Ecotypic variation in a dominant prairie grass affects belowground biomass and ecosystem processes

Background/Question/Methods

Root productivity likely has consequences for the composition, activity, and recovery of soil microbial populations and belowground processes mediated by these organisms.  Variation in root productivity could result from intraspecific variation in plant traits that may have evolved in response to local adaptation to climate. In tallgrass prairie, ecotypic variation exists in response to a strong precipitation gradient across the central US grassland.  Thus, ecotypic variation within a species may differentially affect belowground net primary productivity (BNPP) and the associated soil microbial community, to affect ecosystem processes.  The goals of this study were to elucidate: (1) whether ecotype, environment, or an ecotype by environment interaction regulate BNPP of a dominant species (Andropogon gerardii) collected from and reciprocally planted in common gardens across a precipitation gradient, and (2) whether variation in BNPP scales to affect microbial biomass and ecosystem processes.  We quantified root biomass, BNPP (using root in-growth bags), and soil microbial biomass and nutrient mineralization rates in root-ingrowth cores below six population sources of A. gerardii (2 Illinois, 2 eastern Kansas, and 2 central Kansas) established in southern Illinois, eastern Kansas, and central Kansas.

Results/Conclusions

Root biomass varied among the ecotypes across all sites (P<0.001), with Illinois ecotypes exhibiting highest biomass and central Kansas ecotypes lowest biomass across the precipitation gradient.  Further, root biomass was affected by an interaction between site and source populations when ecotypes were grouped according to region (P=0.0030) that demonstrated a “home site advantage” for each regional source population and root biomass of regional populations was highest in Illinois and lowest in central Kansas sources.  There was an ecotype effect (P<0.001) and an interaction between site and regional source populations (P=0.021) on BNPP, following the same pattern as root biomass, as root biomass was positively correlated with BNPP (r=0.67, P<0.001).  Microbial biomass nitrogen in root in-growth bags showed a main effect of ecotype (P=0.009).   Potentially mineralizable nitrogen showed an interaction between site and ecotype (P=0.028) due to differences among ecotypes in Kansas but not in Illinois.  Lastly, a main effect of ecotype occurred for microbial biomass carbon (P<0.001) and carbon mineralization rates in soil (P<0.001), with eastern KS ecotypes promoting these labile carbon pools more than other ecotypes.  Thus, ecotypic variation, typically characterized by aboveground traits, can also scale to affect belowground structure and ecosystem processes.