  We determine the forecast errors on the absolute neutrino mass scale and the
equation of state of dark energy by combining synthetic data from the Dark
Energy Survey (DES) and the Cosmic Microwave Background (CMB) Planck surveyor.
We use angular clustering of galaxies for DES in 7 redshift shells up to $z\sim
1.7$ including cross-correlations between different redshift shells. We study
models with massless and massive neutrinos and three different dark energy
models: $\Lambda$CDM ($w=-1$), wCDM (constant $w$), and waCDM (evolving
equation of state parameter $w(a)=w_0 + w_{a}(1-a)$). We include the impact of
uncertainties in modeling galaxy bias using a constant and a redshift-evolving
bias model. For the $\Lambda$CDM model we obtain an upper limit for the sum of
neutrino masses from DES+Planck of $\Sigma m_\nu < 0.08$ eV (95\% C.L.) for a
fiducial mass of $\Sigma m_\nu = 0.047$ eV, with a 1$\sigma$ error of 0.02 eV,
assuming perfect knowledge of galaxy bias. For the wCDM model the limit is
$\Sigma m_\nu < 0.10 $ eV. For a wCDM model where galaxy bias evolves with
redshift, the upper limit on the sum of neutrino masses increases to 0.19 eV.
DES will be able to place competitive upper limits on the sum of neutrino
masses of 0.1-0.2 eV and could therefore strongly constrain the inverted mass
hierarchy of neutrinos. In a wCDM model the 1$\sigma$ error on constant $w$ is
$\Delta w= 0.03$ from DES galaxy clustering and Planck. Allowing $\Sigma m_\nu$
as a free parameter increases the error on $w$ by a factor of 2, with $\Delta
w=0.06$. In a waCDM model, in which the dark energy equation of state varies
with time, the errors are $\Delta w_0 = 0.2$ and $\Delta w_a = 0.42$. Including
neutrinos and redshift dependent galaxy bias increases the errors to $\Delta
w_0 = 0.35$ and $\Delta w_a = 0.89$.
