  Let $M$ be a complete non-compact Riemannian manifold satisfying the doubling
volume property. Let $\overrightarrow{\Delta}$ be the Hodge-de Rham Laplacian
acting on 1-differential forms. According to the Bochner formula,
$\overrightarrow{\Delta}=\nabla^*\nabla+R_+-R_-$ where $R_+$ and $R_-$ are
respectively the positive and negative part of the Ricci curvature and $\nabla$
is the Levi-Civita connection. We study the boundedness of the Riesz transform
$d^*(\overrightarrow{\Delta})^{-\frac{1}{/2}}$ from $L^p(\Lambda^1T^*M)$ to
$L^p(M)$ and of the Riesz transform $d(\overrightarrow{\Delta})^{-\frac{1}{2}}$
from $L^p(\Lambda^1T^*M)$ to $L^p(\Lambda^2T^*M)$. We prove that, if the heat
kernel on functions $p_t(x,y)$ satisfies a Gaussian upper bound and if the
negative part $R_-$ of the Ricci curvature is $\epsilon$-sub-critical for some
$\epsilon\in[0,1)$, then $d^*(\overrightarrow{\Delta})^{-\frac{1}{2}}$ is
bounded from $L^p(\Lambda^1T^*M)$ to $L^p(M)$ and
$d(\overrightarrow{\Delta})^{-\frac{1}{2}}$ is bounded from
$L^p(\Lambda^1T^*M)$ to $L^p(\Lambda^2T^* M)$ for $p\in(p_0',2]$ where $p_0>2$
depends on $\epsilon$ and on a constant appearing in the doubling volume
property. A duality argument gives the boundedness of the Riesz transform
$d(\Delta)^{-\frac{1}{2}}$ from $L^p(M)$ to $L^p(\Lambda^1T^*M)$ for $p\in
[2,p_0)$ where $\Delta$ is the non-negative Laplace-Beltrami operator. We also
give a condition on $R_-$ to be $\epsilon$-sub-critical under both analytic and
geometric assumptions.
