  We investigate Cartan subalgebras in nontracial amalgamated free product von
Neumann algebras $M_1 \ast_B M_2$ over an amenable von Neumann subalgebra $B$.
First, we settle the problem of the absence of Cartan subalgebra in arbitrary
free product von Neumann algebras. Namely, we show that any nonamenable free
product von Neumann algebra $(M_1, \varphi_1) \ast (M_2, \varphi_2)$ with
respect to faithful normal states has no Cartan subalgebra. This generalizes
the tracial case that was established in \cite{Io12a}. Next, we prove that any
countable nonsingular ergodic equivalence relation $\mathcal R$ defined on a
standard measure space and which splits as the free product $\mathcal R =
\mathcal R_1 \ast \mathcal R_2$ of recurrent subequivalence relations gives
rise to a nonamenable factor $\rL(\mathcal R)$ with a unique Cartan subalgebra,
up to unitary conjugacy. Finally, we prove unique Cartan decomposition for a
class of group measure space factors $\rL^\infty(X) \rtimes \Gamma$ arising
from nonsingular free ergodic actions $\Gamma \curvearrowright (X, \mu)$ on
standard measure spaces of amalgamated groups $\Gamma = \Gamma_1 \ast_{\Sigma}
\Gamma_2$ over a finite subgroup $\Sigma$.
