Invasive annual grasslands reduce environmental heterogeneity and bacterial diversity

Annual grassland invasions can decrease environmental heterogeneity and reduce the biological diversity of plants and animals. There is a generally positive relationship between environmental heterogeneity and biodiversity. Here we test if the diversity of soil microorganisms, displays a positive relationship with soil environmental heterogeneity. Specifically, we test to see if reduced soil environmental heterogeneity in invasive annual grasslands lead to reductions in microbial alpha- and beta- diversity. Soil profiles were sampled across invasive annual grassland, oak woodland, and coastal sage scrub habitat and characterized for environmental heterogeneity (soil percent carbon, nitrogen, water content, total dissolved solids, and pH in addition to litter percent carbon, nitrogen and C:N), alpha- and beta- diversity. We found that invasive annual grassland habitat had greater soil environmental homogeneity than native woody habitats throughout the soil profile. Annual grassland communities had lower alpha-, but not beta-, diversity than native woody species. Patterns of alpha diversity with depth differed between grassland and woody habitat, and although not significant, woody habitats had higher community heterogeneity. Alpha diversity and beta diversity showed positive relationships with several measures of environmental heterogeneity, suggesting that soil microbial diversity decreases with environmental homogenization, and annual grassland invasions into native woody habitats reduces soil microbial diversity. This was particularly true in deep soil communities.