  Let $1 \leq k \leq n$ be a positive integer. A {\em nonnegative signed
$k$-subdominating function} is a function $f:V(G) \rightarrow \{-1,1\}$
satisfying $\sum_{u\in N_G[v]}f(u) \geq 0$ for at least $k$ vertices $v$ of
$G$. The value $\min\sum_{v\in V(G)} f(v)$, taking over all nonnegative signed
$k$-subdominating functions $f$ of $G$, is called the {\em nonnegative signed
$k$-subdomination number} of $G$ and denoted by $\gamma^{NN}_{ks}(G)$. When
$k=|V(G)|$, $\gamma^{NN}_{ks}(G)=\gamma^{NN}_s(G)$ is the {\em nonnegative
signed domination number}, introduced in \cite{HLFZ}. In this paper, we
investigate several sharp lower bounds of $\gamma^{NN}_s(G)$, which extend some
presented lower bounds on $\gamma^{NN}_s(G)$. We also initiate the study of the
nonnegative signed $k$-subdomination number in graphs and establish some sharp
lower bounds for $\gamma^{NN}_{ks}(G)$ in terms of order and the degree
sequence of a graph $G$.
