from the subspace L_p(A). These techniques permit to recover a recent result of N. Randrianantoanina concerning a Subsequence Splitting Lemma for the general non-commutative L_p spaces. Various notions of p-equiintegrability are studied (one of which is equivalent to Randrianantoanina's one) and some results obtained by Haagerup, Rosenthal and Sukochev for L_p -spaces based on finite von Neumann algebras concerning subspaces of L_p(A) containing \ell_p are extended to the general case.