Do you plan to use germination assays to determine seed bank composition and density? You’re likely to get the highest proportion of seeds in the top 5 cm of soil, so that’s a good sampling depth to aim for. I’ve done some soil seed bank work and would suggest taking a few cores from each location (e.g., 4 corners of a 1/2 m^2 plot frame or somethign like that) and combining the samples from each plot. I’d recommend concentrating your samples in areas where it was known to occur or does occur, unless you have reason to believe that the seeds have a long dispersal distance. We did a soil seed bank study of salt marsh bird’s beak and sampled within recently known, historically known and just outside of historically known areas. We didn’t find any bird’s beak seeds outside of the historically known range. This was surpising to me because I had asumed that those seeds would be mobile with the tide moving in and out on a daily basis. Sampling the seed bank is like a needle in a haystack because soils (and the seeds within them) are so hetergeneous. Here is a citation for a project focused on soil seed banks in the desert: Schneider, H.E. & Allen, E.B. Plant Ecol (2012) 213: 1277. https://doi.org/10.1007/s11258-012-0085-6 I’d be happy to share more about our bird’s beak work offline, if you’re interested.