Invasive earthworms may alter resource availability and create conditions that favor invasive plant species. Ventenata dubia, a common exotic annual grass, is found in agricultural fields and native and conservation reserve program (CRP) grasslands in the inland Pacific Northwest (iPNW). The invasive earthworm, Aprorrectodea trapezoides is the dominant earthworm within these same ecosystems. A greenhouse experiment was conducted to determine 1) if A. trapezoides influences the growth of V. dubia and the native grass, Pseudoroegneria spicata, and 2) the ability of A. trapezoides to influence the allocation of nitrogen between the native and exotic grass species. Replicate mesocosms were filled with homogenized Palouse loam soil and assigned one of six treatments (V. dubia with A. trapezoides; V. dubia without earthworms; V. dubia with P. spicata and A. trapezoides; V. dubia and P. spicata without earthworms; P. spicata with A. trapezoides and P. spicata without earthworms). Soil moisture was monitored and added as needed and plant heights were taken weekly. Above and below ground biomass and total N content are currently being measured.
Preliminary analysis of the data shows that A. trapezoides has a significant impact on plant height in the presence of V. dubia and P. spicata (P < 0.0001). Average heights of V. dubia in the presence of P. spicata and A. trapezoides was greater (4.02 cm) than when it was grown with P. spicata without A. trapezoides (3.45 cm). This shows that A. trapezoides may contribute positively to the growth of invasive V. dubia when in the presence of P. spicata. Above and below-ground biomass and total nitrogen content of both plant species along with microbial biomass and inorganic nitrogen content will be further analyzed to better quantify the impact of A. trapezoides on both grasses.