Isotopic measurements of plant remains from archaeological sites are now routinely used to reconstruct agricultural practices in the Old World. These studies use charred botanical remains as the analytical substrate because (1) these are the materials that are commonly preserved in the archaeological record and (2) the integrity of the isotopic compositions of archaeological plant remains that are uncharred or desiccated has been questioned, particularly for δ¹⁵N. By comparing charred and uncharred plant remains from two Late Moche (ad 600–800) sites in the Jequetepeque valley on the north coast of Peru—San José de Moro and Cerro Chepén—we sought to directly test the assumption that the isotopic compositions of desiccated plant remains are not representative of the isotopic compositions of the living plants. The average δ¹⁵N values of charred (+11.3 ± 4.4‰) and desiccated (+10.1 ± 5.1‰) remains from these two sites did not significantly differ from one another, suggesting that uncharred plant remains can produce reliable isotopic measurements under some circumstances. Furthermore, the relatively high δ¹⁵N values of the plant remains from these two sites are consistent with the ancient use of composted camelid manure as a fertilizer.