sampling design

any advice on sampling design for soil seed bank study?

Our group is considering writing a proposal to test the soil seed bank of an endangered annual (Acanthomintha ilicifolia). There are several clay lenses that this species was previously found to inhabit, but it has not been observed aboveground for many years, and other populations are contracting. We are debating how many samples to take and at what interval across space we should take them across the suspected habitat. Does anyone have suggestions from experience or know of specific papers that relate to this type of sampling?