Many grass-dominated ecosystems around the world have experienced woody plant encroachment during the past century due to livestock grazing, fire suppression, and/or changes in climate and atmospheric chemistry. Our prior research in the Rio Grande Plains of Texas shows that subtropical thorn woodlands dominated by N-fixing tree legumes have largely replaced grasslands, resulting in increased above- and belowground productivity, soil C and N storage, and the size and activity of the soil microbial biomass pool. These profound changes in ecosystem structure and function are likely to influence the composition and function of soil microbial communities. The purpose of this study was to assess the impact of grassland to woodland transitions on the biodiversity of soil microbial communities using molecular methods. Soil samples (0-10 cm) were collected in remnant grasslands and in four different woody community types (discrete clusters, groves, drainage woodlands, and playas) in a subtropical savanna parkland in southern Texas. Ages of woody plant stands were determined by dendrochronology. Microbial DNA was extracted from whole soil, the 16S-23S ribosomal intergenic spacer region was PCR amplified, and the resulting fragment length diversity analyzed with automated ribosomal intergenic spacer analysis (ARISA). For calculation of diversity indices, we considered each fragment length to represent a unique species.

Results/Conclusions

Bacterial species richness decreased by 48% from 144 species in remnant grasslands to approximately 75 species in each of the four wooded landscape elements. Although the evenness component of diversity was comparable across all sites, the Shannon-Weiner index decreased from 4.38 in grasslands to 3.81 in wooded areas due to the large decline in species richness in wooded areas. There was no relationship between any of the bacterial diversity indices and the age of the woody plant stands, suggesting bacterial communities change rapidly and then remain stable following woody plant encroachment into grassland. Bray-Curtis ordination revealed that microbial community composition of grasslands was significantly different from that of all woody plant community types. We hypothesize that the lower bacterial diversity in wooded areas may be a consequence of  reduced diversity of organic matter substrates resulting from lower plant species diversity in wooded areas.