Semiarid soil microbial associations with dominant plants across a shrub-grassland transition

Background/Question/Methods

Direct and indirect interactions between aboveground and belowground systems drive community structure and ecosystem processes. Yet it is unclear how state changes affect plant-soil feedbacks. We investigated plant-soil interactions within semiarid grasslands subject to shrub encroachment, a phenomenon happening around the world as a consequence of climate change. Aboveground, the conversion of grasslands to shrublands alters plant diversity and biomass, with effects cascading through consumer tropic levels. Belowground effects are less obvious. We measured the activity and community composition of soil microorganisms associated with dominant plant species across a vegetation transition from C₄-dominated grassland to creosotebush-dominated shrubland in central New Mexico. Microbial activity was measured via extracellular enzyme activities (EEA) and bacterial and fungal composition examined with phylogenic DNA sequencing. 

Results/Conclusions

Aboveground, plant size (height and percent cover) was similar for grasses in the grassland and ecotones and similar for shrubs in the shrubland and ecotone. Belowground, enzyme activities associated with rhizosphere soils were lower in the ecotone for both dominant grasses (Bouteloua eriopoda, Pleuraphis jamesii) and the dominant shrub (Larrea tridentata) when compared to activities in grassland and shrubland, respectively. For example when compared to activities within the grass-shrubland ecotone, activity of alkaline phosphatase, an enzyme that removes phosphate groups from organic molecules, was 37% higher under P. jamesii in grasslands and 43% higher under L. tridentata in shrublands. Some of the differences in enzyme activity were likely due to variation in resource availability, as soil organic matter content was statistically lower within ecotone soils when compared to both grassland and shrubland soils. These belowground responses were not predictable based on observations of aboveground plant performance, but may nonetheless affect the rate, and perhaps the limit, of ecosystem transition. As climate change increasingly alters community structure and function in ecosystems worldwide, it is important to concurrently study both above- and belowground effects to better understand the full extent of state transitions on ecosystem functioning.