Is there a first order formula $\varphi[x]$ in $(\mathbb Q, +, \cdot, 0)$ such that $x≥0$ iff $\varphi[x]$? In the first-order language $\mathscr L$ having $(+, \cdot, 0)$ as signature, it is easy to define a formula $\phi[x]$, namely $\exists y \; x = y^2$, satisfying : 
$$\text{for all } x \in \Bbb R, \quad x \in \Bbb R_+ \;\text{ if and only if} \;\; \phi[x]  $$
My question is : what happens if I replace $\Bbb R$ by $\Bbb Q$ ? More precisely :

Is there a first-order formula $\phi[x]$ of $\scr L$, such that $$\text{for all } x \in \Bbb Q, \quad x \in \Bbb Q_+ \;\text{ if and only if} \;\; \phi[x]  $$

Said differently, I would like to know if the set of the positive rationals is definable in that language. Related questions are, for instance : (1), (2).
I don't know if the $\scr L$-structure $(\Bbb Q, +, \cdot, 0)$ admits elimination of quantifiers. If this is the case, then this could be helpful ; see this answer.
Thank you for your comments !

Yes. Every rational number $\ge 0$ is the sum of four squares. This is easily derived from Lagrange's Theorem which says that every non-negative integer is the sum of four squares.
Note that the formula we get is existential. 
