Do Invasive Grasses Outcompete Native Species by Altering Soil Carbon Cycling and Microbial Diversity? A Study In the Santa Rita (Arizona) Rangeland.

It is generally acknowledged that invasions by exotic plant species represent a major threat to ecosystem stability through the ability of invasive species to outcompete natives. Invasive exotic species tend to have higher standing crop biomass, higher net primary production, increased soil nutrient cycling, and faster growth rates than the native species that they displace. What is less clear is the mechanism(s) that induce the measurable soil changes. With the hypothesis that shifts in nutrient cycling may be microbially-mediated, we conducted a one-year study in the Santa Rita Rangelands (southern Arizona) to assess belowground effects of an invasive grass on soil microbial diversity and carbon (C) cycling. Soil samples were collected at the bases of two native rangeland grasses, bush muhly (BM; Muhlenbergia porter) and Arizona cottontop (AC; Digitaria californica), and the invasive Lehmann’s lovegrass (LL; Eragrostis lehmannianna). Net soil CO2 efflux was measured at the time of soil collection with a LiCOR 6400, while net community-level physiological profiles (CLPPs) were examined using Biolog C-substrate profile microtiter plates. Additional plates with added cycloheximide and streptomycin were utilized to assess total bacterial and fungal CLPPs, respectively. Net CLPPs showed distinct seasonal differences, as invasive LL showed far higher CO2 production (4.5 and 3.8 µmol m-2 s-1 under invasive and native, respectively), while CLPPs revealed that the LL total C substrate utilization and microbial diversity were highest during the monsoon season (July to September). Total microbial diversity was higher underneath native grasses in the fall and winter, but net CO2 efflux was equal. All sites showed greater bacterial diversity and substrate usage when compared to fungal during all seasons. Little is known about the mechanisms utilized by invasive plants to outcompete natives for available resources; studies such as this will elucidate competitive tools used by invasive plants and may aid in future remediation of disturbed ecosystems.