Short-term nutrient reduction reduces cover of an invasive winter annual grass without negatively impacting the soil microbial community.
Invasive winter annual grasses have degraded ecosystems around the world, forming large monocultures. However, common management methods, such as tillage, herbicide, and prescribed burning, also reduce native plant diversity and harm the soil microbial community. Early-season, short-term nutrient reduction applied to intact vegetation to manage invasive winter annual grasses could potentially decrease their abundance, while not impacting later season species. Low rates of sucrose, a labile carbon, were applied in a Pacific Northwest semiarid grassland in early spring to stimulate microbial growth and reduce nutrient availability to the invasive grass Ventenata dubia. Inorganic nitrogen was tracked throughout the experiment and plant and soil microbial community changes were determined at the end of the growing season. Sucrose application reduced nitrogen at the beginning of the season, but effects did not persist to mid-May when most plants were still active and soil moisture was not limiting. Treatments reduced V. dubia cover, per area seed production, and seed mass with no corresponding impact on perennial or other annual plants, except at the highest application rate when annual cover was reduced by 2%. The soil microbial community, determined via phospholipid fatty acid and neutral lipid fatty acid analysis, and the soil nematode community were largely unchanged at the end of the growing season with no reduction in the abundance of arbuscular mycorrhizal fungi. This short-term nutrient reduction method has the potential to not only target invasive winter annual grasses, but any plants that are active earlier or later than the rest of the community.