  We demonstrate a method to quantify and manipulate nuclear spin decoherence
mechanisms that are active in zero to ultralow magnetic fields. These include:
(i) non-adiabatic switching of spin quantization axis, due to residual
background fields; (ii) scalar pathways due to through-bond couplings between
$^1$H and heteronuclear spin species, such as $^2$H used partially as an
isotopic substitute for $^1$H. Under conditions of free evolution, scalar
relaxation due to $^2$H can significantly limit nuclear spin polarization
lifetimes and thus the scope of magnetic resonance procedures near zero field.
It is shown that robust trains of pulsed dc magnetic fields that apply $\pi$
flip angles to one or multiple spin species may switch effective symmetry of
the nuclear spin Hamiltonian, imposing decoupled or coupled dynamic regimes on
demand. The method should broaden the spectrum of hyperpolarized biomedical
contrast-agent compounds and hyperpolarization procedures that are used near
zero field.

Chemical Physics;Quantum Physics