  We investigate the effects of isocurvature perturbations on the 21cm
radiation from minihaloes (MHs) at high redshifts and examine constraints on
the isocurvature amplitude and power spectrum using the next generation of
radio telescopes such as the Square Kilometre Array. We find that there is a
realistic prospect of observing the isocurvature imprints in the 21cm emission
from MHs, but only if the isocurvature spectral index is close to $3$ (i.e.,
the spectrum is blue). When the isocurvature fraction increases beyond $\sim
10\%$ of the adiabatic component, we observe an unexpected decline in the 21cm
fluctuations from small-mass MHs, which can be explained by the incorporation
small MHs into larger haloes. We perform a detailed Fisher-matrix analysis, and
conclude that the combination of future CMB and 21cm experiments (such as
CMBPol and the Fast-Fourier-Transform Telescope) is ideal in constraining the
isocurvature parameters, but will stop short of distinguishing between CDM and
baryon types of isocurvature perturbations, unless the isocurvature fraction is
large and the spectrum is blue.
