Three Bermudagrass-suppression techniques have little effect on soil-nutrient availability and microbial communities 200 days after application

Abstract

Cynodon dactylon (Bermudagrass) is a highly competitive invasive plant dominating many southern grasslands. Glyphosate (N-(phosphonomethyl)glycine) is often used in conjunction with other herbicides and land-management practices to remove Bermudagrass in preparation for native-grassland restoration projects. Studies evaluating glyphosate-induced effects on soil ecology, however, often disregard potential for herbicide × or herbicide × mechanical treatment interactions. Therefore, our objective was to evaluate three chemical Bermudagrass-removal methods (i.e., repeated glyphosate application, repeated glyphosate application + imidazolinone herbicide use, repeated glyphosate application + mechanical above-ground biomass removal), previously described by Farthing et al. (2018), at research sites in Stephenville and McGregor, TX USA. We sought to determine if inter-treatment differences in soil pH, multiple element concentrations, and microbial diversity existed. We applied treatments to 355-m2 plots throughout summer 2015 and collected soil samples in April–July 2016 (216–249 and 218–259 days following final herbicide applications in Stephenville, and McGregor, respectively). We observed differences (P ≤ 0.05) in soil pH and NO3-N relative to untreated controls at the McGregor site, however, we were unable to observe differences directly related to treatments. Microbial communities differed (P ≤ 0.05) between locations, but treatments had no effect on soil-bacteria or archaea species richness and Shannon diversity. We did find that ≈1% of Operational Taxonomic Units appeared to be differentially abundant following treatments, but it is unclear whether differences are connected to glyphosate application. Our findings were generally consistent with previous research indicating herbicide use induces few-to-no long-term shifts in soil microbial communities, but our results highlight the necessity for microbiological field-study designs to include multiple locations.