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Community ecology remains focused on interactions at small scales, which limits causal understanding of how regional and local processes interact to mediate biodiversity changes. We hypothesise that species pool size and immigration are two regional processes altering the balance between local niche selection and drift that cause variation in plant diversity. We manipulated the richness and number of seeds sown (species pool size and immigration respectively) into 12 grasslands across a landscape soil moisture gradient. Greater immigration and smaller species pools increased the variation in plant composition explained by soil moisture gradients but resulted in greater erosion of plant α-diversity and spatial β-diversity over time. Our results suggest that regional constraints on colonisation make community assembly more variable but help maintain species diversity by limiting biotic homogenisation. This study provides large-scale experimental evidence on how regional contexts can alter the relative importance of fundamental processes shaping biodiversity change across scales.